During the night and till 11 A.M. on the following

Fig. 20. Vicia faba: circumnutation of a radicle, at first pointing vertically upwards, kept in darkness, traced on a horizontal glass, during 14 hours. Movement of bead of filament magnified 23 times, here reduced to one-half of original scale.

morning, the bead moved to a great distance in a nearly straight line, in the direction indicated by the broken line in the figure. This resulted from the tip bending quickly downwards, as it had now become much declined, and had thus gained a position highly favourable for the action of geotropism. Fig. 21. Vicia faba: tracks left on inclined smoked glass-plates, by tips of radicles in growing downwards. Plate C was inclined at 63o, plates A and D at 71o, plate B at 75o, and plate E at a few degrees beneath the horizon. [page 31]

We next experimented on nearly a score of radicles by allowing them to grow downwards over inclined plates of smoked glass, in exactly the same manner as with Aesculus and Phaseolus. Some of the plates were inclined only a few degrees beneath the horizon, but most of them between 60o and 75o. In the latter cases the radicles in growing downwards were deflected only a little from the direction which they had followed whilst germinating in sawdust, and they pressed lightly on the glass-plates (Fig. 21). Five of the most distinct tracks are here copied, and they are all slightly sinuous, showing circumnutation. Moreover, a close examination of almost every one of the tracks clearly showed that the tips in their downward course had alternately pressed with greater or less force on the plates, and had sometimes risen up so as nearly to leave them for short intervals. The distance between the extreme right and left positions of the radicle A was 0.7 mm., ascertained in the same manner as in the case of Phaseolus.

Epicotyl.--At the point where the radicle had protruded from a bean laid on its side, a flattened solid lump projected .1 of an inch, in the same horizontal plane with the bean. This protuberance consisted of the convex summit of the arched epicotyl; and as it became developed the two legs of the arch curved themselves laterally upwards, owing to apogeotropism, at such a rate that the arch stood highly inclined after 14 h., and vertically in 48 h. A filament was fixed to the crown of the protuberance before any arch was visible, but the basal half grew so quickly that on the second morning the end of the filament was bowed greatly downwards. It was therefore removed and fixed lower down. The line traced during these two days extended in the same general direction, and was in parts nearly straight, and in others plainly zigzag, thus giving some evidence of circumnutation.

As the arched epicotyl, in whatever position it may be placed, bends quickly upwards through apogeotropism, and as the two legs tend at a very early age to separate from one another, as soon as they are relieved from the pressure of the surrounding earth, it was difficult to ascertain positively whether the epicotyl, whilst remaining arched, circumnutated. Therefore some rather deeply buried beans were uncovered, and the two legs of the arches were tied together, as had been done with the epicotyl of Tropaeolum and the hypocotyl of the Cabbage. The movements of the tied arches were traced in the usual manner on [page 32] two occasions during three days. But the tracings made under such unnatural conditions are not worth giving; and it need only be said that the lines were decidedly zigzag, and that small loops were occasionally formed. We may therefore conclude that the epicotyl circumnutates whilst still arched and before it has grown tall enough to break through the surface of the ground.

In order to observe the movements of the epicotyl at a somewhat more advanced age, a filament was fixed near the base of one which was no longer arched, for its upper half now formed a right angle with the lower half. This bean had germinated on bare damp sand, and the epicotyl began to straighten itself much sooner than would have occurred if it had been properly planted.

The Power of Movement in Plants Page 17

19th Century English Literature

Charles Darwin

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Charles Darwin

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