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Show 58 CIRCUMNUTATION OF SEEDLINGS. CHAP. I. Cycas pectinal a (Cycadero ).-The large seeds of this plant in germinating first protrude a single leaf, which breaks t~rough the ground with the petiole bowed into an arch and w1th 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 mcn.ns of a glass filament bearing paper triangles attached across its tip. The trn.cing (Fig. 45) shows how large, complex, and rapid were the circum· Fig. 45. C!Jcas 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 sicle which it passed over amounted to between · 6 and · 7 of an inch. Canna Warscewiczii (Cannacero).-A seedling with tho plumule projecting one inch above the ground was observed, but not under fair conditions, as it was brought out of the hothouse 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 0IIAP. I. ALLIUM. 59 by us of the part that first breaks through tho ground not being arched. Fig. 46. Canna. Wa1·scewiczii: circumnutation of plumule with filament affixed obliquely to outer sheath-like leaf~ traced in darkness onhorizontnl glass from 8.45 A.l\1. Nov. 9th to 8.10 A.M. 11th. Movement of bead magnif- ied 6 times . .Allium cepa (Liliacero).-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 tho 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 bene~th the ground, is simply 1·ounded; 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,t and answers the same end as the knife-like white crest on the summit of tho straight cotyledon of the Graminero. * This is the expression used by Sachs in his 'Text-book of Botany.' ~ Haborlo.ndt bas briefly descnbed ('Die Schutzeinricbtunge! l · · · Keimpflanze,' 1877, p. 77) this curious structure and tho purpose which it subserves. He states tho.t good figures of the cotyledon of tho onion have been given by Tittmo.nn and by Sacht~ iu his' ExperiJUento.l Physiologie,' p. 93. |