94 SKETCHES OF BIRD LIFE. 



condition of the sap which fosters the congregating 

 caterpillars, so that they appear upon the trees in 

 swarms, secure during the day in their silken tents, 

 but eating voraciously at night, till not a particle of 

 green is left ; so that if the tree does not perish, it 

 remains actionless for the year. Nor is the mischief 

 confined to the leaves ; for without their action the 

 sap does not change into a healthy cambium fit for 

 producing the new layers of wood and bark, which 

 when a tree ceases to produce, its decay has begun. 

 The whole active surface of the tree thus becomes 

 saccharine, and is infested with swarms of insects 

 of different kinds down to the very roots ; and in 

 the course of the summer the bole and branches 

 become full of canker ; half the top dies, branch 

 after branch, and the miserable remnant of the once 

 fruitful and ornamental tree becomes equally unpro- 

 ductive and unsightly. When a tree is thus attacked, 

 its recovery is always doubtful and generally hopeless. 

 Wood and bark that have stood more than one 

 winter are much less fit for vegetable action, even 

 though they have received no direct injury, than 

 when they have stood only one. But in a tree 

 which has been injured as above described the 



