PHYSIOLOGY 295 



cuius, Quercus), or where their function is to absorb nourishment from 

 the endosperm (in Palms and the scutellum of Gramineae). More fre- 

 quently the cotyledons are pushed above ground, and may then be 

 thick and filled with reserve nourishment, or thin and turning green 

 on exposure to the light. In many Monocotyledons, as also in Ricinus, 

 etc., the cotyledons, even if they afterwards appear above ground, may 

 first take up the nutritive substances of the endosperm ; while in the 

 Conifers the cotyledons perform the same office above ground. The 

 cotyledons are drawn from THE seed by the curvature of the 

 hypocotyl or of the petioles of the cotyledons (Smyrnium, Delphinium). 

 The seed-coverings also are often further ruptured by the swelling of 

 the hypocotyl (Cucurbita, etc.). The unfolding of the first leaves 

 above ground is frequently accompanied by a contraction OF THE 

 ROOT, occasioned by its distension in a transverse direction ; the seed- 

 ling is in consequence drawn deeper into the soil, and its position 

 rendered more secure. Even older plants, particularly those whose 

 leaves form a radical rosette, notwithstanding their upward growth, 

 are held close to the ground through a similar contraction of their 

 roots. 



When its attachment in the soil is properly provided for, and after 

 the first germ-leaves are unfolded, the young plant has acquired the 

 capacity for self-sustenance, its further growth and development being 

 dependent upon its own activity. 



