MANUAL OF BOTANY 



lines which alternate with other darker ones, so that the whole 

 presents the appearance of a series of more or less irregular con- 

 centric shells placed around a common point. The cause of these 

 appearances has given rise to much discussion ; but from' the 

 observations of Schimper and others there appears to be no doubt 

 that these shells are separate layers of starch substance that have 

 been deposited successively by the amyloplast round the first 

 formed portion, which is therefore more or less central. The 

 various shapes of starch grains are due to irregular deposition of 

 these layers. If the grain is formed regularly in the interior of 

 the amyloplast, the layers are concentric ; if the formation com- 



FlG. 6.53. 



Fig. 652. 



Fig. 652. A. Compound ; E. Semi-compouiid 



granule of starcli. Fig. 653. Part of 



a laticiferous cell from Euphorbia 

 splendens ; it contains starch granules 

 of a peculiar dumb-ben and somewhat 

 rod- and bone-like form. 



mences towards some portion of its exterior and quickly bulges 

 outwards, the layers are irregular, being thicker and more 

 numerous on the end of the grain which is nearest to the bulk 

 of the plastid, as in the Potato {fig. 648). Probably the grain is 

 always covered by the substance of the plastid, though there 

 may be only a very delicate film-like layer of the latter over the 

 protruding portion of the starch grain. 



Compound starch grains are frequently met with. These 

 are of Wo kinds : some are due to two or more grains 

 having been pressed together during their development. Such 

 grains usually originate by two or more arising symmetrically 

 in the interior of the same plastid. Others are found to contain 



