CH. XL] LEAVES, AND LEAF^DISEASES. 245 



Now there are very many foes which injure the 

 leaves of our timber trees, and I wish to show, as 

 clearly as possible in a short chapter, how it comes 

 about that injury to the leaves means injury to the 

 timber. The sum total of the matter is that the 

 substances which are to be sent down to the cambium, 

 and converted through its agency into wood, are 

 produced in the cells of the leaves: consequently, 

 from our point of view, when an insect or a fungus 

 consumes the substance of the leaves, it consumes 

 timber in prospective. Similarly, when the leaves are 

 removed from a tree by any agent whatever, the latter 

 is robbed in advance of timber. A leaf, generally 

 speaking, is an extended, flattened appendage of a 

 branch, covered by a continuation of the epidermis 

 of the branch, and containing a continuation of its 

 other tissues — the vascular bundles of the branch 

 being continued as the venation, and the cellular 

 cortex reappearing as the green soft tissue of the leaf. 

 The epidermis of the leaf is so pieced at hundreds of 

 thousands of nearly equi-distant points, that gases 

 can enter into or escape from all its tissues : at these 

 points are the so-called stomatay each stoma being a 

 little apparatus which can open and close according 

 to circumstances. 



These openings lead into excavations or passages 



