GROWTH AND METABOLISM 129 



ature and formation of fructifications being perhaps of most 

 importance. The minimum in laminarin content occurring 

 in July and August is almost certainly the result of the 

 reduction in photosynthetic activity which is known to be 

 brought about by increased desiccation.^^^ The increase 

 in this effect with degree of exposure is shown in Fig. 20, 

 from which it will be seen that the early summer maximum 

 occurs sooner and the summer minimum is more pro- 

 nounced the higher the position which the species occupies 

 on the shore. Judging by the behaviour of simple algae as 

 described earlier in this chapter, it seems unlikely that ex- 

 haustion of nutrients such as nitrate and phosphate from 

 the seawater would affect photosynthesis directly and lead 

 to the summer minimum in laminarin and mannitol, as has 

 been suggested. ^^ Differences in lipide metabolism of 

 various Phaeophyceae are also correlated with depth of 

 immersion, e.g. Pelvetia canaliculata, which grows at high- 

 water mark, has been found to contain 6-2 per cent of fat, 

 whereas Laminaria digitata, which grows below low water 

 mark, has been found to contain only o-i6 per cent.^^^ 

 These differences may depend on the decreasing amounts 

 of nitrogen available to plants growing higher up the shore 

 but other factors besides this are almost certainly involved. 

 Few studies of seasonal variation in chemical constitution 

 have been made with representatives of the Rhodophyceae. 

 In conformity with the results obtained with Phaeophyceae 

 the carbohydrate content of Chondrus crispus has been found 

 to be lowest in early spring and highest from June to 

 September. ^1 The agar extracted from Gigartina in autumn 

 gives a gel of greater strength than that extracted in 

 spring,!^^ a variation that is perhaps related to seasonal 

 changes in the type of sulphuric esters 'present, such as 

 have been observed in Chondrus. ^^ 



