THE METABOLISM OF PLANTS: l8l 



general protoplasm or inside the amyloplasts the spheroidal 

 form is retained and the planes of stratification are concentric 

 around the hilum which is the part of the grain which was 

 first formed : when, however, the grains are formed, as they 

 frequently are, on the outside of an amyloplast, they soon 

 become oval and the layers excentric ; the end of the grain in 

 contact with the amyloplast becomes broad, and the number 

 of layers of stratification is greater there than at any other 

 part; hence the hilum is gradually removed further and 

 further away from the amyloplast, so that the long axis of 

 the grain coincides with the direction of greatest growth 

 (Fig. 26). We naturally conclude from these facts that the 

 grain grows by the deposition of new layers on its surface, 

 and that the successive layers produce the stratified appear- 

 ance of the grain. 



In addition to the planes of stratification, the starch-grain 

 is marked by lines radiating from the hilum, which are planes 

 of striation. Schimper and Arthur Mayer were led by this 

 to regard a starch-grain as a spherocrystal, consisting of a 

 number of radially-placed prisms. According to their view 

 the formation of a starch-grain is effected in this way, that 

 the amyloplast takes up sugar from the cell-sap and converts 

 it into starch, which is deposited in successive layers con- 

 sisting of prismatic crystals. Strasburger, however, finds that, 

 as in the case of cell-walls, each layer of the grain is formed from 

 a layer of protoplasm, and he thinks it probable that the 

 striation of the layers, like that of the layers of cell-walls, is 

 connected with the arrangement of the microsomata in the 

 layers of protoplasm from which the layers of starch have 

 been derived. 



According to Naegeli, a starch-grain increases in bulk not by apposi- 

 tion but by intussusception, that is by the intercalation of new particles 

 (micellae) of starch between those which are already present ; he regards 

 the stratification of the starch-grain not as the result of the deposition of 

 successive layers one upon the other, but as being due to the differentia- 

 tion of the growing starch-grain into layers containing alternately a 

 greater or a smaller proportion of water. The most important of the 

 facts upon which this view is based is that the most external layer of a 

 starch-grain is always a dense layer, whereas the hilum is relatively 



