MOULDS, RUSTS, ETC. 117 



spores. The former are most commonly the means of rapid 

 reproduction, while the latter constitute the so-called resting- 

 spores, by means of which the plant is carried safely through 

 such vicissitudes as drying, freezing, etc. The distinction 

 of summer and winter spores, which we have observed in 

 the wheat rust, is frequently strongly pronounced, and 

 may be of much economical importance. In the case of 

 the rusts, and various other destructive parasites, it is very 

 desirable to know in what form the fungus passes the 

 winter, in order to destroy, if possible, the material in 

 which it harbors, or, at all events, to avoid_ its farther dis- 

 semination. 



The influence of the parasite on its host is naturally 

 injurious, although in this respect different species vary. 

 Some mildews, such as those of the lilac and influence on 

 oak leaf, penetrate their host to so little depth, ^°^^- 

 and absorb so little nutriment, that the leaf shows little or 

 no sign of being injured by the presence of the parasite. 

 Others, as for instance that of the grape-vine, cause great 

 damage, both by choking the tissues on which they have 

 fastened, and by absorbing the nutriment required for 

 the proper development of the fruit. Still others, such 

 as the smuts, utterly destroy the organs they invade. It 

 becomes an extremely interesting subject of inyestigation 

 to determine the actual relation of parasite and host, — how 

 the attack is carried out, by what process an entrance is 

 effected, the resistance offered, the character and extent 

 of the injury sustained, the effort to repair the loss, and 

 the final result. Such studies afford the data by which 

 already some of the most destructive pests have been 

 checked in their course, with the result of preventing 

 heavy losses that formerly followed their uncontrolled 

 depredations. 



