INSECTS AND PREVENTIVES 



these attacks. Among these fungi are the downy 

 mildews, like those on spinach and lettuce, and 

 the powdery mildews, such as attack cucumbers. 



They multiply through the agency of spores, as 

 plants by seeds. Some of these spores develop 

 with great activity in a summer temperature, 

 natural or artificial, when other conditions are 

 favourable, but are readily destroyed when exposed 

 to severe cold, long-continued drought, etc., or to 

 certain artificially created conditions. Others, 

 however (called resting spores), more sluggish in 

 habit but of more endurance, are found to remain 

 unaffected, preserving the species; so that their 

 complete extinction in any locality is not to be 

 expected, in the present state of our knowledge. 



As a rule, when a vegetable parasite fastens 

 upon a plant, it is next to impossible to kill the 

 parasite without destroying the plant by the same 

 operation. It is obvious, therefore, that no treat- 

 ment can be judicious which is not mainly pre- 

 ventive. 



Healthful condition is, as already urged, an 

 important means of prevention. Another, which 

 has been often recommended, but far too little 

 attended to, consists in the removal and destruc- 



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