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. 110.—a, CompounD, B, Srmt- sented in fig. 110, Aands. The 
comrounp SrancH GRAINS former arise by two or more grains 
From Poraro. of : P : 
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, 8, 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, 1s 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 ; the latter 
is an elongated structure, and at first forms a rod of starch 
