420 STORAGE SYSTEM 



the oxidation of fat. Where the starch is stored in the endosperm, it is 

 usual for the embryo to be attached to one side of the storage-tissue, 

 or at most to be partially embedded in the latter. A familiar illustra- 

 tion of this relation of embryo to endosperm is furnished by the 

 Gramineae, and a similar state of things prevails in the Caryo- 



PHYLLACEAE, PORTULACEAE, POLYCOXACEAE, CHENOPODIACEAE, etc. The 



peripheral, and hence exposed, situation of the embryo enables the latter 

 to respond to comparatively slight external stimuli ; like the starchy 

 character of the reserve-material, the superficial location of the embryo 

 seems to represent a device for hastening germination. In the case 

 of seeds with a protracted period of germination, where the embryo 

 withdraws the reserve-materials from the storage-tissue very gradually, 

 the endosperm, when persistent, always surrounds the embryo com- 

 pletely, and moreover often possesses very thick cell-walls. In this 

 instance, also, the advantages entailed by the character and arrange- 

 ment of the reserve-materials are fairly evident. As compared with 

 numerous starch grains, a thickened cell-membrane exposes a far 

 smaller surface to the attack of hydrolysing enzymes. In seeds with 

 thick-walled storage-tissues, therefore, the whole process of germination 

 can be temporarily arrested and subsequently resumed with comparative 

 ease ; storage-organs of this type are also less likely to undergo 

 decomposition, or to suffer from the attacks of insects and other 

 animals, even when they remain buried for weeks and months. There 

 can, in short, be little doubt that thickened cell-walls constitute a more 

 appropriate form of carbohydrate reserve-material than starch-grains, 

 where the process of germination is slow and liable to suffer frequent 

 interruption. By taking into account the several methods of storage, 

 and the various combinations of plastic materials belonging to different 

 chemical classes, it is possible to distinguish a number of types of 

 storage-tissue. These may be tabulated as follows : 



A. All the reserve-materials deposited within a single storage-tissue. 



(i.) Eeserve-materials restricted to the cell-cavities. 



1 . Amides and proteins (dissolved in the cell-sap) plus sugar 



(dissolved in the cell-sap) : Beet-root, Onion. 



2. Amides and proteins (dissolved in the cell-sap) plus starch : 



Potato. Many rhizomes. 



3. Aleurone-grains plus starch : Cotyledons of Pea, Bean and 



Lentil. 



4. Aleurone-grains plus fatty oil : Cotyledons of Soja Bean ; 



endosperm of Bwinus. 



