210 DIVISION OF LABOUR 



exhibiting Brownian movement, and are a peculiar feature 

 of tliis genus. Similar elaborate chloroplasts are found in 

 Cosmarium (Fig. 114, E), where the two halves of the cell are 

 usually rounded or oblong, and Micrasterias (Fig. 114, A), 

 where the outline of the cell as a whole is often very complex. 

 This last feature reaches a climax in those Desmids which often 

 abound in the surface-water of lakes, and where, as in other 

 unicellular floating organisms, the increased surface due to 

 their complicated outline considerably augments their buoyancy. 



The extreme variety of the chloroplasts amongst the Algre 

 is in striking contrast with their comparative uniformity amongst 

 Flowering Plants. It may be remarked, however, that the 

 chloroplast of the simple Alga is as much the assimilating organ 

 as is the leaf in the higher plant, where, too, a great diversity 

 of structure in an apparently uniform habitat is found. 



The Algse afford an excellent illustration of the fact that 

 division of labour is associated with increased complexity' of 

 structure. In many filamentous Green Algte all the cells, except 

 that serving for attachment, may be alike in form and play an 

 equal part in growth and division {e.g. Ulothrix, Spirogym). At 

 the other extreme the larger Brown Algte not only exhibit a 

 relegation of attachment, growth, and reproduction to definite 

 parts of the thallus, but the units of which the latter is built up 

 also show a certain specialisation into conducting, assimilating, 

 and meristematic elements. So too, within the cell itself, definite 

 bodies are organised for the carrying on of particular functions. 

 Whilst in the simply constructed Blue-green Alga; it is difficult, 

 or even impossible, to recognise either chloroplast or nucleus, 

 such division of labour is well marked within the cell in the 

 majority of plants. The complexity which the chloroplast 

 sometimes attains merely affords an extreme illustration of this 

 principle. 



It is probable that the complex tj'pe of cell arose from a much 

 simpler one, and that similarly the multicellular organism 

 had its origin in the unicellular, as is usually the case in the course 

 of the individual development (cf. next chapter). High efficiency 

 for particular conditions of life demand complexity of structure 

 which, however, like all specialisation, tends to diminish the 

 adaptability of the organism, to reduce its cajiacity for meeting 



