physiology; symbiosis 421 



There are other criticisms that have been levelled against this 

 work, e.g. according to Moewus, who reports all this work, there 

 must be a Hnear transformation of the cis- and trans- esters, where- 

 as the pubHshed data shows an exponential curve. Also if ?95/5 are 

 mixed with the $65/35 one should get a homogeneous 80/20 

 mixture in the filtrate of the cis-/trans- ester, and there should be no 

 copulation. Even if the gametes continued manufacture as they 

 move they would leave behind much of their excreted material and 

 would be in the 80/20 diffusion mixture. It would seem that at least 

 one gamete of a pair would need to be sedentary, and that is not 

 what is observed. 



Moewus also claimed that there is a relationship between tem- 

 perature, pH and production of female gametes in Protosiphon 

 botrydioides, where an andro- and a gynotermone are also said to 

 occur. In Monostroma wittrockii it is claimed that an active secre- 

 tion from a brei of the gametophyte thallus results in cells of sporo- 

 phytic thaUi giving rise to 64 bifiagellate gametes instead of 32 

 quadrifiagellate zoospores. It is clear that all this work has opened 

 up an extremely interesting field, but at present there is a need for 

 the work to be repeated and extended to the gametes of other algae. 



Mineral nutrition and chemical composition 



The marine algae are able to absorb most of the salts present in 

 sea water, though Valonia and Halicystis apparently cannot take up 

 sulphates. The availabiUty of major nutrients, expecially nitrogen 

 and phosphorus, largely determines the plankton maxima (see p. 

 343). Trace elements are also present in the ash of algae and in the 

 case of Ulva increased growth takes place in the presence of zinc, 

 iron, boron and manganese. 



In view of the fact that so many algae deposit calcium carbonate 

 in or around the thallus it is remarkable how little we know about 

 the phenomenon. It is generally assumed to be a result of the ex- 

 cessive use of CO2 in photosynthesis with a resulting rise of alka- 

 linity in the immediate neighbourhood of the algae to a point where 

 CaCOa is no longer soluble. It is doubtful whether this simple ex- 

 planation will suffice and it is possible that enzymatic action, pos- 

 sibly involving enzymes, is involved. A similar problem must exist 

 in the case of the coral polyps. 



In recent years a considerable volume of work has been carried 

 out in great detail on the chemical composition of algae, especially 



