THE GENETIC RELATIONSHIP OF PARASITES 121 
mental problems of parasitism remain unsolved, that *'we have scarce- 
ly begun the study of the intimate relations of parasite and host, the 
conditions and results of parasitism." He comments further that 
the very simplicity of the plant's organization makes these relations 
more difficult to investigate than with the animal, the cell being the 
unit, not the organism, the study of chemical interrelations is rendered 
highly dijhcult. "Such problems," he says, "call for the combined 
skill of pathologist, physiologist, cytologist, and chemist." These 
forces, however, have not been mobilized adequately as yet. 
Chemotropism is frequently put forward as a factor involved in 
the origin of parasitism in the lower plants. Massee goes so far as to 
assert infection is due to positive chemotaxis. If it is the pulling 
power of some substance within a host which causes fungi to enter, 
it is difficult to explain highly restricted parasites. Either very few 
hosts contain the proper substance or all others must contain some- 
thing to neutralize this attraction or something of a more powerful 
repellant nature. Neither view seems adequate to give us a concep- 
tion of a large percentage of the world of plants with parasites more or 
less specialized to themselves. It must be kept in mind also that 
mere entrance of a fungus is only preliminary and not necessarily in- 
dicative of parasitism. The theories of immunity come to mind, pass 
in review, and leave us unsatisfied. One investigator states that 
"immunity depends chiefly (perhaps entirely) upon the ability of the 
cytoplasm of the host-cells to resist infection by secreting anti-toxins 
which will kill the mycelium of the fungus." Tannin, vegetable acids, 
and oxidizing enzymes have been named as possible toxic substances. 
Others would not place the burden of keeping invaders out upon the 
cell-contents of the host, but explain the failure of parasites to grow 
promiscuously upon their lack of development of cytolytic enzymes 
sufficient to break down the barrier offered by the cell wall. It makes 
little difference which of the theories has the most supporters or which 
one appears most plausible; not one of them, or any combination of 
them, gets at the real intimate relation which must exist between host 
and parasite. Chemical, food, or structural features, as far as we are 
able to appreciate them, seem unable to explain why one parasite can 
attack only one variety of a single host species while another similar 
parasite may attack all the varieties, other species of the genus, and 
perhaps even extend to other closely related genera. 
Our problem would be sufficiently baffling if we were left to con- 
