12 THE SMALL GRAINS 



endosperm, consisting of large parenchyma cells, has an 

 abundance of starch grains, except in the outer layer. 

 Very small protein bodies are distributed throughout the 

 seed, but especially in the outer portion of the endosperm. 

 In the starchy portion, these bodies occur between the 

 starch grains. If they fill the intervals more or less com- 

 pletely, the endosperm becomes translucent or corneous 

 and the kernel is said to be flinty. If the intervals are 

 not closely filled, air spaces remain, the endosperm becomes 

 opaque, and the kernel is called mealy (Fig. 5). Both 

 conditions (in the case of wheat) often occur in the same 

 variety and even in the same season's crop. There are 

 corresponding differences in color of the kernel, and the 

 mealy kernels are commonly referred to commercially as 

 " yellow berries." The whole phenomenon is one of en- 

 vironment, in which heredity has little or no part, and in 

 which climatic changes have apparently the chief influence, 

 as shown by recent experiments (316317). 



11. Starch grains. In different cereals, the starch 

 grains vary greatly in size and shape. They are often 

 spherical. In wheat starch they are either lenticular and 

 rather large, or comparatively small and many-sided, or 

 compound, composed of 2 to 25 partial grains. Those of 

 barley are similar to those of wheat but a little smaller, 

 while those of rye kernels are larger. In rice they are 

 either single or in compound oval form, containing 2 to 

 100 parts. Commercial rice starch, however, contains 

 no compound grains. In oat starch, which is otherwise 

 similar to the latter, such compound grains are present. 

 The exact mode of starch grain formation in seeds is not 

 well understood, but it occurs within plastids, called 

 amyloplasts specialized portions of the protoplasm. In 

 structure starch grains appear stratified in all cases, the 



