236 VEGETABLE PHYSIOLOGY 



frequently only be detected by delicate staining as the starch 

 grain grows. If the point of deposition is near the side 

 of the leucoplast, as is generally the case, the successive 

 shells of starch are not of equal width, but are wider on 

 the side of the grain which is in relation with the greater 

 bulk of the plastid. The amount deposited on any part 

 of the first-formed portion is proportional to the thickness 

 of the plastid in contact with that part. An eccentric shape, 

 often approximating to that of an oyster-shell, is conse- 

 quently arrived at. Even the most eccentric grains can 

 be shown by delicate staining to be covered entirely by 

 the leucoplast, even the small free end which appears to 



protrude from the latter being 

 clothed by a thin film of its 

 substance. 



Some grains which occur in the 

 potato are not so simple in their 

 structure. Two types are repre- 

 FIG. no. A, COMPOUND, B, SEMI- sented in fig. 110, A and B. The 

 COMPOUND STARCH GRAINS f ormer ar i se by t wo or more grains 



FROM POTATO. J 



originating in the interior of a 



leucoplast : as each grows by deposition of new layers, they 

 become closely pressed together, and constitute a compound 

 grain. Fig. 110, B, shows what is often called a semi-com- 

 pound grain. In such a formation an ovoid leucoplast com- 

 mences deposition at two points, one towards each end. As 

 the starch is deposited round each, the concentric grains come 

 into contact, and the bulk of the leucoplast is reduced to 

 a shell surrounding the mass. Its subsequent continued 

 activity then forms new sheaths overlying the whole. The 

 leucoplast, as in the first case, is gradually used up by its 

 own activity, and it is finally reduced to a film of extreme 

 tenuity, which surrounds the whole grain. 



A very curious starch grain occurs in the latex of certain 

 species of Euphorbia, having the appearance of a dumb-bell 

 (fig. 111). This also is formed by a leucoplast ; the latter 

 is an elongated structure, and at first forms a rod of starch 



