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Appendix Part LXII: Adventitious Roots.

(INCLUDING NOTES ON STILT-ROOTS, LENTICELS, PNEUMATOPHORES; SWAMP PLANTS.)

*

Adventitious roots are roots arising from stems or leaves. The rarer phenomenon of developing adventitious roots on leaves is taken advantage of to a small extent in horticulture, and need not be further referred to here. In practice all dicotyledons, and most monocotyledons, given favourable conditions, which can always be forced by the various horticultural methods of cuttage, can be made to produce this phenomena of root action.

It does not matter whether the material for the purpose is selected from the stem, root, or leaf -from growing wood, as in Euphorbia, or ripened wood, as in Salix; from short pieces of root, as in Bouvardia, or long pieces, as in Rubus (Blackberry), or from the leaf, entire as in Echeveria, divided as in Begonia, or from bulb scales, as in Lilium. In all these cases roots can be produced without the aid of the radicle from the seed.

Stilt-roots. — Some adventitious roots are, for obvious reasons, called "Stiltroots," "brace-roots," "prop-roots."

Stilt-roots are attached to the main trunk, being developed from the lower part of the stem, to which they are attached obliquely and stilt-like. These are well-known in the case of the Screw Pine (Pandanus) and certain Mangroves of our coasts, which are useful in fixing mud-banks. They are common in Maize (Indian Corn).

We have also spinous and non-spinous adventitious roots on Palms. Verschaffellia splendida H. Wendl. is a good example. The root-system often consists of a succession of roots produced farther and farther from the original base of the stem after the disappearance of the ordinary or tap-root. Some Palms and Screw Pines are ultimately borne several feet clear of the ground, in consequence of the lower ones dying away, by the stout adventitious roots which succeed each other higher and higher up the trunk. We notice this form of Stilt-root in certain Orchids in our glass-houses. For example, Aerides odoratum Lour., Angraecum eburneum Bory, Vanda teres Lindl.

Common Aroids illustrating this phenomenon are Alocasia macrorrhiza Schott., Philodendron Andreanum Devans, Anthurium ornatum Schott.

In quite a number of well-known economic plants we have adventitious roots. For example, in the Onion, the single primary root lasts but a short time, and is succeeded by others, which do not arise as branches from the primary one, but spring from the very short stem of the plant. In the case of wheat and barley, when the plants have begun to unfold a few leaves above the ground, the primary roots are succeeded by


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adventitious roots, which grow from the lower nodes or joints near the surface of the soil. Such roots are common in the potato, runners of the strawberry, couch-grass, buffalo-grass, &c.

There is a remarkable development of adventitious roots on the trunks of some tree-ferns (e.g., Dicksonia), where they way appear in such abundance as to enclose the trunk in a spongy mass. Other ferns showing this well are Alsophila Cooperi Hook., Cyathea dealbata Swartz (both tree ferns), Nephrolepis cordifolia Pr., and also Todea barbara L. (Squatty Fern).

The tendency to form adventitious roots may be taken advantage of to raise plants from cuttings, by placing them directly in the ground (Roses, Willows), in water, or by packing with a cushion of damp moss (Figs). This is aided sometimes by incisions in the stem. Adventitious roots may be seen in the Dragon Tree (Dracæna Draco) and Mr. E.N. Ward, Superintendent of the Botanic Gardens, Sydney, has suggested that they may be taken advantage of to save several years in the propagation of this ornamental tree.

Coulter, Barnes and Cowles ("Text-book of Botany," ii, 503) emphasise the value to a tree of a tendency to produce adventitious roots when coming into contact with wet soil, thus Willows and Poplars possess this character in a pre-eminent degree, and this is economically useful in reclamation work, for when they are partly buried by sand or soil, as fast as the stems are buried, new adventitious roots appear at higher levels. In this connection note the lists of plants recommended for reclamation of river banks in Vol. vi, pp. 141 — 143 of the present work. Almost invariably they were selected because of their tendency to produce adventitious roots, a few exceptions being plants selected to clothe the normal banks. Indeed, the study of plants which produce adventitious roots is indispensable to workers in problems of river erosion, and, to a less extent, sand drift (see Part LVII of the present work).

Now Pines (Pinus), Cypress Pines (Callitris), Oaks (Quercus), &c., are unable to develop adventitious roots in this manner, and hence they are readily killed when partly buried. From this cause we lost our finest Cork Oak (Quercus Suber), and other valuable trees in the filling-up operations in the Botanic Gardens, Sydney, in the year 1903. The subject has a very practical aspect to the landscape gardener and tree-planter generally in his operations of filling in soil around the stems of trees, and one realises that very little attention has been given to it.

Adventitious Roots in Native Trees.

