42 PLANT LIFE 



a smooth stalk, and is attached to stones, etc., 

 by a slightly expanded disc-like foot. 



An individual Hydrurus is made up of a 

 colony of unicellular algae, 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 role of determining the fashion 

 of the branching, the rest of the cells merely 

 building up the plant on the lines laid down 



