All five tracings resembled each other to a certain extent, and it will suffice to give two of them. In A (Fig. 48) the seedling was only .45 of an [page 62] inch in height, and consisted of a single internode bearing a bud on its summit. The apex described between 8.30 A.M. and 10.20 P.M. (i.e. during nearly 14 hours) a figure which would probably have consisted of 3 ellipses, had not the stem been drawn to one side until 1 P.M., after which hour it moved backwards. On the following morning it was not far distant from the point whence it had first started. The actual amount of movement of the apex from side to side was very small, viz. about 1/18th of an inch. The seedling of which the movements are shown in Fig. 48, B, was 1 3/4 inch in height, and consisted of three internodes besides the bud on the summit. The figure, which was described during 10 h., apparently represents two irregular and unequal ellipses or circles. The actual amount of movement of the apex, in the line not influenced by the light, was .11 of an inch, and in that thus influenced .37 of an inch. With a seedling 2 inches in height it was obvious, even without the aid of any tracing, that the uppermost part of the stem bent successively to all points of the compass, like the stem of a twining plant. A little increase in the power of circumnutating and in the flexibility of the stem, would convert the common asparagus into a twining plant, as has occurred with one species in this genus, namely, A. scandens.

Phalaris Canariensis (Gramineae).--With the Gramineae the part which first rises above the ground has been called by some authors the pileole; and various views have been expressed on its homological nature. It is considered by some great authorities to be a cotyledon, which term we will use without venturing to express any opinion on the subject.* It consists in the present case of a slightly flattened reddish sheath, terminating upwards in a sharp white edge; it encloses a true green leaf, which protrudes from the sheath through a slit-like orifice, close beneath and at right angles to the sharp edge on the summit. The sheath is not arched when it breaks through the ground.

The movements of three rather old seedlings, about 1 inch in height, shortly before the protrusion of the leaves, were first traced. They were illuminated exclusively from above; for, as will hereafter be shown, they are excessively sensitive to the * We are indebted to the Rev. G. Henslow for an abstract of the views which have been held on this subject, together with references. [page 63]

action of light; and if any enters even temporarily on one side, they merely bend to this side in slightly zigzag lines. Of the three tracings one alone (Fig. 49) is here given. Had the observations been more frequent during the 12 h. two oval figures would have been described with their longer axes at right angles to one another. The actual amount of movement of the apex from side to side was about .3 of an inch. The figures described by the other two seedlings resembled to a certain extent the one here given.

Fig. 49. Phalaris Canariensis: circumnutation of a cotyledon, with a mark placed below the apex, traced on a horizontal glass, from 8.35 A.M. Nov. 26th to 8.45 A.M. 27th. Movement of apex magnified 7 times, here reduced to one-half scale.

A seedling which had just broken through the ground and projected only 1/20th of an inch above the surface, was next observed in the same manner as before. It was necessary to clear away the earth all round the seedling to a little depth in order to place a mark beneath the apex. The figure (Fig. 50) shows that the apex moved to one side, but changed its course ten times in the course of the ten hours of observation; so that there can be no doubt about its circumnutation. The cause of the general movement in one direction could hardly be attributed to the entrance of lateral light, as this was carefully guarded against; and we suppose it was in some manner connected with the removal of the earth round the little seedling.

The Power of Movement in Plants Page 32

19th Century English Literature

Charles Darwin

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