DISEASES OF CULTIVATED PLANTS 441 



twines about and robs the wild herbs and shrubs of the woods and 

 fields as well as the cultivated flax and clovers. 



HOW PARASITIC FUNGI AFFECT THE HOST. 



We know the cumulative effects of insufficient food supply; 

 these effects must hold for plants attacked by parasitic fungi. Aside 

 from the nutriment diverted to the parasite, there is reduced func- 

 tional vigor of leaf, stem or root, and the loss becomes increased in 

 this way. Let all the leaves be parasitized, or let even three-fourths 

 of them be entirely so attacked, and we may look for great loss of 

 foliage, possibly entire loss of fruit and the detailed effects of dimin- 

 ished vigor, unripened wood, or by repetition, entire destruction of 

 the host. Usually the effects are of many gradations, but in all cases 

 of leaf parasites the entire plant must suffer. We have learned that 

 bacteria may, in a suitable medium, destroy themselves by the form- 

 ation or emission of poisonous products which are fatal alike to the 

 bacteria and to animals, or even man; that such takes place within 

 plants parasitized by fungi remains in doubt, and may be disre- 

 garded for the present. The results of impaired function in the 

 parts are serious enough to demand our attention. It is altogether 

 probable that future investigations will modify our views upon some 

 points. 



There are many curious transformations and malformations re- 

 sulting from the attacks of parasitic fungi, simply by the multiplica- 

 tion of cells of wound cork or other tissues in the effort of the host to 

 shut off the fungus, not because the fungus consists of such a mass 

 of tissues. (See leaf-curl of peach.) 



While exceedingly interesting to trace the effects of the white 

 mold on shepherds purse and on the garden purslane, as well as the 

 effects of bramble rust, cabbage club-root and a number of others, 

 the principle above pointed out will be found generally applicable, 

 and it is to the reactions of the host plant that the excrescences or 

 malformations are chiefly attributable. It may further be stated 

 that artificial cultures of parasitic fungi, either upon culture media 

 or living plants are constantly adding to our knowledge in these lines. 



BENEFICIAL ORGANISMS : ROOT NODULES, ETC. 



While realizing the losses caused by parasitic fungi and bacteria 

 we may not hastily condemn all fungi and bacteria. One of the 

 most profound influences of aging culture of the soil is the beneficial 

 effects in nitrogen fixing, exerted by the root nodule bacteria of legu- 

 minous plants. The well known beneficial effects of the root nodule 

 bacterium upon clover has made rotation in clover an agricultural 

 necessity. The species or forms of root nodule bacteria required on 

 alfalfa, cowpeas, vetches, etc., have become recognized as factors of 

 consequence in our efforts at seeding and new species of legumes 

 on the farm. 



A less understood relation between certain fungi which develop 

 as mycorrhiza upon the roots of some deciduous trees and notably on 

 conifers may not be passed. Herein we may find an explanation of 

 rotation in forest species when reforestation crops arc to be grown. 



