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PRODUCTION OF TUBERS WITHIN THE POTATO.
By A. B. RENDLE, M.A., F.L.S.
(PLATE 336.) In the Gardeners' Chronicle of January 22nd, 1870, is a short note on the anomalous production of young tubers or shoots in the interior of a potato, resulting in the bursting of the rind and their protrusion through the clefts, when grown too large to be any longer contained within. There is also a rough figure, which is reproduced in the same journal of Nov. 29th, 1879, in connection with a short article on the subject called forth by some experiments of M. Lachaume who, in the Revue Horticole,* tells how he was able to bring about the phenomenon artificially. He placed potatoes in the spring on a table in a cellar, and every week removed any shoots which had appeared. On September 1st the skin split, and a few days after the little potatoes protruded.
No certain explanation of the origin of these internal shoots is given in any of the above notes, but the Gardeners' Chronicle suggests as the most probable one, the formation of advertitious buds within the tuber in connection with its vascular ring; these feed on the plant, grow and produce young tubers, which ultimately rupture the rind and so become exposed. Perforation of the tuber and inversion of the young shoots whose tips are thus thrust into the interior, where they develop new tubers, is cited as a less probable cause.
Having received from Mr. Carling, of Norwich, & specimen of a potato in the state in question, I thought it would be of interest to ascertain the real origin of these “intrasomatal" shoots. The tuber, while lying with others in a cellar, had sprouted, but the shoots had been broken off and nothing left of them but the damaged bases. In the course of the summer it was noticed that the rind was splitting in several places, the clefts increased in width, and soon little potatoes appeared through the apertures. This went on for some weeks, and when given to me the potato had the appearance shown in the plate (fig. A.), and was literally bringing forth young at several points. It was also rapidly wasting away, having to supply nourishment for its large family from its own tissues. By following the internal shoots through the substance of the tuber, it was seen that each was attached to the base of one or other of the damaged aërial shoots, and examination in longitudinal section showed the connection to be a very real one, and explained the anatomical origin. The internal shoot is produced at the base of the aërial shoot, and its vascular cylinder is connected at the narrow point of union with a small network of bundles, with which those passing down the adjacent side of the aërial shoot also anastomose (fig. C). The shoot is very narrow at its point of origin, but rapidly widens out, and begins at once to give off roots and
* Revue Horticole, 1879, p. 397.
JOURNAL OF BOTANY.--VOL. 31. [July, 1898.]
branches, showing that life is going on with great activity. Another evidence of this vigour is seen in the great number of undeveloped buds and roots all round the base of the aërial shoot, which at once give the clue to the physiological cause of the phenomenon. What has happened is this. When the “eyes” or buds of the motherpotato sprouted, an active metabolism was set up in their immediate neighbourhood. The reserve-material, stored in the insoluble form of starch, was changed by the diastatic ferment into sugar, and thus becoming soluble and capable of easy transport, passed up into the growing shoot to supply material for formation of tissue and production of the energy necessary for growth. But this growth, which in the dark would be exceptionally vigorous, was suddenly stopped altogether by removal of the shoot. The chemical processes, however, still went on; and to find some channel for the use of the material produced, numbers of adventitious buds were formed round the bases of the shoots, one or more of which pushed through the substance of the tuber, giving rise on its course to numerous roots and branches. It is difficult to see what purpose the colourless thread-like roots can serve, as the entire nourishment is obtained through the attachment with the aërial shoot, the suberisation of the internal cavities as well as the surface of the young shoot and tubers precluding any idea of its assumption from outside. We must suppose that the habit of producing roots is so deeply graven on the constitution of tuberbearing shoots that it continues even when roots are useless.
The branches either produce or themselves become young tubers in which the reserve-material of the old tuber is again stored in the form of starch. The new growth being entirely at the expense of the mother, naturally causes the latter to shrivel.
Mr. Carruthers has drawn my attention to a case somewhat parallel physiologically, where supply of soluble food-material goes on after the necessity for it has ceased as far as the plant is concerned. The Indians of Mexico have discovered and use it for their own convenience. They cut off the inflorescence of the Agare at the base, close to the thick, fleshy leaves, and scoop out the open wound into a sort of basin. Of course a large supply of soluble carbohydrate (sugar) was necessary for the very active metabolism and growth going on in the huge opening inflorescence. This supply does not cease with the removal of the stalk, and the sugary liquid wells up into the artificial basin, whence it is removed by the Indians and fermented to make a drink. This goes on until the plant has exhausted the large quantities of starch which it had stored in its leaves with a view to flowering and fruiting. It then withers and dies.
There is one other point worthy of mention as regards the relation between the two generations of tubers. In a paper entitled * “Ueber Vernarbung und Blattfall,” Bretfeld cites a remarkable example of an internal wound found in a kind of dry-rot of the potato, and induced through penetration by a parasite. The pene
* Pringsheim's Jahrbuch, Bd. xii. p. 138,
tration starts at the eyes; those nearest the diseased spot decay, and thence, mostly without perceptible alteration on the exterior, the disease passes inwards and becomes localised in various parts of the tuber. Suitable sections show plainly that the area of infection is surrounded by cells actively engaged in formation of periderm.” Similarly in our potato, the channels formed by the penetration of the shoots and the larger cavities in which are contained the young tubers, singly or often massed together in clusters, have their walls suberised, while the surface of the penetrating shoots and tubers is protected in the same way. This occurs not only where, through formation of a cleft, the interior becomes exposed, but right in the heart of the mother-tuber, which thus protects itself against the inroads of its own offspring, much in the same way as against a parasitic fungus. There is this difference between the protecting layer of the vigorously-growing young shoot and that of the old wasting tuber, where life must be at a much lower ebb. While the former, with its young tubers, gives rise to a phellogen producing layers of periderm, in the latter we find simply suberisation of the walls with more or less disappearance of the contents of the outer layers of cells.
It may be asked why the adventitious buds at their formation should not break out and grow freely in the air, rather than force their way through a resistent tissue. But we can understand that, dealing as we are here with shoots, which in the natural order of things have to push their way through the soil, the relations to the substratum will thus be more truly expressed, and the response to the external stimuli of contact and moisture better satisfied.
DescRIPTION OF Plate 336.-A. An old potato bringing forth young tubers through clefts in its skin. B. Lower part of the same specimen cut open and the upper part of the section removed, laying bare a large cavity containing several young tubers, of which the greater number are borne on a shoot whose origin at the base of an aërial shoot is indicated at x; x' is near the origin of another shoot. C. Longitudinal section of the origin of an intrasomatal shoot (a) at the base of an aërial shoot (V), showing the arrangement of the bundle system.
BY EDWARD F. LINTON, M.A., AND Wm. R. LINTON, M.A.
(Concluded from p. 182.) Hieracium eustales, n. sp. A plant gathered in Glen Derry, S. Aberdeen, 1889, stood alone for some time, the material being too scanty to deal with. No name was even suggested. Dr. Lindeberg, to whom it was sent, observed on it, “Species pulchra, bene ut videtur distincta.” In 1891 two gatherings were made in MidPerth, viz., on Meall Ghaordie, on the Glen Lyon side, and on Meall-na-Saone, on rocky sides of the Allt Dubh Galair, of a plant which was eventually found to be practically identical with the Glen Derry form. The latter, probably from growing on granite,