ALG.E AT DIFFERENT SEASONS 211 



changed conditions. It is in harmony with this that only a small 

 part (viz. the relatively unspecialised gametes, see next chapter) 

 of highly specialised organisms persists from one generation to 

 the next, whereas in the case of a simple organism, the whole 

 may survive in the bodies of its offspring. We may see in this, 

 in the case of the more highly differentiated forms, a provision 

 whereby the next generation is temporarily relieved of the 

 trammels of the specialisation of its parents, and thus probably 

 becomes better fitted to meet the extremely varied conditions 

 to which the different individuals are subjected during their 

 development. 



The diverse freshwater Algae are not found in equal quantity 

 all the year round, many disappearing more or less completely 

 during the warmer months or during the winter. A maximum 

 abundance is usually attained during the spring (the time at 

 which sexual reproduction occurs in many forms), whilst 

 renewed development may take place in the autumn. The 

 different genera and species frequently succeed one another 

 to a more or less marked extent, so that a number of phases 

 can be distinguished in the algal vegetation of small ponds, 

 etc. This periodicity is probably conditioned by a multi- 

 plicity of factors, among which varying temperature and gas- 

 content of the water play an important role. Some forms 

 (e.g. Cladophora, several Diatoms), however, are encountered 

 all the year round, although in varying abundance. 



Many marine Algae appear to persist throughout the year, 

 although their development is often retarded during the summer 

 or winter, as the case may be. The late winter is often the period 

 at which sexual reproduction occurs. The power of with- 

 standing low temperatures, which is so manifest in the case 

 of the Seaweeds, also appertains to many freshwater Algae, 

 even such delicate forms as Desmids, for instance, often occurring 

 as healthy individuals beneath the ice-sheet in the depth of winter. 



