88 Minnesota Plant Diseases. 



produced which is especially fitted for storage of food materials. 

 The cells therefore are large, thin-walled, closely crowded and 

 contain m^ich starch and other storage food. 



Effects of hosts on parasites. In the unequal partnership of 

 host and parasite, where the fungus is the dominant partner, the 

 latter is often profoundly affected by the host plant. It be- 

 comes so accustomed to peculiarities in the life history of cer- 

 tain plants or groups of plants that it has learned to shape its 

 own course in harmony with these peculiarities. First of all, 

 then, one finds a parasite capable of living on but one particular 

 kind of host — it is found on no other, and an attempt to culti- 

 vate it on even the most closely-related plants fails. Such a 

 condition of parasitism, though by no means unknown, is not 

 very frequent. Far more common is that condition where the 

 parasite has learned to shape its general habits to comply with 

 the peculiarities of each of a group of plants very closely re- 

 lated and is capable of infecting any of them. It is found, how- 

 ever, that the previous habitation of a fungus has in some rusts 

 at least an important eiTect upon the spore of that rust in the 

 infection of other host plants. In general, infection succeeds 

 best upon the identical kind of host upon which the spores were 

 formed, while the nearest relatives of this host are more easily 

 infected than are distant relatives. One must infer from these 

 facts that the effect of nutrition, etc., received during habitation 

 on a host are far reaching and influence the fungus towards a 

 preference for this same host. 



Certain fungi, again, are able to attack any of a number of 

 host plants which are but widely related. Such fungi show 

 general abilities and no special education in selection of host. 

 In other words, they are not so deeply affected as the previously 

 mentioned specialists. 



It has already been noted that certain rusts, in order per- 

 haps to produce spores continuously throughout the season, 

 have learned to live on different hosts at different times of the 

 year. Such fungi may also exercise exact preference for their 

 hosts, though, of course, two hosts are necessary. The influ- 

 ence of the host-effect may here be carried over through the life 

 on the second host until the fungus again inhabits the first host 

 plant. Such an impression must indeed be a profound one. 



