Dr Fleming's Remarks on the Defoliation of Trees. 75 



exercise fewer functions, than the same organ in trees with de- 

 ciduous leaves ? 



When growing too much in the shade, or when subject to 

 the influence of the shears, annual leaves may have the period 

 of their life prolonged, so as to exercise their functions after 

 the new shoots have evolved their leaves. 



In trees of the third class, the leaf may be termed "Jblium 

 perenne^ Its duration is not influenced, directly, by the per- 

 fection of the bud, nor the new supply of leaves during the 

 following season. The leaves of two or more seasons, exer- 

 cise, in trees of this class, their functions at the same time, and 

 appear to be requisite for the prosperity of the stem. Our 

 ordinary evergreen firs furnish very obvious examples of this 

 persistent leafing. In trees of this description, the leaves seem 

 to exercise functions even of a higher order, and continue to 

 exercise these longer than in those of the two preceding groups. 

 The tree here requires the leaves, not for a few months, or 

 until new leaves be produced, but leaves of different ages, — 

 two, three, or more years old. 



In the trees of this class there is less regularity in the fall- 

 ing of the leaf, as to season, than in the two preceding ones. 

 Few of the old leaves drop off, when the tree produces the 

 shoots, and new leaves in spring. A greater number seem to 

 perish about midsummer, and again on the approach of win- 

 ter. 



The succession in the fall of the leaves is, as in the other 

 classes, in the order of their seniority on the same stem and 

 branch. But sometimes only a portion (the first formed ones) 

 of the leaves of one season drop off, the remaining portion 

 continuing to exercise their functions during a longer period. 

 On the same tree may even be observed leaves of three, four, 

 five, six, or even seven years of age ; and while a part of those 

 three years old may be changing colour and dropping off", 

 those seven years old may remain green and fresh. Those 

 leaves which are placed in the shade live longest ; yet even in 

 this respect I have witnessed many anomalies. 



These remarks apply to the duration of the leaves of the 

 trees of this country. The influence of climate on the dura- 

 tion of leaves has often been stated as considerable, and 



