physiology; symbiosis 4^5 



Enter omorpha. Chryptomonas ovata and Synura uvella will only 

 grow if an extract of soil or lake sediment is added to the culture 

 medium. 



Some species of algae are as rich in vitamin C as lemons, though 

 the content is affected by season, temperature, location and pH. 

 Vitamins D and E also appear to be present in at least some algae. 



Growth hormones 



Auxin (indole-acetic acid), presumably identical with that found 

 in higher plants, has been reported in Valonia macrophysa^ Lamin- 

 aria agardhii and in a number of Pacific coast algae, including 

 Macrocystis, Desmarestia and Fucus evanescens. Growth substances 

 have been detected in the eggs, sperm and fruiting tips of European 

 species of Fucus^ and the presence of these may be partly respon- 

 sible for the fact that the first rhizoid of a germinating zygote grows 

 out on the side adjacent to neighbouring eggs. It is also clear that 

 acidity is an important factor in this effect. Although auxin has not 

 been detected in Codium tomentosum from Naples, nevertheless 

 indole-acetic acid does promote growth in Codium^ and a further 

 investigation would seem desirable. 



In addition to growth promoting substances there maybe growth 

 inhibiting compoimds. Such substances are probably produced by 

 Nostoc punctiforme^ Nitschia palea and Chlorella vulgaris. The in- 

 hibitor from Chlorella is known as ^chlorelHn' and appears to be a 

 mixture of fatty acids. It is active against bacteria and this is one 

 reason why Chlorella is so effective in purifying sewage effluent 

 (see p. 453). 



Phototropic phenomena are known to occur in Bryopsis, Aceta- 

 hularia and Derhesia whilst rather weaker effects are found in 

 Cladophora and Griffithsia. In Bryopsis the tips of the branches 

 grow towards the Hght and the rhizoids away from it : this may well 

 be associated with auxin, as in higher plants, but although auxin 

 occurs in Bryopsis there is no clear gradient along the plant, nor is 

 there any effect from external appHcations until inhibitory con- 

 centrations are reached. 



Sex hormones 



Since 1926 it has been known that the gametes of certain algae 

 secrete sexual substances into the surrounding water. This has been 