The photo of Eucalyptus rostrata Schlecht., by His Honor Judge Docker, is, so far as I am aware, the first published of this particular Eucalypt with adventitious roots.

Eucalyptus resinifera Sm. — In the Botanic, Gardens, Sydney, there is a tree of this species ("Forest Mahogany") not planted by the hand of man, which has sent forth, at a; distance of 4 1/2 feet from the ground, an adventitious root, which is now 2 1/4 inches in diameter, and which has now entered the ground, thus forming a small auxiliary stem.




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For the following three photographs of a Flooded Gum (Eucalyptus rudis), Western Australia, I am indebted to Mr. W. C. Grasby, of the Western Mail. It inhabits the low-lying flats and banks of the rivers between the Swan and Blackwood, and is sometimes to be met with east of the Great Southern Railway. This tree is of great interest to settlers, first, because the timber is of no commercial value, and is poor firewood; second, because it is exceedingly difficult to kill; and third, because it is equally difficult to burn. Of all trees which have to be destroyed by the settler in clearing his land, the Flooded Gum is the hardest to destroy. After ringbarking it takes longer than any other to die, and is the most persistent in throwing up base shoots or suckers. Even when the suckers are kept regularly knocked off, the stump will remain green for years; but the photographs reproduced serve to illustrate the reason why this tree is so hard to destroy.

Photograph No. 1 represents an ancient Flooded Gum, about 4 feet or more in diameter, which has been thoroughly ringbarked about eight years, standing on. the swampy edge of the brook at Ferndale, Balingup. It will be noticed that while the larger portion of the tree is dead, there is still a fair amount of green top, although the tree has been ringbarked about seven years. At the foot of the tree in the foreground will be seen a big limb, and just above the thick end of the limb a large scar showing the place from which the limb has fallen. The falling of this limb exposed the reason for the tree remaining partly alive.

No. 2 is a photograph of the lower portion of the trunk of the same tree. In the foreground is the thick end of the big limb, and on the front of the tree is a large scar made by the breaking away of the dead limb. The mark of the ringbarking can be seen about a foot above the fallen limb. When this limb fell the cause of the tree remaining alive was explained. It will be noticed that from the green sapwood under the upper portion of the scar left by the fallen limb, a number of roots have grown, and one, fully 3 inches in diameter, has found its way down the crack between the old limb and the main portion of the trunk, and lower down has entered through the rotten interior of the tree into the ground. There must have been a crack between this limb and the main tree at the time when the ringbarking was done, and in this crack, of course, the water would run, and there was a certain amount of rotted wood. During the wet season in that part of the State a mass of roots grew from the upper portion of the wound, and although only two, the big one and one other, are now alive, the remains of the smaller ones are to be seen in considerable number. As long as the big limb remained on the tree none of these roots could be seen.

On the bank of the same brook, a few hundred yards from the big tree, is a smaller Flooded Gum, about 9 inches in diameter, and perhaps 18 or 20 feet high. Some years ago this tree was well ringed about 6 inches from the ground, which remains wet through the whole of the summer, and in the winter time is covered with water for months. Indeed, the water probably covers the ringbarked area for several months in the winter. It will be noticed (Photograph No. 3) that this tree, instead of dying as the result of ringbarking, has put out a number of aerial roots above the ringing, and is now in a


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fairly vigorous condition of health. The largest root, which is facing the camera, is quite 3 inches in diameter. It starts from just above the ringbarking, and has completely grown over it so as to hide it altogether. In addition to this big root are a number of smaller ones from as thick as a pencil to three-quarters of an inch. All these roots are on the west, south, and east sides of the tree.

Mr. Grasby's photos, published in 1914, are the first published, so far as I am aware, of adventitious roots in a Eucalypt.

E. robusta Sm. — Mr. T. Ormond O'Brien informed. me that thirty-five to forty years ago he drove a stake of Swamp Mahogany (Eucalyptus robusta) 3 inches in diameter, and 9 feet from the ground. The stake took root, and was (1905) a spreading tree of 25 feet, and diameter of 14 inches below the fork. It forked at 7 feet from the ground. Water has encroached on this tree, and its present roots are at a higher level than the original roots. I saw the tree at. Mr. O'Brien's house, at Bondi, a suburb of Sydney, and published an account of it in the Sydney Morning Herald, of 15th March, 1905, this being the first published account of adventitious roots in Eucalyptus, so far as I am aware.

