24 SOUTH AFRICAN BOTANY 
The aleurone layer is stained a yellowish-brown colour 
by iodine, and so stands out from the starch cells, which 
are stained blue. 
15. Form of Cell.—The original form of the cell was 
probably spherical, but in. multicellular organisms the 
newly developed cells are always polygonal in shape 
owing to the pressures set up by the adjacent. cells. 
Fig, 13. 
A. Sieve tubes. s.y. Sieve plate, B, Tracheides. a. Thickened in 
rings. 6. Thickened spirally. cv. Thickened reticulately. 
Subsequent growth tends to change the form of the 
cell completely. 
When the cells have the same dimensions in all direc- 
tions, are thin walled, and possess a lining of protoplasm 
they make up a tissue called ParrENcHYMA. They are 
usually six-sided and fit tightly together like the cells 
in a beehive. If, on the other hand, the cells are elon- 
gated, and possess thickened walls the tissue is called 
ProsgencHyMa. Usually prosenchymatous cells have 
pointed ends and little or no contents. These thick 
