150 PLANT ANATOMY. 
from their variety and slight regularity they preclude a more 
precise definition (Figs. 29, 30, 31, 56, 63, 65, 76). 
When the deposition of new cell material does not take place 
chiefly in a direction tangential to the cell-wall, but in such a 
manner that the latter grows in thickness, this growth can take 
place more largely either toward the exterior or toward the 
interior. In the first case, prominences of various kinds are 
formed (spores and pollen cells, the outer wall of epidermis 
ells), in the latter, the cavity of the cell becomes contracted, 
Oy 
if 
Fie. 65, Fie. 66, 
Fie. 65.—Polyhedral parenchyma from Rhizoma Graminis. 
Fie. 66.—Uncoiling spirals and an annular vessel from Bulbus Scilla. 
often almost entirely filled up (some bast cells and stone-cells). 
A perfectly uniform thickening, however, never takes place, but 
the cell-membrane retains in some places its slight thickness. 
The appearance of the cells which are subjected to a considera- 
ble extent to the growth in thickness is chiefly determined by 
the relative extent of the thickened places and those which have 
remained thin. If the thickened places are in about the same 
