i6 



constitution of their substratum? If, however, as previously shown, the 

 closest parasite is thus very dependent upon its host plant, it only goes to 

 show how completely it agrees with non-parasitic plants in its demands for 

 zrry definite nutritive conditions, and that with a change in these the para- 

 site changes its character and either adjusts itself or disappears. Stahl's 

 observations^ on myxomycete plasmodia show that we must take 

 these phenomena of adjustment into consideration. If the water in the cul- 

 ture glass was replaced by a 3-2 per cent, grape sugar solution, the plasmodia 

 either died from this sudden change or shunned the sugar solution. Grad- 

 ually, however, they accepted it, having accustomed themselves to a more 

 concentrated solution (perhaps by a certain loss in water) and indeed in 

 such a way, that, replaced in pure water, they showed considerable injury. 



In regard to the formation of races, Pfefifer- expresses himself 

 thus ; "Present discoveries . . . make it clear that the tropistic reaction 

 of the same species of bacteria, flagellates etc. gradually changes in accord 

 with the existing cultural conditions. Thus it should be understood that in 

 the same species in nature and in artificial cultures there is found at times 

 a very appreciable ability to respond to reactions and changes, varying to a 

 disappearing point, according to a definite stimulus. Indeed after wide ex- 

 perience it seems possible to breed races in which a definite reaction to 

 tropism has been partially or entirely lost." 



Parasitism is nothing extraordinary. Possibly it is not a factor which 

 has newly appeared since plant cultivation was begun. It should be con- 

 sidered as a nutritive form which arose gradually with the development of 

 organic Ufe and a necessary one, to be looked upon as the last link in the 

 chain formed by the mutual interaction of organisms. This last link begins 

 with those organisms which have the ability of forming organic substances 

 from inorganic material through the action of light. Joined to these are the 

 plants with the lesser need of light, such as are found among the bacteria 

 living in humus where an addition of quickly decomposible organic sub- 

 stances presents essential aid to the nutritive process. As the struggle for 

 light gains in importance with an increasing numl)cr of organisms, the more 

 pertinent becomes the development of groups of organisms requiring but 

 little light and an ever greater need of a method of nutrition by which the 

 raw material is ofifered in the form of organic, easily re-worked substances. 

 Such conditions are found at present in saprophytism. 



With the struggle for light in the case of a constantly increasing num- 

 ber of individuals comes also the struggle for space. In the course of time 

 the lack of space will lead finally to those forms of adjustment in the plant 

 world which require soil for their habitat only in the beginning, if at all, and 

 have chosen some other organism as a centre of colonization. The mutual 

 interrelations forming under such conditions are partly friendly, partly hos- 

 tile, just as they occur in mutualistic and in antagonistic symbiosis. 



Stahl in Bot. Z. 1884, pp. 163-66. 



PfefEer, Pflanzenphysiologie, 2 Edition. Vol. II, p. 763. Leipzig 1904. 



