INSECTS AND DISEASES AFFECTING PLANTS. 275 



aphis, or of sulphur, which is death to the spider, fall 

 harmlessly on this microscopic insect. 



There is hardly a doubt but that the fumes of sulphur 

 or tobacco would destroy it, if it had not the power of 

 imbedding itself in the leaf. This is evidently the case, 

 as on subjecting affected plauts to a severe fumigation 

 with tobacco for thirty minutes no insects could be dis- 

 cerned on the leaves ; but after a short time they again 

 appeared on the field of the microscope, apparently un- 

 scathed. We also find that an excellent preventive 

 against this insect is to syringe the plants twice a week 

 with a weak solution of fir-tree oil ; one-half pint to 

 five gallons of water. This seems like tobacco smoke to 

 check it somewhat, yet it is not a com- 

 plete remedy and if plants are severely at- 

 tacked, there is nothing for it but to throw 

 those affected out as there is but little 

 doubt that it quickly spreads. Now, al- 

 though we have no direct remedy against 

 this insect, which produces the black rust, 

 we have, I think, a preventive, by keeping 

 the plants in that healthy condition which seems to be 

 repellant to its attack. For the means used to get that 

 healthy condition, see chapter on the culture of the Ver-. 

 bena, which is, with slight modifications, equally ap- 

 plicable to all other plants affected by this insect. 



The microscope reveals that this particular species, 

 which is so destructive to our Verbena, Heliotrope, Petu- 

 nia, and scores of other plants cultivated in the green- 

 house or garden, is the same or one that closely resembles 

 that which gives the roughness to particular parts of the 

 cherry, plum, and peach trees, and no doubt is to 

 be found on thousands of other kinds of plants, when- 

 ever a lessened vitality takes place. Corroborative of 

 this view, I had a lot of about 500 plauts of Heliotrope 

 growing in two-inch pots in one of our greenhouses one 



