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 foundin the 
potato are not so simple in their 
structure. These are represented 
Fic. 110.—a, Compounp, 8, Semr- in fig.114,a ands. The former 
Pommren STARCH GRAINS FROM arise by two or more grains 
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, 8, shows what is often called a semi-com- 
pound grain. Insuch 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 Huphorbia, having the appearance of a 
dumb-bell (fig. 111). This also is formed by a leucoplast ; 
