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The Prudential Insurance Co. of America INCORPORATED AS A STOCK COMPANY BY THE STATE OF NEW JERSEY. JOHN F. DRYDEN, President. DEPT. 25 Home Office, NEWARK, N. J. BOTANICAL GAZETTE OCTOBER, 1905 REGENERATION IN PLANTS. II.' CONTRIBUTIONS FROM THE HULL BOTANICAL LABORATORY. LXXIX. WILLIAM BURNETT MCCALLUM. (WITH NINE FIGURES) WOUND STIMULUS. THE influence of the wound has been put forward many times, either as the direct cause, or as an important factor in regeneration. WIESNER (19) has suggested that between the wounded part and the new structure formed there is a direct causal connection, due to substances developing in the wounded cells and passing to other parts, there inciting the reversion of mature cells into the meristematic condition. GOEBEL (4, p. 204) also believes the wound stimulus to be a factor. KLEBS (9), on the other hand, rather discredits this idea and thinks that the wound in itself is of no importance in regeneration. The possibility of the wound having a far reaching influence is not at all improbable, for many well-known cases of traumatropism show how cells may be affected at a distance from those actually concerned in the wound. Of the many experiments to determine if such an influence is operating, only a few need be mentioned, and in these cases only the results will be given. It was seen that with Phaseolus the removal of the stem above the basal primordia is followed by the development of the latter. The removal of the cotyledons, while causing wounds still closer to the young bud, produces no such development; nor does the severing of the stem as close as possible below the primordia. Wounding the stem by cutting notches immediately above the primordia, as 1 The first paper was published in BOTANICAL GAZETTE 40:97-120. 1905. deep as four-fifths through the stem, has no influence. String was tied tightly around young stems, just above the primordia, and as the stems grew the string cut in deeply on all sides; no results followed. Longitudinal slices, the length of the epicotyl and three-fourths through the stem, produced no results. Experiment 38.-Notches cut at different points in a spiral around the stem, so that all the bundles were severed, failed to incite the buds below. Experiment 39.-Five plants 70-100cm high, 6-9 internodes long, and with lower internodes old and hard were used. The tip of each plant was cut off and also the buds from all the nodes below. In three plants the basal primordia produced shoots; one plant died; and the others remained alive, but no shoots formed. Here the wound effect, if there was such, traveled through a distance of nine internodes. A wound, however severe, seems unable to cause the buds to develop if it does not include the complete removal of the growing apex; and even on a large plant the removal of the very tip is all that is necessary. Here, as in the willow and other plants. (see experiment 43), the effect of the wound passes only down the stem; a wounding or a complete severance at any point along the stem has no effect on the buds above this point. As will be brought out later, the opposite is true of the roots; that is, the influence of the removal in inciting new roots only passes upwards. This would hardly be true if it were due to the diffusion of substances formed. in the wounded cells, as WIESNER supposes. GOEBEL (5) has found that in Bryophyllum no wounding at all is needed to produce shoots on the leaves. He encased all the buds on the shoot in plaster so as to prevent further growth, and after a time the buds on the leaves developed. If the leaf blade of Cyclamen is cut off new leafy structures arise along the margins of the petals. But WINKLER (19) has shown that this removal is not necessary. He left the blades intact, but incased them in plaster, and soon the leaf-like outgrowths appeared as when the blade was removed. The blade was not injured or wounded in any way by this treatment, though undoubtedly some of its activities were suppressed. As will be described in experiment 40, I inhibited the growth of the apex of Phaseolus by placing it in a hydrogen atmosphere. The basal primordia in the air below promptly developed. When the hydrogen was removed, the apex continued growing. Placing the roots in plaster, which inhibited any further growth on their part, also resulted in the development of roots along the stem. In these cases there was no wounding anywhere. It appears that while regeneration follows the removal of certain parts, neither this removal nor the wound incident to it are necessary, since the regeneration occurs equally well when the part is left uninjured and certain of its activities suppressed. The wound, therefore, is not in itself any part of the stimulus. CORRELATION. By correlation is meant the influence which one organ or part may exert over another. That the removal of certain parts leads to changes within the plant that may modify markedly the growth or function of other structures is a matter of common observation. Examples of this interdependence among the different members of the plant body are abundant. JOST (8) has shown that in Phaseolus the mere presence of the leaf is a necessary condition to the development of the bundles of the leaf trace. PISCHINGER (16) determined that if the large cotyledon of Streptocarpus be removed the small functionless one will develop into a large one. According to GOEBEL (6, p. 809) the early removal of foliage leaves induces the bud scales to develop into the foliar structures. If the upper end of a Taraxacum root be cut away, removing all the buds, new buds soon arise from the cortex below; this is true of many roots. IRMISCH and others (10) have shown in many species that if seedlings be cut off below the first node, new buds arise out of the tissue of the hypocotyl. The removal of the growing tip on many shoots is followed by the development of the dormant axillary buds. As is commonly observed in cultivation, if the lateral roots be destroyed, their place is taken by new roots, which otherwise would not have developed. GOEBFL (2 and 5) showed that in Begonia and Bryophyllum, and this is probably true for many other plants, the removal of all the buds on the shoot will result in the development of buds on the leaves. The process by which such an influence is exerted by one part over another is the main problem to be solved in regeneration. GOEBEL attempts to explain it in one of two ways: either (1) the one part monopolizes the nutritive material to such an extent that the other parts concerned cannot obtain sufficient to enable growth to go on; or (2) he applies SACH's Stoff-form hypothesis. According to this (17), there are formed in the plant small quantities of different substances, presumably of enzyme-like nature, each one having the capacity to incite the formation of a definite structure. These substances are supposed to move in definite directions, and where they accumulate in sufficient quantities start the development of the particular structure they are concerned with. In GOEBEL's opinion (4, p. 204) the influence of external conditions are of little account in regeneration, the important cause being "the direction in which the constructive material moves." GOEBEL says (3, p. 42) "the vegetative points act as centers of attraction for the plastic material, their influence being stronger or weaker according to their position." In Bryophyllum, for example, the apex of the shoot is the strongest, then the lateral buds, and last of all the vegetative points on the leaves; so that the apex is able to draw to itself the greater part of the "constructive material;" but if this apex is removed, the lateral buds will be able to "attract" this substance; and in the absence of these lateral buds, the growing points on the leaf are able to appropriate it. In Begonia no growing points are present on the leaf, but when it is removed GOEBEL says bud-forming material accumulates at the base and induces the formation of buds there. If this material, formed in the leaves, moves toward the base of the leaf and passes out because it is "attracted" by the growing points on the stem, just why it should continue to flow in that direction and accumulate at the base, when all connection with these "centers of attraction" is broken, is one of the unexplained difficulties that beset this hypothesis on every hand. MORGAN (12) has strongly objected to this theory, but his evidence against it does not seem to me to be necessarily fatal. GOEBEL in a recent paper (5) is inclined to lay less stress on it than formerly, asserting that the non-development of the buds on the leaves is due to a checking influence exerted by the buds of the shoot; "but," he adds, "whether we are here dealing with a stimulus transmitted along the conducting system, or whether the building material (Baustoffe) flowing in the conducting channels is attracted more strongly by the shoot vegetative points than by those on the leaves remains uncertain." |