A few days afterwards a gentleman from Kincumber stated that stakes of Eucalyptus robusta "sprout readily if stuck in the ground." In the Herald of the 17th March, 1905, a correspondent, "Farmer," wrote:—

I can state that some years ago I erected a barn on the Richmond River. The upright posts were some form of the Eucalyptus (Mahogany) I believe. I squared them on two sides with the adze, and finished the buildings. For some considerable time afterwards young sprouts used to push through and grow about 6 inches long. These posts would be about 12 inches through at the butt end, and about 11 feet out of the ground. The subsoil was sandy and moist.

I believe these posts to have been Eucalyptus robusta also.

Dr. G. V. Perez, of Teneriffe, Canary Islands, wrote to me in 1915:—

E. tereticornis Sm. — "Stout stakes of E. tereticornis grown here hammered into the ground have produced shoots, but they have died after a few months." This is, of course, an Australian tree — our Forest Red Gum.

E. paniculata Sm. — Mr. F. Cridland gives me the following information concerning an Ironbark, the first Ironbark to be thus recorded:—

In June or July, 1916 (eighteen months ago), I had some Ironbark trees cult, down on my property near Cronulla. I had them carted to my house about a mile away, where they lay for a week or two, then I put them in the ground as upright posts to build a rose bower. Some time after one of the posts, about 9 inches in diameter, threw out a few shoots. These died off later, but others have since taken their places, and at the present time the post has several green shoots up to 6 inches in length.

Macadamia ternifolia F.v.M. — The "Queensland Nut." Mr. W. F. Blakely has drawn my attention to a stubby mass of roots in a tree of this species in the Botanic Gardens, Sydney, a little east of the Refreshment Room. They are at the forking of some branches, and about 7 feet from the ground. During a wet season these roots are obviously alive. This is the first instance of the kind in the Proteaceæ known to me.




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Melaleuca. — Now we come to the Tea-trees, which are closely allied to Eucalyptus. Mr. T. Ormond O'Brien, the well-known landowner, of Bondi Beach, wrote the following letter, which appeared in the Sydney Morning Herald of 9th February, 1905:—

I would like to draw the attention of those who understand such matters to what appears to be a novelty in regard to this tree. When I speak of the tea-tree I refer to that tree which, so far as I have seen it, grows actually in or near the margins of the fresh water lagoons on the coast. It grows to a good size, say thirty feet in height, with a bole two feet in diameter, having a bark very suitable for use where shingles are not available. I have seen it used by ' Old Wingle ' for making his gunyah. It bears a white cockade flower, which appears to be greatly relished by the bees, flying foxes, and paroquets. And now to the feature about the tree to which I wish to draw attention. At a lagoon on the coast where a number of these trees grew, the depth was greatly increased by reason of partial filling up, and this increased depth remained for some years. The result was that the tea-trees threw out a new set of roots-say from 3 to 6 feet from the ground. For some reason — probably a tunnel having been driven by the Sewerage Department in the vicinity — the waters of the lagoon went down below the new set of roots thrown out by the tea-trees, with the result that most of them have died. As it appears to me, the trees had ceased to live on their original roots, and the moisture being taken from their new sets of roots, death was the result. And there the trees may be seen standing, some in the water, sonic in the marsh, with the stems clear for a few feet above the ground, and then with a large bunch of tangled and matted roots, full of soil, which has been gathered out of the water. Two of these dead trees must be of great age, as I have known them above 50 years, and, so far as I can judge, they have not increased in size since my recollection of them.

I visited the trees, and they are the reputed Melaleuca Leucadendron (M. Smithii, R.T. Baker), and the formation of new sets of roots emanating from the trunk, was as Mr. O'Brien stated.

A few years later (1910) Messrs. C.T. Musson and W. M. Carne published a full paper, "The adventitious roots of Melaleuca linariifolia Sm." in Proc. Linn. Soc. N.S.W. xxxv, 662, with two excellent plates. The trees figured and described are on the Hawkesbury Agricultural College Farm, and also at Rickaby's Creek, near Richmond, N.S.W.

It is probable that adventitious roots will be found in all the Melaleucas (which are swamp-loving plants). I have found them in M. styphelioides Sm.

Owing to the facility with which they form adventitious roots, cuttings and stakes of this species, when forced into moist ground while in the resting season, readily form independent plants. The Chief Justice (Sir William Cullen) has propagated Melaleuca ericifolia on his property at Mosman, near Sydney, somewhat extensively in this way, for about twenty years to my knowledge.

Ficus. — To the category of plants with adventitious roots belong the Figs (Ficus), and these roots are sometimes so well-developed that they become devastating, and are hence called "Strangling Figs." I have dealt with the subject at Part LVIII, p. 225, of the present work at some length, and have given figures showing their development. In that paper I chiefly refer to F. rubiginosa, the Port Jackson or Illawarra Fig, but the Moreton Bay Fig (Ficus macrophylla) more frequently produces roots and an illustration is given of the adventitious roots of F. Henneana, a New South Wales Fig, already dealt with in Part XIV of the present work.

