48 TYPES OF BRITISH PLANTS 
duction by conjugation, a process which may be thus 
described. Two cells, apparently similar to all the other 
cells in the plant, separated from one another, show a 
swelling on the near sides of their cell-walls. The swell- 
ings grow until they meet and then fuse, forming a tube 
from one cell to the other. The protoplasm flows together — 
into one or other and forms what is called a zygospore. . 
This slips away from the plant and starts a fresh existence. 
SPIROGYRA. 
A good example of this method is seen in “ Spirogyra” 
(it has no English name), a plant with long threads of 
cells, which unite with one another until two threads 
look like a long ladder, the tubes forming the rungs, A 
very well-known group in this division is formed by the 
diatoms (a word meaning “cut through”). They get 
their name from the fact that their cell-walls consist of 
two halves, fitting closely to one another like the halves 
of a pill box. These are distributed in enormous numbers 
