EEPOET OF THE BOTANIST. - 77 



one alone. It not nnfrequently happens, also, that the symptoms of 

 a disease caused by insects are very similar to those induced by ruii<;i, 

 making- it a difficult matter to trace the malady to its real source. Tlic 

 closest microscopical examination is sometimes necessary to determim.- 

 wliich are the aggressors. AVhen insects and fungi are both present in 

 a particular disease, as they not infrequently are, who is to say \vliii.-h 

 ;ire the most active agents in the case ? The closest study of the ento- 

 mologist and the mycologist may result only a conflict of opinions. 



Unquestionably insects, by wounding the'^bark or leaves, or by other- 

 wise affecting the vitality of the tree or plant, pave the way for the at- 

 tacks of fungi, whose growth and activity continue the injury already 

 begun. A debilitated condition of the plant, arising from the lack of 

 I^roper or insufficient nourishment, an excessive vegetative growth, sud- 

 den changes of temperature, or an undue amount of moistiue in the soil 

 or air, and even certain electrical conditions of the atmosi)here, may, 

 singly or together, invite the growth of these minute parasites, which no 

 one doubts are the ultimate cause of the injury occasioned. It is now 

 weU known that many plant diseases are directly due to the attacks of 

 these parasites, for certain plants are affected with peculiar fungous 

 forms, whatever may be their physical condition or surroundings. If 

 these diseases be whoUy due to atmospheric causes, why were not the 

 vineyards of Europe " struck" with mildew before the iutrodftction of 

 the Peronospora of the vine iuto that country ? Or why has the black 

 rot of the grapes, so destructive here, never been observed across the 

 Atlantic ! 



It is true that many diseases can be avoided by keeping the plants in' 

 full vigor of growth : as a man may escape a contagious disease by care- 

 fully attending to his physical condition, acquiring a degree of vitality 

 sufficient to resist infection. The plant diseases in question are chiefly 

 infectious, and the laws that ajjply to animals in such cases, are, in the 

 main, applicable to plants. 



When we have learned that a disease, in its aggravated form at least, 

 is due to a species of fungus, it must not be implied that we are pre- 

 pared to advise remedies or preventives. To determine these, one 

 must learn the life history of the parasite, its method of nutrition, 

 growth and propagation, and the varied forms or conditions under which 

 it perfects its spores or fruit; the manner of distribution, exactly how 

 it comes ujjon or enters the affected plant, and its means of continuing 

 its existence from year to year. These are all points of the greatest 

 economic and practical importance in this connection, and yet they are 

 the very points about which the mycologist knows least. The reason 

 for this is that the vegetative portion of the fungus, the mycelium, af- 

 fords few points for classification, so the systematic student has confined 

 himself chiefly to the fi'uitiag forms which present the more positive 

 characters. When the fungus begins to show its fruit, however, the 

 disease has passed beyond control, the mischief is done, for in mo>t 

 cases the mycelium has been long at work absorbing unto itself the pi e- 

 pared juices of the host plant. 



Thanks to the patient industry of a few scientific workers, chiefly 

 private students, the bfe history of a number of injurious fungi has 

 beeu fully traced. This is the case with the grape mildew {Pcronoapora 

 riticola) and the rose mildew {Spkacrotheca pannosa), so that, these are no 

 longer the dreaded foes they once were. Efficient remedies for keeping 

 these diseases in check are now generally known. 



Every fungus is produced from a spore (seed), and wherever a fun- 

 gus is found it is as certain that its development was preceded by a 



