STARCH AND FAT 419 



richer source of carbon than starch ; or, in other words, taking equal 

 volumes and equal carbon contents, fat is much lighter than starch. 

 The circumstance that the non-nitrogenous reserve-material of seeds 

 consists far more frequently of fat than of starch, is probably to some 

 extent connected with the fact that any reduction in -the weight of a 

 seed increases its chances of dispersal. This argument applies more 

 particularly to seeds and fruits with parachute- or wing-arrangements, 

 which are in fact almost invariably of the " oily ? ' type. For the seeds 

 of water-plants are mostly distributed by the aid of appropriate floating 

 devices, which generally depend upon the presence of numerous large 

 air-containing cells in the testa. The period of dispersal ends when 

 these floats become water-logged ; such seeds will then most readily sink 

 to the bottom (where, after a period of rest, their germination takes 

 place), if their specific gravity is fairly high. 



Bulky storage-organs, such as tubers, bulbs, rhizomes, and large 

 seeds (spanish chestnut, horse-chestnut, acorn, bean), also generally 

 accumulate their store of non-nitrogenous reserve-material in the form 

 not of fat, but of starch, or some other carbohydrate. Here the under- 

 lying cause seems to be of a chemico-physiological nature. The 

 quantity of oxygen absorbed during germination is much larger in the 

 case of oily seeds than in that of seeds and fruits which contain starch. 

 In the former case, oxygen is required, not merely for respiration, but 

 also to a large extent for the oxidation of the fat, which has to be trans- 

 formed into one or other of the more highly oxygenated carbohydrate 

 substances before it can be translocated. As storage-tissues are, how- 

 ever, never provided with very abundant air-spaces, oxygen penetrates to 

 their interior chiefly by diffusion from cell to cell ; the supply of oxygen 

 to the deep-seated cells of a storage-tissue is thus most liable to become 

 insufficient, where large quantities of this gas are used up at the time 

 of germination. Hence, if a large proportion of the reserve-material of 

 bulky storage-organs consisted of fat, it would be a difficult matter for 

 the necessary oxygen to be absorbed with sufficient rapidity ; this 

 would constitute a particularly serious disadvantage in the case of 

 plants with rhizomes, tubers and bulbs, which are designed to produce 

 their leafy shoots and flowers as soon as possible after the beginning of 

 the vegetative season. 



It has already been stated that carbohydrate reserve-material is 

 most frequently stored either in the form of starch or in that of 

 reserve-cellulose. The selection of one of these substances rather 

 than the other, for this purpose, is again determined by ecological 

 considerations. As a general rule, seeds which germinate quickly 

 contain starch, which may either be present as such in the resting 

 seed, or else may only appear during germination as a result of 



