CHAPTER I 

 INTRODUCTION 



The term 'alga' is a difficult one to define. In the broad 

 sense in which it is used by modern authorities^^^' ^59 it 

 includes a number of types of organism (see Table i) which 

 differ profoundly one from another in cell organization and 

 which have little in common except that their characteristic 

 mode of nutrition is photosynthetic and that they cannot 

 be included in any other division of the plant kingdom. 

 This grouping together of phylogenetically remote classes 

 of organisms, while artificial from some points of view, is 

 nevertheless convenient, particularly if the transformations 

 of matter and energy which comprise their metabolism are 

 to be considered. Regarded in this way, these organisms 

 form a more homogeneous group with sufficiently marked 

 characteristics to justify special consideration. 



On the one hand algae are distinguished from the 

 morphologically more complex higher plants by the variety 

 and flexibility of chemical activity characteristic of the more 

 primitive forms of life. On the other hand they differ from 

 organisms such as bacteria, fungi and protozoa, in having a 

 chemical economy based upon photosynthesis, in which the 

 accumulation rather than the breakdown of organic matter 

 predominates. In bulk of material involved this algal type 

 of metabolism perhaps exceeds any other. The total yield 

 of photosynthesis in the oceans, in which algae are the only 

 photosynthetic organisms, has been estimated to be from 

 1-6 to 15-5x1010 tons of carbon fixed per year, and is 

 evidently at least as much as that of land plants. ^47, 209 i^ 

 soil and in freshwater algal metabolism is generally on a 

 lesser scale but is nevertheless of considerable importance. 



It is not altogether surprising, then, that the metabolic 

 activities of algae should have attracted attention while the 

 description and classification of species was yet in a chaotic 



I 



