SECT. II PHYSIOLOGY 323 



251) ( 126 ). Similar contrivances are found in the case of Pandanus, Canna, 

 Typha, Potamoycton, and many Dicotyledons J Tetragonia expansa, Medicago, 

 and some species of Onobrychis and Portulaca). 



SEEDLINGS PENETRATE THE SOIL by means of the elongation of 

 the primary root, or of the hypocotyl, or also, as is the case with 

 many Monocotyledons, through the movements of the geotropic 

 cotyledons. After the descending part is firmly attached to the 

 soil, by either root- hairs or lateral roots, THE UPWARD GROWTH 

 COMMENCES. In this process the cotyledons may either remain 

 within the seed (hypogeal) or unfold above ground (epigeal). The 

 first is often the case where the cotyledons are full of reserve 

 material (Phaseolus multiflorus, Aesculus, Quercus), or where their 

 function is to absorb nourishment from the endosperm (in Palms and 

 the scutellum of Grramineae). More frequently 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 Eicinus, 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 (Cucurbitaceae, Fig. 231, etc.). 



The unfolding of the first leaves above ground is frequently 

 accompanied by a CONTRACTION OF THE ROOT. This is caused by 

 the cortical cells of the root becoming more stretched in a transverse 

 than a longitudinal direction by their turgescence. In consequence 

 of this the root becomes thicker and shortens considerably. The 

 seedling 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. The shortening is indicated by the transverse wrinkling of 

 the surface of the root. According to the researches of RIMBACH the 

 shortening amounts to 30 per cent of the length of root in Allium 

 ursinum, to 50 per cent in Arum maculatum, and even to 70 per cent 

 in Oxalis elegans ( 12: ). 



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. 



