INFLUENCE OF EXTERNAL STIMULI. THE MEDIUM 259 



exhaustion of the mycelium. Other species of Coprinus remain quite 

 sterile in darkness, for instance, C. niveus, C. nycthemerus. The primor- 

 dium of the fructification is directly dependent upon light, and especially 

 upon the more highly refrangible rays. Once the primordia have been 

 laid down under the influence of light and have reached a certain stage 

 of development, the ripening and unfolding can go on in the darkness, just 

 as is the case with flowers, whilst the still small primordia of fructifications 

 dwindle coincidently with the elongation of their stipe. Light must then 

 act here up to a certain stage of development ; if this be the case, then the 

 process of ripening may take place in the darkness. It is remarkable that 

 in Coprinus stercorarius the formation of the pileus is not suppressed, 

 as is usual, in darkness if the temperature be high, or at least it is only 

 partially suppressed, and complete pilei may be formed, although much 

 more slowly than they are in the presence of light. 



C. INFLUENCE OF THE MEDIUM. 



We have in this chapter to consider all those conditions of the 

 plant which are brought about by the state of aggregation, or the 

 chemical composition of the surroundings or ' medium ' in which it 

 grows. We leave out of view then purely quantitative differences as 

 well as the deviations which can be designated ' malformations ' l ; we 

 also pass over among other things the phenomena of dwarfing or 

 ' nanismus,' especially as the deviations that appear in dwarf-forms and 

 which are usually the result of an insufficient water-supply require 

 further investigation in their relation to normal forms. When Frank 2 , 

 for example, says : ' Leaf-form may also essentially change, thus dwarf 

 Capsella Bursa-pastoris and Teesdalia nudicaulis may occur with simple 

 entire instead of pinnate leaves,' we learn nothing about the beha- 

 viour of the dwarf from such a statement. In the first place the 

 primary leaves in Capsella are always undivided, and the dwarf might 

 have remained in the stage of primary leaves, and in the second 

 place the form of the leaves in Capsella is a fluctuating one, therefore 

 one must, in order to get accurate results, compare the leaf-formation 

 in dwarf plants and normal plants which are sister-plants. The changes 

 of form which are brought about by such mechanical influences as 

 pressure do not fall to be considered here. 



1 These have been spoken of in the Fourth Section. See p. l 77. 



2 Frank, Die Krankheilen cler Pflan^en, 2. Aufl. p. 274, 



S 2 



