physiology; symbiosis 425 



Scottish plants is almost certainly the result of a lowering of photo- 

 synthetic activity brought about by increased desiccation. 



Outside of Europe rather similar results have been reported for 

 Fucus vesicidosus, F. evanescens and Ascophyllum from the Canadian 

 maritime provinces, except that laminarin tends to remain rather 

 constant. The summer mannitol maximum appears to be depen- 

 dent upon temperature. In the Mediterranean an analysis of 60 

 species for their iodine content showed that the maximum values 

 do not coincide with the period of maximum development and is 

 also independent of habitat. 



Sap, permeability and osmotic pressure 



The Siphonales and Charales have formed excellent material for 

 studies in permeabilit}^, sap composition and osmotic pressure be- 

 cause of the size of their cells. There is considerable variation in 

 the rate of penetration by the different ions ; iodine, for example, 

 entering far more rapidly than chlorine, and potassium more 

 rapidly than sodium but less rapidly than ammonia. Oxygen also 

 moves in and out of cells extremely rapidly. The behaviour of salts 

 and gases in the case of other algae with a higher and more complex 

 organization of tissues may well be different from the Siphonales 

 and Charales, but so far it has been but Httle studied. A recent 

 study on ion movement in the brown fucalean alga Hormosira 

 hanksii has shown that there is an influx pump favouring accumula- 

 tion of potassium and an efflux pump removing excess sodium. 

 The extent to which these two mechanisms operate depends, at 

 least in large part, upon the degree of desiccation (see p. 390). 



The cell sap of many fresh water algae has an osmotic pressure 

 of about 5 Ats., whereas that of many marine algae, e.g. Halicystisy 

 Valonia, Nereocystis, is only slightly higher than that of sea water. 

 The osmotic pressure in Fucales would appear to be rather higher 

 and also that of some of the Rhodophyceae. In general, however, 

 the osmotic pressure in red algae is somewhat lower than in green 

 or brown algae. The tolerance of algae to changes in osmotic pres- 

 sure (caused by water loss) is important in the case of Httoral algae 

 (cf. p. 404). Some algae appear to respond almost instantaneously: 

 at all events there is no plasmolytic response to sudden changes, 

 e.g. Monostroma, Dunaliella salina. This suggests that cells of such 

 plants may be very permeable to the salts rather than impermeable 



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