I 3 o TEXT-BOOK OF FUNGI 



other hand, if a well-nourished mycelium is placed in a 

 poor medium that is solid, thus precluding the formation 

 of zoospores, sexual organs will be formed in abundance. 



In the formation of zoospores the relative value of food 

 depends on the available amount of carbon. Inorganic 

 acids and their salts are of little value. The formation of 

 sporangia is dependent on the presence of potassium 

 phosphate. 



As regards temperature the optimum for sexual organs is 

 26; for sporangia, 32. 



The formation of gemmae is of no phylogenetic signifi- 

 cance. They are formed under conditions when the forma- 

 tion of other reproductive bodies is rendered impossible, 

 owing to unfavourable environment. On starting growth 

 they behave like ordinary hyphae, and produce sexual 

 organs or sporangia, depending on conditions. 



There is no fixed inherent law determining an alternation 

 of generations, but the conditions are such, that under 

 ordinary natural conditions, an alternation is effected, due 

 to the exhaustion of nourishment in each host-plant 

 attacked. Klebs agrees with cytologists generally in con- 

 sidering that morphological characters furnished by an 

 organism at any one period is not sufficient to stamp a 

 species. In future the systematist must determine the 

 limits of variation in a plant, also the causes that determine 

 such variations. 



In Thamnidium elegans^ one of the Mucoraceae, two 

 forms of sporangium are usually present. The terminal 

 one, of large size, is many-spored, with a conspicuous col- 

 umella and a diffluent sporangial wall. The second form, 

 of which there are usually several, situated laterally on the 

 sporangiophore, is much smaller than the terminal one, 



