42 PLANT LIFE 
a smooth stalk, and is attached to stones, etce., 
by a slightly expanded disc-like foot. 
An individual Hydrurus is made up of a 
colony of unicellular alge, the walls of which 
have become swollen and rather firmly 
gelatinous. The whole organisation of the 
plant depends on the different mode of 
development followed by the various in- 
dividual cells. The apices of all the branches 
and hair-like protuberances are occupied by 
single cells, and it is to these that the alga 
owes its definite form. The terminal cells 
of the branches multiply by dividing longi- 
tudinally ; one of the two daughter cells then 
gradually slides in front of the other and 
continues to function as the growing apex, 
the other one, which has taken a rearward 
position, contributes to the building up of 
the plant body. Some of these cells behind 
the apex extend outwards from the cylindrical 
surface and become the starting-points of 
new branches; or, if their growth is but 
limited, they merely give rise to the villous 
hairy outgrowths. 
The important lesson to be learned from 
Hydrurus is that a definite co-ordination 
exists amongst the individuals composing 
the colony or association. In this way it be- 
comes possible to speak of the terminal cells 
as “‘ apical cells’; that is to say, they have 
assumed the réle of determining the fashion 
of the branching, the rest of the cells merely 
building up the plant on the lines laid down 
