3i6 LIFE : OUTLINES OF GENERAL BIOLOGY 



further efficiency possible, and this continues cumulatively till the 

 limit of growth is reached. As a tree grows older, it enlarges its 

 leafage area at a ratio far beyond its actual needs for growth and 

 repair. This organic momentum — a veritable growth by compound 

 interest, and thus rewarding afforestation — is the condition for 

 storage, which is so characteristic of plants. Moreover, as contrasted 

 with animals, plants do not oxidise much of their daily nutritive 

 gain as a source of energy for work. The vital expenditures in 

 animals tend to come much nearer their nutritive income. Further- 

 more, the processes of anabolism tend to increase the size of the 

 thus elaborated molecules; and a general result of this is in the 

 direction of reducing mobility. Hence the abundance of reserve- 

 products in plants — a fortunate fact for man and beast alike. 



There are not many cases of ordinary leaves becoming laden with 

 reserves, partly because this would encumber the photosjTithetic 

 and other actively anabolic processes, and also because most leaves 

 are organs with a limited length of life. Or, stating the fact less 

 tcleologically, we may say that the presence of diastatic and other 

 ferments in the green leaf secures the mobilisation of starch and 

 other synthetic products. Stores are accumulated at some distance 

 from the seat of manufacture, e.g. in roots (as in carrots), in spread- 

 ing underground stems or rhizomes (as in bracken), in compacted 

 portions of underground stem (as in potato tubers and crocus 

 corms), in subterranean buds (as in crucifers), in pith (as in sago 

 palms), and notably in seeds. A process of fermentation or digestion 

 is needed to precede the transport from leaf to store, as when starch 

 becomes sugar, and also for the re-mobilisation of the reserves after 

 a resting phase. The forms taken by the stored food are very varied, 

 and several different forms may occur in one and the same plant. 

 Commonest of all are the starches which arise, from glucose and the 

 like, as characteristic grains in the interior of minute leucoplasts. 

 In the tubers of potatoes, about 80 per cent, of the dry weight 

 consists of starch; in seeds of rice and wheat, 68 per cent. ; in peas, 

 52 per cent.; yet in almonds, only 8 per cent. Less complex than 

 starch, and arising from fructose, is inulin, which occurs dissolved 

 in the sap of storage-cells, as in dahlia tubers. The commonest 

 storage form of the sugars is saccharose or cane sugar, as in grasses, 

 of which sugar cane is but the extreme instance. Remarkable for 

 their solidity and durability are the hemi-celluloses that are deposited 

 inside the cell walls of some seeds, cotyledons, etc., e.g. in date- 

 stones and coffee-beans. Yet these, too, are fermented before 

 germination into the readily transformable sugars known as mannose 

 and galactose. 



Fats and oils are very common reserve stuffs, especially in seeds 

 (e.g. cotton and flax), and usually occur as droplets in the cyto- 

 plasm. Proteins in solution may lie in reserve in the cytoplasm of 



