22 SOUTH AFRICAN BOTANY 
84), and of the substances elaborated by the activity of 
the cell, such as sugar. The sap is usually acid in nature 
owing to the presence of various organic acids or acid 
salts. 
Other substances found in the cell are, starch, calcium 
oxalate, fats and oils, tannin, and aleurone grains, 
Starch is present in the form of small granules (fig. 11) 
= 
Fig. 11.—Starch Grains of Potato, 
attached to the leucoplasts. It is the first visible pro- 
duct of carbon assimilation, and the food reserves of 
many plants consist of millions of these starch granules 
stored away in tubers (e.g. potato) in corms, in bulbs, 
in rhizomes (eg. iris), and in most seeds. The starch 
grains are always stratified (fig. 11) and have a distinct 
centre round which the successive strata have been 
deposited. The size of starch grains varies from -002 
mm, to°170 mm. The amount stored away in different 
