physiology; symbiosis 



427 



en 



Nostoc enclosed in the colourless, pear-shaped vesicles that arise 

 from an underground weft of rliizoidal threads. Reproduction by 

 the formation of new vesicles is said to occur only in the presence 

 of the Nostoc. The presence of chitinous material in the vesicular 

 wall suggests a fungal nature for Geosiphon, the vesicles perhaps 

 being galls that are formed on the threads as a result of the presence 

 of the alga. 



The principal genera taking part in lichen synthesis are Nostoc, 

 Scytonema, Gloeocapsa, Cephaleuros and Trentepohlia. Under 

 normal conditions the partnership is truly symbiotic, but under 

 abnormal conditions the fungus may become a parasite and 

 devour the algal component. The green bodies which are found 

 associated with the cells of Coelenterates and Radiolarians are 

 usually placed in Torm' genera, Zoochlorella and Zooxanthella 

 (cf. Fig. 225). Most of the species belong to the Cryptophyceae, 

 but in certain of the Coelenterata 

 the motile phases of some of the 

 algae suggest an affinity to the Dino- 

 phyceae, whilst Chlorella (Chloro- 

 coccales) is also regarded as a sym- 

 biont of this group. The non-motile 

 cells are usually found in the peri- 

 pheral layers of the polyp, the 

 larval stages of the host commonly 

 being devoid of the alga. Most 

 of the algal symbionts are known 

 to have a motile phase and hence 



are ranahle of an inHpnenrlpnt ^^§-225 Symbiosis. Zooxanthellae 



are capaoie oi an maepenaent in thetissuesofacoeienterate^Po- 

 existence. The function and rela- cilloporabulbosa(x^j^).ec=ecto- 



tions of these svmhintir alcrap in derm, en =endodermis, ^2= dead 

 uons 01 mese symoiOtlC aigae m zooxanthellae,/^ = fat globule, m^ 



the coral polyps have been dis- = glands, 5w= structureless lamella, 



cussed at great length by Yonge ^ = zooxanthellae. (After Yonge.) 



(1932), and on the whole there would appear to be evidence for a 

 symbiotic relationship, the alga obtaining food from the animal, 

 and the animal oxygen and perhaps nitrogenous material from the 

 alga. The problem of the relationships between algae and animals 

 is by no means completely worked out, and it is not impossible 

 that in some cases we really have an animal parasitizing the alga. 

 This is probably especially true in the case of the worm-like Con- 

 voluta roscoffensis and its algal associate Car^^r^a, because the animal 