In their highest development we have the tropical Banyans, with their columnar stems covering large areas by the same tree. In Lord Howe Island a single tree of F. columnaris covers over three acres in this way.




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Lenticels.

An excellent popular article on lenticels entitled, "How the bark breathes" will be found in the Journal of Heredity (Washington, U.S.A.), 15th November, 1915, p. 490. It points out that in connection with the elaborate respiratory system of plants connected with the taking in of air and giving out of carbon dioxide we have three general types of external openings, viz.:—

1. Stomata or valves on the surfaces of leaves and young shoots.

2. Ventilating pores, which occur in certain aerial roots.

3. Lenticels, pores in the older wood, whose presence can be noted by the unaided eye in almost any plant.

The earlier naturalists were quite in the dark as to the functions of these pores. Guettard, who described them in 1745, designated them merely as glands; De Candolle (1826) thought they were a kind of bud, from which roots later put forth; Unger (1838) believed they had something to do with reproduction; but as early as 1809, Dupetit-Thouars declared their purpose was ventilation, and the work of several students during the next half century demonstrated that this opinion was well founded.

It has been found that lenticels are in some plants functionless, some for a season of the year; others are permanently closed, and of no value to the plant for breathing.

Following are the legends to two admirable photographic illustrations to the above quoted paper:—

The ventilators of a rose twig. — The irregular openings or 'eruptive craters' in the bark, photographed under high magnification, are known as lenticels, and serve as pores through which air is admitted to the inside of the plant. By channels and passages of various kinds between the interior cells of the plant, the air passes to even the most distant parts. The plant is thus enabled to renew its supply of oxygen, and at the same time it discharges carbon dioxide through the lenticels.

Twig of a Chinese Magnolia, highly magnified. — The dry, powdery cells which fill the breathing pores of the bark have absorbed moisture from the air until they have swelled out and protrude like warts. One of the functions of the lenticels is to regulate the transpiration of moisture between the interior of the tree and the outside air.

The author (unnamed) of the paper includes the following comment:—

These facts have led many plant physiologists to think that, although the lenticels undoubtedly do fulfil in many cases the function of breathing pores for the bark, that is not really their purpose. Such a solution of the problem accords well with the interpretation of nature of certain scientists, who hold on philosophical grounds that nothing should be said really to have a purpose. (l.c. p. 492).

Those who desire to pursue the subject further are invited to peruse "A Textbook of Botany" by Coulter, Barnes and Cowles, Vol. II, Ecology, p. 660, under the main heading, "Carbohydrate synthesis and aeration in stems," and the sub-headings, "The structural features of lenticels," "The causes of lenticel development," "The role of lenticels." In regard to the last, they say (p. 663):—

Lenticels are regions of gas exchange, taking the place of stomata in stems after the inception of secondary growth, and making possible the continued activity of the chlorophyll after cork formation has begun. Only a somewhat structureless organ such as a lenticel, consisting of an indefinite patch of loose cells, is fitted for gas exchange in bark, where growth and rupture occur continually.

At figure 974 is a clear photograph showing the markings with which we are familiar in Birch (Betula) bark, which consist of numerous transversely elongated permanent lenticels.




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Swamp Plants and Respiration.

In Proc. Australian Assoc. Adv. Science, i, 327 (1888) the late Dr. Joseph Bancroft read a paper, "Respiration in the roots of Shore-plants," which, like a good deal of his work, was in advance of his time, and it remained, at least in Australia, little added to for a number of years. He took cognizance of the aerating roots, or breathing-roots, or pneumatophores of certain Mangroves, and "knees" in certain Conifers. Their function is to supply oxygen to the trees anchored in mud.

He also took cognizance of certain adventitious, or aerial, or stilt-roots in Mangroves. It is not always convenient to strictly separate the two classes of roots. For example, at p. 328, Dr. Bancroft refers to Rhizophora mucronata, the Red Mangrove, and says, "it throws up no aerial (breathing) roots, but those sent downward, tripod-like (stilt-roots),. apertures are seen with elevated edges, circular, one-twentieth of an inch in diameter, and filled with reddish-brown powder." At Plate xxiii he figures these stilt-roots in this species.

Schimper, p. 401, says, "The species ofRhizophora do not possess special pneumatophores, yet the upper part of their stilt-roots that are above the mud perform the same function."

The stilt-roots have even an economic value. The aerial roots, being very elastic, afford good material for bows, of which the Fijians avail themselves. (Seemann.) Arched roots are similarly used by Solomon Islanders in Port Curtis, district, Queensland. C. Hedley (Proc. Roy. Soc. Qld. v, 11).

