THE STORAGE OF RESERVE MATERIALS 229 
cell taken from the interior of a potato tuber. These 
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). 
In 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 chloro- 
plasts, except that they are colourless. They are known 
for this reason as lewcoplasts; like the chloroplasts they 
Fic. 108.—CrLtu or Potato Fic. 109.—Starcu GRAIN OF 
CONTAINING STARCH GRAINS. Potato, 
occur in considerable numbers in each cell, being situated 
usually near the nucleus. Their relationship to chloro- 
plasts 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 
