CELLS AND THE ORGANISM § 48 
at these apices. Although Hydrurus recalls 
other alge already described, in so far as it 
consists of an organised cell colony, it is very 
far removed from a near relationship with 
them, for it belongs to quite a distinct group. 
The cells are of a yellow colour, and when 
the protoplasts escape from their containing 
gelatinous walls they only possess one cilium 
instead of two as in Chlamydomonas. 
The majority of the filamentous alge are 
composed of cells of an elongated form, placed 
end to end, and the colonial origin of such 
plants is more and more obscured owing to the 
specialisation which takes place amongst the 
cells, for these gradually cease to form merely 
coherent but obviously distinct units. They 
come to exist as mere parts of a higher 
organisation, the latter more and more control- 
ling the arrangement and development of the 
constituent cells. Thus the relative import- 
ance of the cell and the organism is gradually 
reversed. In the lower types it is not always 
easy to discover the organism in a congeries 
of cells, whilst in the higher ones the control- 
ling organisation of the complex individual 
may almost completely override the independ- 
ence of the constituent cells. 
As an example of a plant the cells of which 
have still retained a considerable measure of 
autonomy, we may name Spirogyra, one of 
the commonest of the thread-like algz to be 
met with in ponds and ditches, where it is 
easily recognised by its bright green colour and 