Dr. Bancroft, at p. 327, began his observations on Avicennia officinalis, the common Grey Mangrove (found pretty well round the Australian coast), near Brisbane, and Plates xxv and xxvii represent Mangroves from Moreton Bay. He refers to an earlier paper submitted to the Royal Society of Edinburgh, which was not published, and following is an abstract of the same.

Prof. Dickson read a paper by Dr. Joseph Bancroft on respiration in the roots of certain shore plants. His observations referred chiefly to the remarkable rootlets of Avicennia. These rootlets grow vertically upwards from the larger roots which extend themselves horizontally in the mud of salt-water creeks. The mud bank around the stem is covered by a brush of such rootlets to a distance of from 4 to 6 yards from the bole of the tree. This brush, by entangling débris, protects the bank from destruction by stream or tide. The rootlets are studded with pits or pores emitting powdery matter which consists of cells, and which may be observed floating on the surface of the brackish water of the creek. These pores he regards as corresponding to lenticels, and he finds that when air is forced into the out end of a rootlet it issues by the pores. Hence he conjectures that the function of the pores is to contribute to the aeration of the plant, a view coinciding with that held by several botanists as to the lenticels, which they regard as structures affording, like stomata, a communication between the atmosphere and the interior of the plant. (Nature, Vol. xxv, 403 [1882]).

In his 1888 paper Dr. Bancroft goes on to say:—

Amongst various things that interested me, a white powder floating on the brackish water, presented itself as an object of inquiry. This powder looked at a distance like the male Vallisneria flower, but on closer inspection it was observed to issue from openings in the aerial roots of neighbouring Avicennia trees, the habitat of which is restricted to the muddy banks of salt waters. Some of these upright roots in rapid growth, found casting off the white powder, I drew up, together with the horizontal white pithy parts that were in process of extension to an unoccupied mudbank. Having secured a number of specimens,


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I carried them home for further examination. This powder, by the aid of a microscope, was found to consist of cells, iodine tinting them brown. This substance could have no relation to the reproductive system of the Avicennia tree as the flowers are high up on the branches, followed by fruits like garden beans.

The aerial roots of Avicennia are from a foot to 18 inches long, covered with green epidermis, on which the tides deposit mud and confervas. They never throw out leaves, but occasionally become forked. The muddy bank around the Avicennia stem is covered by a brush of these roots to a distance of from 4 to 6 yards from the bole of the tree.

This brush, by entangling debris, protects the bank from destruction by stream or tide. The roots are as thick as a pen-holder, and are covered with pores, 500 or more to be counted on a single specimen. The pores just opened are surrounded by broken epiderm, looking like the sepals of a flower, but having no regularity. The horizontal portions of the root system to which the aerial upright parts just described are attached are white, pithy, and full of air, and though living in undrained mud are quite free from any waterlogged condition. As the upright roots appear to rise out of the mud to obtain air, could the powder-discharging pores contribute anything towards aeration? Might they be mouths to admit air? After considering how this could be determined, I attached the indiarubber head of a pipette used for eye-drops to the cut part of a root, tied it, and immersed the aerial portion in water. On compressing the rubber cap, air was found to issue freely from the pores, and at no other parts.

This, then, seems to me to be the function of the pores, to supply air to the root system of the mud- inhabiting Avicennia tree; the office of the discharged powder being to establish a communication between the air vessels of the plant and the outer atmosphere, by bursting open the cuticle of the root.

The lenticels of the generality of trees differ somewhat from the root-pore in having no cup-like margins, and the corky mass does not fall freely in the form of powder, as is found in the root of Avicennia. Yet air can be blown through these organs among the foliage of Aegiceras, less freely in Excaecaria., With Paquelin's bellows and patience, it maybe seen to issue from the lenticels on the young shoots of the peach, and by the same apparatus can be made to pass through the stomata near the midrib of the common Oleander. So far, I have seen air issue through the stomata of no other leaf, though experimenting with many.

Excaecaria agallocha has a large well-formed aperture, in which a brown powder is to be seen. I can blow air by the mouth applied to the out stem, through all these apertures, but find the bellows of Paquelin's thermo-cautery a very convenient instrument for such experiments. (Bancroft, p. 328).

The roots that show the greatest resistance to the passage of air, are those of the Excaecaria. The habitat of this tree is not in such close proximity to the shore as that of Avicennia, Rhizophora, or Aegiceras.

The same organ I saw on dried stems of Acanthus ilicifolia, another shore plant.

Guided by the appearances on the roots of Aegiceras and Excaecaria, the pores of which are found to extend also higher up among the foliage, the conclusion is forced on me that these root-pores are only modifications of the organs called lenticels (p. 329).

