THE STORAGE OF RESERVE MATERIALS = 285 
grains of starch are much larger than those which occur 
in the chloroplasts of the leaf, and they have a complicated 
structure. Most of them are irregularly oval in shape, 
and their surfaces are marked by nearly concentric lines 
of striation, dividing them apparently into layers. The 
centre of these layers is not usually the geometrical centre 
of the grain, but lies near the small end, and the rings or 
layers are much narrower at that end than at the other 
(fig. 109). , 
Tn most cases the deposition of starch in these and similar 
cells is brought about by the agency of small protoplasmic 
corpuscles, which closely resemble the chloroplasts, except 
that they are colourless. They are known for this reason 
Fic. 108.—CELL oF Potato Fic. 109.—Starcu Grain 
CONTAINING STARCH GRAINS. or Potato. 
as leucoplasts; like the chloroplasts, they occur in con- 
siderable numbers in each cell, being situated usually near 
the nucleus. Their relationship to chloroplasts is shown 
by the fact that they turn green when they are exposed for 
a considerable time to light. 
The leucoplasts behave very much like the chloroplasts. 
When a solution of sugar reaches the cell in which they 
lie, they absorb it as the chloroplasts do the excess of sugar 
manufactured in the cells of the leaf. They then secrete 
starch, which is at once deposited in their substance. If 
the point of deposition is the centre of the leucoplast, 
successive shells of starch are deposited concentrically upon 
the first-formed portion, and a symmetrical grain is produced 
which ultimately attains a relatively considerable size. 
It remains, however, surrounded by the leucoplast, which 
gradually becomes much stretched until there is merely 
a thin film of it surrounding the striated grain. It can 
