TWENTY-tilXTIi ANNUAL MEETING. 4.5 



VARIATIONS IN DOMINANT SPECIES OF PLANTS.— H. 



By M. A. CARLETON. 



At a previous meeting cf the Kansas Academy of Science I presented a 

 paper on this same subject, having reference to phanerogams only. I wish 

 here to extend the discussion, considering at this time the group of parasitic 

 fungi. 



Of the elements of environment that tend to the modification of flower- 

 ing plants, only two — heat and moisture — affect parasitic fungi to any ma- 

 terial degree. Others do so, if at all, only indirectly, through their action 

 upon the host plants. But there is an element more effective, probably, 

 than any or all these in modifying the structure of parasitic plants, and that 

 is the structure of the host plants themselves. Parasites become greatly 

 modified often in passing from one host to another. Puccinia graminis be- 

 comes more narrowed, or shorter, or more rounded, according to the host on 

 which it is found. The formation of mesospores is an excellent example of 

 modification, and these are particularls'- more apt to occur on some hosts than 

 on others. Puccinia ruhigo-vera almost invariably produces only one-celled 

 spores on Hordeum pratense. Macrosporium, Helminthosporium, Clado- 

 sporium, Cercosj^otxt, etc., have all probably varied from a common parent. 



