FALL OF LEAVES. 299 



in their bulk. The bark is elastic, and not only 

 follows the lessening of the stem, but co-operates 

 in bringing about that lessening. This condensa- 

 tion in the wood of the tree, and it extends to all 

 the wood which is in a state of activity, necessa- 

 rily generates heat, for heat is produced in all 

 cases of condensation; and thus, the colds of 

 early winter, which would destroy leaves, unless 

 those leaves had the glossy epidermis of ever- 

 greens, has no injurious effect upon the stem. In- 

 deed, the epidermis of an evergreen leaf has more 

 resemblance to that of a twig of the first or second 

 year than to that of a deciduous leaf. The cold 

 acts in it in nearly the same way, and it becomes 

 rigid ; so that there is little or no action of any 

 kind in it during the winter. And, as even the ever- 

 green leaf is but a temporary organ, which accom- 

 plishes one purpose and then decays, the leaf of the 

 evergreen does not revive in the revival of spring. 

 If, after the winter is over, the leaves of a com- 

 mon evergreen those of the laurel, for instance, 

 be tried by a proper test, it will be found that 

 their vitality is gone, although their colour re- 

 mains. The very best test of life in vegetables, 

 as well as of health in animals, is the thermome- 

 ter. The living thing, be it what it may, has 

 always one temperature which is the most favour- 

 able to healthy action, and it struggles to pre- 

 serve that ; but when the principle of life fades, 

 the mere matter obeys the laws of matter ; and 

 thus, when the evergreen leaf has ceased to per- 

 form its functions as a leaf, it has no longer the 

 uniform temperature of the growing plant, but 

 gets heated in the sun and cooled in the shade, in 

 the same manner as though it were a portion of 

 inorganic matter. 



