NUTRITION. 



143 



more important (because the longer) does this transfer be- 

 come. In many plants, also, it is desirable that a supply of 

 reserve food be stored for use when a supply is no longer 

 available from the outside or by manufacture. 



196. Storage form. — In the higher plants, storage places are secured 

 by the enlargement of roots, stems, and leaves, to form reservoirs. 

 Similar specialization of parts in the lower plants occurs. The most 

 common form of reserve food, especially in thickened stems, roots, etc., 

 is starch. This is deposited in the form of rounded or oval grains, 

 which often show layers due to different composition and density (e.g., 

 in the potato tuber, A, fig. 114), and are sometimes adherent into com- 

 pound grains, e.g., the oat {B, fig. 114). Ins eeds also, much reserve 



Fig. 114.— Reserve starch. A^ two cells of a potato, showing enclosed starch grains. 

 The other contents not shown. B^ compound starch grains from a grain of oats. 

 Three of the component granules of a large grain are shown separately. C, starch 

 grains from a bean. All highly magnified.— After Kerner. 



food may take the form of starch, and fats are common. Fats occur in 

 liquid form, as droplets of various size (e.g., cotton seed), and are only 

 rarely solid. In some seeds the cell walls are enormously thickened, so 

 that the seed is of bony hardness (e.g., the date). Reserve proteids are 

 stored in the form of aleurone grains. These are small granules, often 

 packed in between the larger starch grains, as in the cotyledons of the 

 bean. 



197. Digestion and transfer. — When solid foods, insol- 

 uble in vt^ater, are to be moved from one part of the plant to 



