DISEASES. 171 



modes of prevention. The best of these is to provide 

 against the evil by reserving the lower branches to shade 

 the stem. There are other excellent reasons for this prac- 

 tice, which will be brought forward in the chapter on 

 Pruning. 



Frequently, however, the nurseryman, or perhaps the 

 injudicious efforts of the planter himself, may have re- 

 moved all the side branches of the young tree, and as 

 these cannot be replaced, we may substitute for them a 

 shelter from the scorching sun to which the newly planted 

 tree is exposed. This may be done by tacking two nar- 

 row boards together at their edges, like a gutter spout, 

 and setting them upright on the south side of the tree to 

 shade it. A wisp of straw, tied loosely to the stem, will 

 answer a very good purpose ; but both of these appli- 

 ances are objectionable, because they furnish a shelter for 

 insects, and thus they fall short of the natural shading 

 of the stem by the foliage of its own branches. 



It is not only the scorching suns of summer that damage 

 our young trees that are thus exposed by injudicious trim- 

 ming. Even the bright rays of a mid-winter sun, falling 

 upon the frozen stem, will often effect the most serious 

 damage, and should be guarded against with equal care ; 

 but here the natural protection wilt answer, for the shade 

 of the naked spray of the laterals is found all-sufficient in 

 the well-trained tree. 



2d To resume the consideration of Lankester's causes 

 of disease, it must be admitted that some diseased condi- 

 tions may be produced by poisonous gases, but the usual 

 result will be the death of plants confined in such an at- 

 mosphere. . The natural power of diffusion of all gases 



