230 VEGETABLE PHYSIOLOGY 



merely a thin film of it surrounding the striated grain. It 

 can 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 succes- 

 sive 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 consequently 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 often found in the 

 potato are not so simple in their 

 structure. These are represented 

 FIG no A, COMPOUND, B, SEMI- in fig. 114, A and B. The former 



COMPOUND STABCH GRAINS FBOM^^ ^ ^ ^ ^^ ^.^ 



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. 114, B, shows what is often called a semi-com- 

 pound grain. In such a formation the leucoplast commences 

 deposition at two points, one towards each side. 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; 