The last is the "Blind-your-Eyes" of Australia because of its acrid juice. Blatter, at p. 653 of a paper to be presently cited, refers to some of its morphological characters, but does not refer to its pneumatophores.

At p. 330, with Plates xx, xxii and xxvi, Bancroft refers to the "breathers" ( pneumatophores) of Sonneratia acida Willd. (Lythrariaceæ). They are up to 6 feet high.

Similar remarks as to breathing roots and stilt-roots may be made in regard to another Mangrove (Bruguiera Rheedii), from Queensland, in Dr. Bancroft's paper, p. 331, with good illustrations at Plates xix and xxi. The whole of Dr. Bancroft's paper will well repay perusal.




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We now turn to Schimper's "Plant Geography on a physiological basis," Fisher's English translation (Groom and Balfour), 1903. At p. 73 we have-

In many cases certain lateral roots are differentiated as oxygen-pumps, and in accordance with this function differ structurally from other roots. Such respiratory roots or pneumatophores (Jost) have been studied by Schenck in species of Jussieuea inhabiting in numbers, as shrubs or undershrubs, the shallow waters of warmer districts.

At fig. 47 are shown the pneumatophores of J. peruviana L. Inconspicuous pneumatophores (spongy bodies) may be seen in the common J. repens L. about Sydney. Then we may turn to the section "The Eastern Mangrove" at p. 395, the term Mangrove having more than a generic significance, including species of various families, but having a common habitat.

On the other hand, the roots of most Mangrove-trees are characteriscd by the possession of highly peculiar pneumatophores (figs. 223, 224, 225). These are displayed in their simplest form by Carapa obovata (fig. 223, 3), where the serpentine creeping roots project above the mud with their upper edge, like the blade of a thick knife, but studded with lenticels. In Carapa moluccensis the secondary growth in thickness in the upper part is irregular, so that the root terminates in finger-like outgrowths. (Schimper, p. 401).

The young branch of the root of Carapa obovata seen at fig. 223 (3) shows admirably that the lenticels in a pneumatophore apparatus may be abundantly present for breathing purposes without either the normally-shaped pneumatophore such as we see in Avicennia or the stilt-root, such as we have in Bruguiera. These have been differentiated by an author in the "Journal of Heredity," quoted at p.69, as "Ventilating Pores."

Concerning pneumatophores, Schimper goes on to say (p. 403).

That pneumatophores supply subterranean parts of the trees upon which they occur with oxygen was proved by G. Karsten ard Greshoff, as has been already explained. All these structures are accordingly provided with devices for absorbing oxygen (lenticels, stomata, thin cork), and for transporting it (intercellular passages in the primary cortex or bast). . . .

Avicennia officinalis (figs. 223, 4; 224) together with two American species, Sonneratia acida and S. alba, Ceriops Candolleana, and the American Combretaceous Laguncularia racemosa, all have negatively geotropic lateral roots protruding from the ground like asparagus; these are as long as one's finger, or, in Sonneratia, one's arm. See also "The Mangrove of the Bombay Presidency and its Biology," by E. Blatter, S.J. (Journ. Bomb. Nat. Hist. Soc. xvi, 644 (1905) ) for Pneumatophores.

This is an especially valuable paper. He follows Schimper in distinguishing a Western and an Eastern Mangrove (or, more fully, a Mangrove formation), the western one covering the coasts of Western Africa and America, and the eastern one those of East Africa, Asia, Australia and Micronesia. He adds Excæcaria agallocha L. to Schimper's list.

He gives an account of Rhizophora mucronata (which he styles "The True Mangrove") in some detail, and describes the long aerial roots.

Rhizophora mucronata forms sometimes tangled thickets by the interlacing of its roots, sometimes it is more isolated; but in any case it always occurs on the outer border of the mangrove formation towards the open sea, thus serving as a protective outpost of the less favoured representatives of the same formation. When the tide is out, the ground occupied by the mangrove shows a bluish-black mud, from which


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innumerable short stems and longer roots arise. The "true Mangrove" may easily be distinguished from its neighbours by the long aerial roots which raise the main trunk above the level of its origin and give the tree the appearance of being supported on stilts. These arise from the usually short stem on all sides, growing first for a short distance in a horizontal direction and arching down afterwards into the water. Soon the base of the stem, with its original roots, dies and now the only support to the upper stem and its branches are those stilt-roots which reach to a height of 2 or 3 yards, and which, on account of their great elasticity, are the best possible protective system against the continuous dashing of the waves. The tree may be moved forwards and backwards by the force of wind and water, but, ultimately, it will always assume its former position. In this way the aerial roots are like as many strong anchors which would not allow the tree to be carried away even by the wildest play of the waters. We may very often observe that the growing point of such a root loses its vitality, whereas behind the apex a forked root makes its appearance. It is evident that such a change of growth can have a beneficent influence only under the conditions of existence in a soft and muddy substratum. Another means of furnishing the tree with considerable resisting power is the circumstance that not seldom a row of secondary roots breaks through the under surface of the primary aerial root, descends immediately in a vertical direction into the mud, and, by a luxurious branching into roots and rootlets, helps to strengthen the primary root. (pp. 645 — 6).

