Relations betiueen Host and. Parasite. 59 



immunity and parasitism when studied as they affect the pUint, with the 

 wider outlook derived from considering their hearing upon and connexion 

 with men and animals, are more Ukely to be fruitful in results and less liable 

 to be misunderstood. Bacteria, in their relation to plants, and plant 

 sanitation, are now receiving attention, and this is but the beginning of a 

 new era in the mode of regarding plants as affected by disease. 



There is a sort of reciprocal action going on between host and parasite. 

 On the one hand, the more perfect the fungus becomes as a parasite, the 

 more deadly will be its effects on the host-plant, and, on the other hand, 

 the more resistant the host-plant becomes, the less chance will the parasite 

 have of surviving, unless it changes its host. But there is a common ground 

 of agreement in the fact that, while it is to the interest of the parasite that 

 the host should not become extinct, it is also naturally to the interest of the 

 host that it should survive, and the final result is that the general tendency 

 is in favour of the preservation of the host. The question is not so much 

 Which is to conquer in the struggle for existence ? but if one organism is 

 favoured at the expense of another, how is that other to maintain its ground 

 notwithstanding the struggle. 



There are various ways in which this may be accomplished, and a few 

 may be recorded here. 



1. The most permanent means would be the production or de- 

 velopment of immunity in the plant itself, so that, instead of being 

 predisposed to the disease, it would have hereditary tendeucies in an opposite 

 direction. This immunity may be more or less complete, and it is strongly 

 developed in the Durum varieties of wheat. 



2. The germination of the seed may be so rapid that the young seedling 

 will outstrip the germinating spore of the parasite, and thus be too far 

 advanced for successful infection to take place. This property of rapid 

 germination is associated with various varieties of wheat, and it has been shown 

 experimentally that an Ohio variety of wheat, also Florence and Genoa, 

 which are highly bunt-resistant, are relatively rapid in their germination, 

 while a well known bunt-liable variety such as Warden is comparatively 

 slow. 



3. There may be a form of symbiosis established between parasite and 

 host which will enable both to thrive in partnership. This is apparently the 

 case with the seed fungus of Lolium temulenticm or Darnel, which has been 

 investigated by Freeman, ', -, -^ and is now considered to be a smut. 



In some of the smuts, such as the loose smut of oats, there is a profuse 

 development of spores, and consequently a relatively large destruction of 

 the host-plant. In loose smut of wheat, where infection takes place through 

 the flower, the parasite passes the first stage of its existence in the seed. If 

 the seed germinates and the conditions are favorable, there will be a production 

 of spores in the plant produced, but it is conceivable that spores may not be 

 formed, and then this would lead up to the case of the smut in Lolium. There 

 the mycelium of the fungus occurs in the seed, and a mutual understanding 

 is set up between the smut and its host-plant by the production of s})ores 

 being kept in abeyance. " The Lolium fungus symbiosis is therefore apparently 

 to be explained as a development from a smut parasitism of the loose smut 

 of wheat type, in which the spore formation is entirely lost or of very rare 

 occurrence, and in which a compensating mycelial infection of the host-plant 

 embryo has been substituted for it." In short the parasite here has to be 

 contented with a purely vegetative existence, and seems to exert a beneficial 

 influence on the host, for cultures showed that mycelium infected seeds 

 generally produced more vigorous plants and a greater number 

 of seeds than the uninfected. It is known that some Darnel seeds 



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