ELISHA MITCHELL SCIENTIFIC SOCIETY. 35 



the leaves have fallen to the ground and become more or 

 less disoro^anized or frag^mentarv and the evidences of the 

 Cercospora have disappeared. 



While the species are not autonomous, and we thus possess 

 only fragmentary evidence, as it were, of the characters of 

 the complete individual, the peculiarities of form, group- 

 ing, markings, color, dimensions and effect upon their 

 hosts are such as to offer comparatively satisfactory data 

 for the systematist to characterize and arrange them. It 

 is fortunate that this is so, because of their parasitic habit 

 it is quite important that we can arrive even approximately 

 at the limitations of the species on the different hosts. 



It may seem surprising at first, to one unfamiliar with 

 the growth of these forms and the reactionary influence of 

 their hosts, that so many species are at present known, 

 and that the probability is the number will even yet be 

 increased. The specific physiological differences of the 

 various hosts as w^ell as the structural variations of their 

 leaves, the differences in texture, thickness, and the varying 

 powder which the different species possess through their vital 

 processes to resist the growth of the parasite, all exert a 

 powerful influence upon its form and characteristics. Here 

 we have the coincidence of several quite effective agencies, 

 all which tend to produce variations in the parasite. It is 

 quite possible to conceive how during a long period of time 

 a few forms wndely distributed over a great number of hosts 

 have become more and more unlike each other and finally 

 more firmly fixed in the possession of peculiar characteris- 

 tics. This is even more probable when we consider that 

 quite likely during much of this time the hosts themselves 

 have been differentiatino- more and more so that now w^ell- 

 marked specific differences appear in hosts that long ago 

 were alike and harbored the parasite which has kept pace 

 with them in descent. 



The action of the Cercospora parasite on the host results 



