412 STORAGE SYSTEM 



is mainly due to the researches of Schimper. In the case of storage- 

 tissues, it is chiefly the amyloplasts that act as starch-formers. Every 

 starch-grain develops in the interior of a chromatophore : whether it 

 remains permanently enclosed in a layer of chromatophore substance, 

 as A. Meyer supposes, is not so certain. In many cases, appearances 

 suggest that a portion of the grain may burst through the chromato- 

 phore-envelope. Schimper has shown that the shape of the grain and 

 the character of its stratification vary, according as the envelope is 

 uniformly distributed or not ; in other words, they depend upon the 

 position of the developing grain within the chromatophore. If the 

 grain arises at the centre of a spherical amyloplast, the successive 

 strata will be laid down concentrically, because the conditions of growth 

 are the same at every point of the surface. If, on the contrary, the 

 point of origin of the grain lies near the surface of the amyloplast, so 

 that the chromatophore-envelope is distributed unevenly around the 

 young grain, the deposition of starch will be favoured on one side, and 

 the stratification will consequently be excentric in character. 



The swelling of starch-grains which may be artificially induced by 

 treatment with water at a temperature of 60 to 70 C, with caustic 

 potash or soda, etc., need not be considered here. Some attention must, 

 however, be given to the natural process of solution, which takes place 

 when storage-tissues are emptied of their starchy contents, or, more 

 generally, when starch is transported from one place to another. This 

 so-called " solution " is really a process of hydrolysis, effected with the 

 aid of diastatic enzymes. It results in the transformation of the starch 

 by successive stages into amylodcxtrinc, dextrine, isomaltosc, and, finally, 

 maltose. The changes of form which starch-grains undergo during 

 solution, vary considerably in different plants. When the grains are 

 large and excentric, as in the Potato or in I/ilium candidum (in the 

 bulb-scales), they dissolve gradually from the surface inwards. The 

 rate of removal of the starch does not, however, stand in any 

 direct relation to the orientation of the layers, but takes place, to begin 

 with, at the same rate at every point of the surface. As a result, the 

 successive layers soon begin to " crop out " at the thicker end of the 

 grain ; the more watery among those exposed then begin to dissolve more 

 rapidly, so that annular grooves are formed. Except in the very earliest 

 stages of the process, therefore, solution goes on more rapidly at the 

 thicker end of the grain. A dissolving starch-grain finally acquires 

 the shape of a rod, with constrictions at frequent intervals. The 

 starch-grains of the Bean and other Leguminosae are traversed by 

 radial fissures. Here solution proceeds uniformly all over the surface 

 until the cracks are laid open ; the latter then rapidly enlarge, owing 

 to the entrance of diastase, and the more watery central portion of the 



