126 THE METABOLISM OF ALGAE 



point, as is shown by the continued increase in mannitol 

 and laminarin contents. A temporary fall in mannitol con- 

 tent which is sometimes observed in August may be due 

 to the effect of high temperatures in increasing the rate of 

 respiration relative to that of photosynthesis. In the autumn, 

 decreasing light intensity brings about a reduction in the 

 rate of photosynthesis but the concentration of nitrate 

 available is then rising so that the mannitol and laminarin 

 contents fall whereas that of protein increases. ^^' ^^' ^^ 



The extent of the variations in the chemical composition 

 of L. saccharina depends on environmental conditions, being 

 least in the open sea and greatest in lochs and other situa- 

 tions where temperature and the composition of the sea- 

 water are less constant.^' In general the mannitol content 

 increases with depth of immersion of the weed down to 

 6 to 10 metres and decreases below this. The maximum in 

 mannitol content appears to correspond w^ith a maximum 

 in intensity of photosynthesis. Laminarin content tends to 

 decrease progressively with depth but other constituents, 

 such as alginic acid, protein and ash, remain at a level which 

 is more or less independent of depth of immersion. ^^ 



These variations in chemical composition shown by L. 

 saccharina are less extreme than those shown by unicellular 

 algae. For example, the extreme recorded limits of variation 

 of the protein content of L. saccharina and other brown sea- 

 weeds are 3 and 15 per cent of the dry weight^^' ^^' ^'^ which 

 is a much narrower range than that shown by Chlorella 

 pyrenoidosa (Table 11). However, the variations just des- 

 cribed for L. saccharina are those taking place in whole 

 plants. If blades and stipes are analysed separately it is 

 found that the variations occurring in the blades cover a 

 wider range than those of the plant as a whole. Fluctuations 

 in the chemical composition of the stipes follow those in 

 the blades but their range is smaller. Laminarin is absent 

 from the stipes throughout the year.^^' ^' This state of 

 affairs is to be expected since growth and photosynthesis 

 are more intense in the blade. Appreciable translocation of 

 material can evidently occur in species of Laminariales 

 and in the largest, e.g. Macrocystis spp., the holdfast and 



