THE STOEAGE OF EESEKVE MATERIALS 235 



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 

 (%. 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 chloroplasts, except 

 that they are colourless. They are known for this reason 



FIG. 108. CELL OF POTATO FIG. 109. STARCH GRAIN 



CONTAINING STARCH GRAINS. OF 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 