Father Blatter (p. 651) goes on to say that -

An interesting feature of Avicennia officinalis, Sonneratia acida and Ceriops Candolleana are the pneumatophores, which exhibit an aspect widely different from those of the Bruguieras. As soon as the shrub reaches certain height, in Avicennia officivalis, e.g., 1 foot or l 1/2 feet, there appear in great number around the stem within a large circle, erect shoots with a soft, elastic texture like cork. They resemble very much the young shoots of Asparagus, except in colour, which, in our case, is a brownish black. They are very seldom observed developing leaves and growing up into bushes. If we follow them downwards we find the point of origin to be the subterranean roots of Avicennia officinalis, of which they are the negative-geotropic branches. In this plant they reach 1 to 1 1/2 feet above the mud or the shallow water and do not exceed in thickness 1/2 or 3/4 of an inch, whereas in Sonneratia acida they reach 18 to 24 inches in length, by 3 inches in diameter. As they do not develop into a shrub, it is evident that they serve some other purpose. A transverse section of such a root-branch gives us the looked-for explanation. In Avicennia officinalis our attention is drawn to a large, white ring which occupies nearly the whole plane of the section, leaving room only for a small, darker ring in the centre and a comparatively disappearing, protective skin. The white, loose portion is easily recognised as the parenchymatous tissue of the primary cortex and in it the naked eye is able to distinguish little holes which, by microscopic examination, prove to be lenticels. Those roots, therefore, represent respiratory organs like the over-ground roots of Bruguiera. But why do the pneumatophores reach beyond the water-level, as there is oxygen in the water? We must admit that the air dissolved in water shows, on the one hand, a higher percentage of oxygen than the atmosphere, but, on the other also a higher percentage of carbonic acid. In consequence of it the quality of oxygen available to the plant is much smaller in the water than in the air. Besides, the air diffuses very slowly in water, and thus it may easily happen that the slow movement of the water causes a want of oxygen. It is for this reason that woody plants, the stem-bases and roots of which are submerged in mud and stagnant water, are furnished with special adaptations for the absorption of oxygen from the atmosphere. That there are really graduations as to the percentage of oxygen available to the plant in different media, may be shown to evidence by the examination of a pneumatophore of Avicennia Officinalis. The respiratory root is very thin at the base, where it is covered by mud; it grows thicker, where it is submerged in water, and it reaches its maximum, where it is surrounded by the atmosphere. And if we examine the anatomical structure, we find that the various degrees of thickness are due to the respective development of the parenchymatous tissue, which contains the lenticels, i.e., the respiratory organs. The same may be observed in the species of Rhizophora. They are not possessed of special pneumatophores, but the modified tissue. of their "stilt-roots" takes upon itself the function of respiration and here again it is not the portion buried in the mud, but the one emerging from the mud and still more the upper part which is accessible to the atmosphere.

Then follow biological notes (p. 652) on Carapa obovata Bl. (Meliaceæ), Lumnitzera racemosa Willd. (Combretaceæ) and Ægiceras majus Gaertn. (Myrsinaceæ), a wellknown Mangrove shrub of the Sydney district and New South Wales.




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Then a paper, "Some Queensland Mangrove Barks, and Other Tanning Materials," by J. C. Brünnich and F. Smith (Queensland Agricultural Journal xxvii, 86, 1911) may be referred to. It contains useful information in regard to the Mangroves of Northern Queensland, and incidentally to the bark-collecting. There are notes as to the area covered, and to the relative abundance of the stilt-roots.

Mr. Charles Fledley refers to Avicennia officinalis in his most interesting Presidential address before the Royal Society of New South Wales, Journal, Vol. xlix, 45, 46 (1915), and published two plates, 1 and 2, the latter, and Mr. Baker's xlviii, showing the breathing-roots, or Cobbler's Pegs, excellently.

A few months later, op. cit., p. 257, Mr. R.T. Baker published an exhaustive and copiously illustrated monograph on Avicennia officinalis. I am only incidentally referring to this species at this moment, and content myself with drawing attention to those portions of the paper referring to breathing-roots (pneumatophores). At p. 263 Mr. Baker makes the interesting observation that the shade of the tree is necessary for the welfare of the breathing-roots underneath. At p. 264, &c., he deals with the anatomy of these roots.

