XL] LEAVES, AND LEAF-DISEASES. 251 



correlations implied by this competition of the various 

 organs and tissues for the food supplies from the 

 leaves ; but probably the following proposition will be 

 generally clear : If the leaves are stripped, the 

 cambium suffers starvation to a greater or less extent, 

 depending on the intensity of its competition with 

 other tissues, &c. ; of course a starved cambium will 

 form less wood, and, it may be added, the timber will 

 be poorer. 



Again, even if the leaves are not stripped quickly 

 from the tree, but the effect of some external agent is 

 to shorten their period of activity ; or to occupy 

 space, on or in them, and so diminish the amount of 

 leaf-surface exposed to the light and air ; or to block 

 up their stomata, the points of egress and ingress for 

 gases and water ; or to steal the contents of the cells 

 contents which should normally be passed on for 

 the growth, &c., of other parts of the tree in all or 

 any of these ways injury to the timber may accrue 

 from the action of the agent in question. Now there 

 are numbers of parasitic fungi which do all these 

 things, and when they obtain a hold on pure planta- 

 tions or forests, they may do immense injury before 

 their presence is detected by any one not familiar with 

 their appearance and life-histories. 



The great difficulty to the practical forester who 



