This lateral movement was not caused by the attachment of the glass filaments, nor by the action of light; for no light was allowed to enter when each observation was made, except from vertically above.

The hypocotyl of a seedling was secured to a little stick; it bore nine in appearance distinct cotyledons, arranged in a circle. The movements of two nearly opposite ones were observed. The tip of one was painted white, with a mark placed below, and the figure described (Fig. 44, A) shows that it made an irregular

Fig. 44. Pinus pinaster: circumnutation of two opposite cotyledons, traced on horizontal glass in darkness, from 8.45 A.M. to 8.35 P.M. Nov. 25th. Movement of tip in A magnified about 22 times, here reduced to one-half of original scale.

circle in the course of about 8 h. during the night it travelled to a considerable distance in the direction indicated by the broken line. A glass filament was attached longitudinally to the other cotyledon, and this nearly completed (Fig, 44, B) an irregular circular figure in about 12 hours. During the night it also moved to a considerable distance, in the direction indicated by the broken line. The cotyledons therefore circumnutate independently of the movement of the hypocotyl. Although they moved much during the night, they did not approach each other so as to stand more vertically than during the day. [page 58]

Cycas pectinata (Cycadeae).--The large seeds of this plant in germinating first protrude a single leaf, which breaks through the ground with the petiole bowed into an arch and with the leaflets involuted. A leaf in this condition, which at the close of our observations was 2 inches in height, had its movements traced in a warm greenhouse by means of a glass filament bearing paper triangles attached across its tip. The tracing (Fig. 45) shows how large, complex, and rapid were the circum-

Fig. 45. Cycas pectinata: circumnutation of young leaf whilst emerging from the ground, feebly illuminated from above, traced on vertical glass, from 5 P.M. May 28th to 11 A.M. 31st. Movement magnified 7 times, here reduced to two-thirds of original scale.

nutating movements. The extreme distance from side to side which it passed over amounted to between .6 and .7 of an inch.

Canna Warscewiczii (Cannaceae).--A seedling with the plumule projecting one inch above the ground was observed, but not under fair conditions, as it was brought out of the hot-house and kept in a room not sufficiently warm. Nevertheless the tracing (Fig. 46) shows that it made two or three incomplete irregular circles or ellipses in the course of 48 hours. The plumule is straight; and this was the first instance observed [page 59] by us of the part that first breaks through the ground not being arched.

Fig. 46. Canna Warscewiczii: circumnutation of plumule with filament affixed obliquely to outer sheath-like leaf, traced in darkness on horizontal glass from 8.45 A.M. Nov. 9th to 8.10 A.M. 11th. Movement of bead magnified 6 times.

Allium cepa (Liliaceae).--The narrow green leaf, which protrudes from the seed of the common onion as a cotyledon,* breaks through the ground in the form of an arch, in the same manner as the hypocotyl or epicotyl of a dicotyledonous plant. Long after the arch has risen above the surface the apex remains within the seed-coats, evidently absorbing the still abundant contents. The summit or crown of the arch, when it first protrudes from the seed and is still buried beneath the ground, is simply rounded; but before it reaches the surface it is developed into a conical protuberance of a white colour (owing to the absence of chlorophyll), whilst the adjoining parts are green, with the epidermis apparently rather thicker and tougher than elsewhere. We may therefore conclude that this conical protuberance is a special adaptation for breaking through the ground,** and answers the same end as the knife-like white crest on the summit of the straight cotyledon of the Gramineae.

* This is the expression used by Sachs in his 'Text-book of Botany.'

** Haberlandt has briefly described ('Die Schutzeinrichtungen...Keimpflanze,' 1877, p.

The Power of Movement in Plants Page 30

19th Century English Literature

Charles Darwin

Free Books in the public domain from the Classic Literature Library ©

Charles Darwin

All Pages of This Book