THE STOKAGE OF EESEEVE MATEKIALS 243 



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. 113). 



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 leucoplasts ; like the chloroplasts they 



FIG. 112 CELL OF POTATO FIG. 113. STARCH 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 



B 2 



