GROWTH AND METABOLISM I27 



lower part of the stipe are entirely dependent on products 

 of photosynthesis translocated from above. ^^'^ 



Both seasonal and diurnal fluctuations of the mannitol 

 content of Phaeophyceae are less extreme than those in 

 laminarin content. ^^^' ^^' ^'^' ^^ The enzymic interconver- 

 sion of mannitol and laminarin does not appear to have been 

 demonstrated, but it seems from this behaviour that they 

 constitute a reserve substance system in which their roles 

 are analogous to those of sugar and starch respectively in 

 higher plants. Fluctuations in the amount of another carbo- 

 hydrate, alginic acid, however, parallel those in protein 

 content rather than those in mannitol and laminarin, apart 

 from a tendency to a second maximum in autumn (Fig. 19). 

 Two maxima in the amounts of cellulose expressed upon a 

 dry weight basis occur during the year. The cellulose con- 

 tent of L. saccharina is at a maximum in spring, falling to a 

 minimum during the period of rapid growth, reaching a 

 second maximum in autumn and falling again in the 

 winter.^® These fluctuations in cellulose and alginic acid 

 are evidently due to the rate of deposition of these cell wall 

 constituents being considerably slower than that of forma- 

 tion of protoplasmic material during the period of rapid 

 growth. The ash constituents of L. saccharina are present 

 in the living plant as salts accumulated by the protoplasts 

 and bases associated with the intercellular sulphuric acid 

 esters. It is to be expected therefore that the seasonal varia- 

 tion in their amount should follow those of protein and 

 alginic acid (Fig. 19). The substances that have so far been 

 mentioned in this account of variations in chemical com- 

 position of L. saccharina together make up over 90 per cent 

 of its dry weight. Fluctuations in minor constituents such 

 as lipides do not appear to have been studied. 



The pattern of metabolism in other Phaeophyceae 

 appears to be substantially the same as that which has been 

 described for L. saccharina^ both in sub-littoral, e.g. L. 

 digitata, L. cloustoni 2Lnd Saccorhiza bulbosa,^^' ^^ and littoral 

 species, e.g. Ascophyllum nodosum and Fucus spp.^^' ^^' ^^® 

 In the littoral species the tendency seen in the algae of 

 the sub-littoral zone towards more than one maximum in 



