228 DIFFERENTIATION OF TISSUES. FUCUS 



cylindrical threads from six to twenty cells thick. The 

 surface cells are small and densely filled with phseo- 

 plasts, the central cells are larger and especially 

 longer, and possess many fewer phaeoplasts per unit 

 volume (Fig. 33, C). 



We can only relate this difference between surface 

 and central cells, which is quite a general feature of 

 bulky algae, to the much more favourable conditions 

 for active growth and division in which the surface 

 cells are placed. They get more oxygen and more 

 dissolved salts, for all these are obtained directly from 

 the surrounding water ; they are also better illumi- 

 nated. Thus it appears th^t the differentiation of 

 tissues in these algae depends directly on the different 

 conditions under which the cells develop. The dif- 

 ferent functions of the tissues in the adult alga are 

 determined by their structure and position in relation 

 to the source of food and oxygen and to the growth 

 of the thallus as a whole. Since growth is localised 

 at the tips of the branches, the medullary cells, as 

 explained on p. 220, owing to their structure and posi- 

 tion, are the natural channels of conduction of the 

 organic foodstuffs elaborated by the photosynthetic 

 cells. 



This is a case in which the origin of differentiation 

 in organisms appears to be directly due to differences 

 in the conditions in which different cells develop, 

 i.e. to physical and chemical differences of environ- 

 ment. The differentiation so initiated is, if it results 

 in a workable mechanism, fixed and further specialised 

 in higher forms. 



In the Larainariacese, the group of Brown Sea- 

 weeds which have the longest bodies, including the 

 gigantic Macrocystis and also the big " tangles " 