Knees in Cypress trees. Aerating roots. Origin and meaning of "knees" in Conifers (Cypresses, &c.).

The effect of the quantity of water in the soil or of growing in a water medium is very marked on most plants, but has not been of great importance in inducing variations in cultivated plants. Serious lack of water (a condition which is found in deserts and sandy regions) has given rise to various devices by plants to prevent loss of water by evaporation from the leaves, water storage reservoirs in the tissue, specialised glands to absorb dew, &c. Desert trees and shrubs are commonly stunted, gnarly-stemmed plants, with large root systems. The fact that these characters almost invariably disappear (frequently in the first generation) when the plants are grown where there is an abundance of water and food, shows that they were assumed because of a lack of these materials.

The bald cypress (Taxodium distichum) furnishes an interesting illustration of the effect of excess of water. The cypress, as is well known, grows usually at the present time in swamps and very wet places. Geological records, however, show that centuries ago, previous to the Glacial epoch, the cypress tree grew in the present Arctic region, associated with oaks, maples, &c. As it was forced southward by the gradual change in climate, competition with other trees evidently resulted in its present habit of, growing only in swamps. Plants growing on dry land secure the necessary oxygen needed in root growth from the air, which is always present in the soil. Plants growing in the water or on very welt soil, however, frequently find it difficult to secure sufficient oxygen, and this has led to the development of devices to facilitate the aeration of the tissue. Cypress trees growing in water form numerous protuberances on the roots known as "cypress knees," which extend above the water into the air. By growing numerous seedlings of the cypress under varying conditions, Dr. Wilson has shown that these roots are invariably formed by plants growing in water, and are never formed when the plants are grown on fairly dry soil which contains sufficient air. He concludes, therefore, that these peculiar organs enable the roots of the tree to secure the necessary oxygen, and are developed as a direct result of the habit assumed by the cypress of growing in swamps. It is an interesting fact that this habit of forming knees, which was acquired centuries ago, has not become hereditary, being totally lost the first generation if the tree is grown on dry soil. In swamps and on lake margins, which places are now its natural home, the bald cypress forms a ragged, spreading growth, with large limbs and sparse foliage, and is very different from the common type of closely related pine trees, This also is the, result of a lack of oxygen and food, as before stated. When the tree is grown on dry soil, as it frequently is in parks, where it secures abundant air and nutrition, it


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reverts to the normal type, forming a tall, symmetrical, columnar top. In this case no knees are developed. The difference in the form of the top developed in the swamp and that developed on uplands or in parks is evidently due to the difierence in food supply, as in the case of the Juniper. (Year book, Dept. of Agric., U.S.A., 1896, pp. 94 and 95.)

Speaking of "knees" in Cypress-trees, Coulter, Barnes, and Cowles, ii, 508, say:- "Oddly enough, knees do not develop in deep water, but only in shallow water or in swamps. If these peculiar structures are regarded as reactions to a slight oxygen content, it is difficult to account for their absence in deep water, unless it is supposed that the life-conditions there are too poor to permit of growth."

Knees are not formed on Taxodium in the Botanic Gardens, Sydney, under any of the conditions under which we grow them.

PHOTOGRAPHIC ILLUSTRATIONS.

Three photos by W. C. Grasby, Western Australia, of Eucalyptus rudis, viz. -

No. 1. Flooded or Blue Gum. Ferndale, Balingup, W.A. Has been ringbarked seven or eight years, and kept alive by developing an aerial root 7 feet from ground, and sending it down through decayed interior. See No. 2 of base of the tree showing the root. No. 2. Base of Blue or Flooded Gum, showing aerial root developed 7 feet from ground, and sent down through decayed centre of tree. See No. 1.



No. 3. Base of small Blue or Flooded Gum at Ferndale, Balingup. Ringbarked a number of years and has developed roots from above the ringing. (See fuller references at p. [sic]).



A giant Fig on the Brunswick River showing adventitious roots. (W. F. McLean, photo).



Roots from River Gum Stumps (Eucalyptus rostrata). Menindie, Darling River, N.S.W., (Judge Docker, photo).



Edge of clearing. Native forest removed for pasture. Lord Howe Island. Shows adventitious roots of Screw Pines (Pandanus). (A. R. McCulloch, photo).



First of Two sketches showing adventitious roots of Fig Trees. Illawarra Ranges, 1853. (From original pencil drawings by the late J. F. Mann).



Second of Two sketches showing adventitious roots of Fig Trees. Illawarra Ranges, 1853. (From original pencil drawings by the late J. F. Mann).



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