210 Transactions British Mycological Society. 



and Wood(38) on the genus Glomerella, evidence is rapidly 

 accumulating to shew that under recurrent identical conditions 

 the morphological variation of a particular fungus is definite 

 and constant. 



Many detailed examples could be given but perhaps two or 

 three from one's own experience may suffice. 



If Fusariiim arcnosporum and Fusaritim angustmn be grown 

 on Czapek's medium with five per cent, glucose, the former is 

 pink and the latter is colourless. If, however, the same medium 

 contain thirty per cent, of glucose, the former is clay-coloured 

 whilst the latter assumes a bright-purplish vinaceous hue. 

 In a neutral Czapek medium Fusariiim sclerotioides possesses 

 a greyish-white aerial mycelium, but in a like medium plus 

 0-4 per cent, by weight of citric acid the aerial mycelium is 

 pink- vinaceous. These and many similar colour reactions may 

 be obtained with the most perfect constancy. Again in certain 

 fungi which have been investigated the expectation of identity 

 of reaction under constant conditions is fulfilled with mathe- 

 matical accuracy. Thus a certain pedigree pure line strain 

 of Botrytis cinerea growing under rigidly standardised con- 

 ditions on the media — steamed sugar-beet root, fresh onion 

 bulb scales and potato agar has the following respective values 

 of its mode spore: 9-3 /x x 6'6/x; 9-3 ju. x 6jLt; and 9-3/x x 4-6 ju,. 

 These are extremely slight differences, but they are mathematic 

 constants. 



All the more recent and accurate work indicates that the 

 fungi like the bacteria respond to particular environmental 

 conditions in definite and characteristic ways; and the present 

 position of this question may fairly be summarised by the 

 following quotation from Thom(43). "The question at issue was 

 not whether or how variations could be produced, but whether 

 a particular variation is constantly produced by a species in 

 a particular environment." " In those species most thoroughly 

 studiL'd both the physiological and morphological reactions 

 have appeared to be very reliable." 



A further development of the fact of plasticity and constancy 

 of response to definite environmental conditions, is the pheno- 

 menon of morphological convergence or divergence of two or 

 more organisms under the influence of external factors. This may 

 perhaps most clearly be illustrated by the diagram on p. 211. 



The numbers i, 2, 3, 4 represent distinct organisms growing 

 under the influence of environmental conditions represented by 

 the capital letters A, B,C, D, E. The small letters shew the 

 growth forms or morphological variations of the organisms under 

 the differing conditions. Thus organism 2 under external factors 

 C exhibits the morphological facies represented by " n." 



