SUMMARY AND CONCLUSIONS 



131 



algal classes. Light absorbed by certain of these pigments 

 can be transferred to chlorophyll a and used in photo- 

 synthesis so that the efficiency and manner of utilization 

 of light of different wavelengths varies considerably from 

 class to class. Similarly, while it appears that the primary 

 photochemical reaction in photosynthesis always results in 

 the splitting of water, the ultimate hydrogen donor, although 

 usually water as in higher plants, may, in many algae under 

 certain conditions, be elementary hydrogen or hydrogen 

 sulphide. There is also a related tendency among algae to 

 utilize reduced inorganic substrates such as these as a 

 source of energy for growth, a tendency which is shown by 

 no higher green plants. 



Another characteristic of the metabolism of algae is its 

 flexibility, which is apparent both in the variety of sub- 

 strates which can be assimilated and the considerable 

 variation which can occur in the proportions of the various 

 products of metabolism accumulating within the organism.. 

 Many algae, although capable of photosynthesis, can also 

 grow in the dark if provided with a suitable substrate and 

 for many algae these substrates may be of very varied 

 chemical nature. A similar flexibility is evident in the 

 assimilation of nitrogen. Among the final products of meta- 

 bolism the proportions of such materials as fats, carbo- 

 hydrates and proteins, which accumulate within the cells, 

 may vary within wide limits according to the stage of 

 growth attained and the conditions under which it has taken 

 place. During growth the products of photosynthesis or of 

 the assimilation of organic substances are used for the 

 synthesis of protoplasm but on cessation of growth the 

 continuation of assimilation results in the accumulation of 

 storage products the nature of which varies according to 

 the species concerned and to other circumstances. The 

 limits within which the chemical composition of an alga 

 may vary as a result of this are particularly wide in the 

 simpler forms; as a general rule structurally more complex 

 algae are more exacting, both in their internal and external 

 environmental requirements, and this limits the amount 

 of variation which can occur. 



