CELLS AND THE ORGANISM 43 



at these apices. Although Hydrurus recalls 

 other algae 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 algae 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 algae to be 

 met with in ponds and ditches, where it is 

 easily recognised by its bright green colour and 



