THE DISEASES OF PLANTS. SOI 



CONVERSATION XXVII. 



THE DISEASES OF PLANTS CONTINUED. 



Mrs. B. Plants suffer much from their leaves being 

 devoured, either by quadrupeds or insects. The former 

 not only wound the branches in obtaining the leaves, 

 but, if the soil be of a loose nature, they disturb the 

 young roots ; hence pasturage is esteemed injurious in 

 loose and wet soils. But the insect tribe is a far more 

 insidious and fatal enemy. Insects not only perforate 

 the plant, in order to deposit their eggs, but, when these 

 eggs are hatched, the larvae or grubs prey upon the plants 

 which have afforded them shelter, devouring their leaves, 

 and often rotting the wood by their acrid juices. Most 

 of these insects bear the name of Cynips: that which 

 produces the swollen excrescence called gall-nuts, from 

 which ink is made, is one of the most remarkable. The 

 smoke of tobacco and washes made of infusions of that 

 plant are the best preservatives against these minute but 

 inveterate enemies. The insect called Cochineal fastens 

 itself to the bark of trees, and sucks the juice through it. 

 The black spots on orange-trees are insects of this class: 

 they are generally pernicious in green-houses, and should 

 be brushed off. 



Caroline. I thought that cochineal was of a bright red 

 color, and that the insect was peculiar to hot climates. 



Mrs. B. The species you refer to comes from Mexico, 

 and feeds on the Cactus Qpuntia, from which it derives 

 the name of cochineal ; but there are many other species 

 of this insect, which are not confined to tropical climates. 



The fifth class of diseases results from the action of 

 plants on each other. Plants being destined by Nature 

 to produce a much greater quantity of seed than they can 

 possibly bring to maturity, we may consider them as con- 

 stantly struggling with their neighbors to obtain nourish- 

 ment for their numerous offspring : thus they impoverish 

 each other, and check that vigor of vegetation, which 

 would take place, had every plant sufficient space and 

 food not to interfere with the wants of its neighbors. 



1621. How do insects injure plants'? 1622. What are these in- 

 sects called, and what one is the most remarkable 1 ? 1623. What is 

 the best preservative against them'? 1624. Of the Cochineal what iff 

 said 1 ? 1625. From what does the fifth class of diseases result! 

 1626. How is this explained! 



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