596 TRA NSL OCA TION 



development of the characteristic vital activity. Hence beetroots, onions, 

 &c., in spite of their great store of sugar, arc just as capable of a winter 

 resting-period as are starch-laden trees or potatoes, and the rapidity with 

 which the reserve-materials are consumed in spring is entirely dependent 

 upon the inherent character of the plants in question and their activity of 

 consumption. 



It has already been mentioned (Sect. 78) that plastic and reserve- 

 substances in general differ somewhat from the actual constructive materials 

 of which the plant is composed, and that any given cell may according to 

 circumstances store certain substances temporarily or for prolonged periods. 

 Hence the differentiation of special secretory cells and tissues is not neces- 

 sarily essential, although for purposes of maintenance and reproduction the 

 formation of special storage receptacles, such as seeds, tubers, bulbs, &c., 

 has become of almost essential importance l . Similarly in trees the 

 restriction of the reserve-materials to particular tissues has arisen by 

 secondary adaptation in correspondence with economic necessities. 



Germination of seeds 2 . The reserve-materials are either stored up in 

 the cotyledons of the embryo, or to a greater or less extent in accessory 

 tissues as endosperm or perisperm. The cotyledons may subsequently 

 function as green leaves, or simply as storage receptacles, or may act 

 as haustoria by means of which food-materials are absorbed from the 

 albumen-tissue. The same natural order may contain plants with cotyledons 

 which become capable of photosynthesis (Litpinns), and others whose 

 cotyledons do not acquire this power (Piswii). The former come above 

 ground, but those of Pisnm usually remain buried, while the cotyledons of 

 PJiaseolns if exposed to light may turn green and exhibit a feeble power 

 of photosynthetic assimilation before they shrivel 3 . 



The nutrient materials are always absorbed from the accessory tissues 

 of the seed through or by means of the cotyledons, and these in Mirabilis 

 jalapa surround the endosperm (c, Fig. 69) and when the latter is exhausted 

 unfold as green foliage-leaves. Similarly the cotyledons of Ricintis escape 

 from the emptied sac-like endosperm, and they may become partially green 

 while still enclosed by it. In Allinni ccpa the cotyledon also functions 

 as a foliage-leaf and withdraws its apex from the endosperm after the 

 whole of the food-material has been transferred to the developing embryo. 

 In grasses the cotyledon forms the scutellum, functions solely as an 

 absorbent organ (s, Fig. 70), and does not increase in size, whereas the 



1 Cf. Haberlandt, Physiol. Anat., 1896, 2. Aufl., p. 345. 



2 On the morphology and biology of the process, cf. Klebs, Unters. a. d. Bot. Inst. z. Tubingen, 

 1885, Ed. I, p. 536; Lubbock, Contrib. to Knowledge of Seedlings, 1892; H. Haberlandt, Schutz- 

 einrichtungen d. Keimpflanze, 1877; Nobbe, Samenknnde, 1876. 



3 Even though green, cotyledons assimilate but little or not at all while still loaded with reserve- 

 materials (cf. Ewart, Journ. of Linn. Soc., 1897, Vol. XXXI, p. 557). 



