676 CONSTANCY AND MUTABILITY OF FUNGI. 



that from the most simple spore-cells rods and threads 

 proceed, and that spores again form in these threads ; 



Stages of we observe, further, in the mould fungi, such as asper- 

 gillus or penicillium, a transition into a completely 

 different form of fructification we see the ordinary yeast 

 pass into spore-bearing cells of a totally different form. 

 External influences often lead to the formation of the 

 one or the other form of development ; but under these 

 circumstances there always arise only the definite forms 

 which are characteristic of the species, and not all sorts 

 of variations differing according to the external con- 

 ditions which have been at work. According to Zopf 's 

 views there is in the case of many bacteria a particularly 

 wide cycle of vegetative forms ; but in spite of all these 

 differences in form definite species can nevertheless be 

 made. We have only, in this respect, to learn all the 

 stages of development, and to arrange them among the 

 characteristics of the species. 



Degenerative In the second place, in the lower fungi modifications of 



modifications form also occur under the influence of external agencies. 



according to T^ & cer t a j n degree the nutrient conditions, for example, 



nutriment. . ... 



can influence the form in a similar manner as in the case 

 of higher beings. Slight increase or diminution in length 

 and thickness, a more marked swelling of the cells and 

 threads, is not uncommonly observed (Buchner) ; and 

 according to the external conditions of existence, now the 

 one, now the other stage of development may be most 

 prominent, and may be observed in a particularly com- 

 plete form. But on more careful examination we can 

 ascertain that these variations scarcely ever go far 

 enough to alter the types of form which are looked 

 on as characteristic of the individual species. The re- 

 lation of the length of the rod-shaped bacteria to the 

 breadth, the form of the ends of the rods, the character 

 of the thread form, the mode in which the individuals 

 are grouped together, remain, as a whole, untouched by 

 such variations. Under the practically abnormal ex- 

 ternal conditions more marked alterations in form, and 

 the production of pathological and involution forms, often 

 occur. These variations in form render it difficult to 



