88 COMPOUND ORGANS OF PLANTS. 



leaves, and we cannot help admiring the interesting and won- 

 derful provision by which nature heals the wounds even before 

 they are absolutely made, and affords a safe covering from 

 atmospheric changes before the parts can be subject to them. 



The decay and fall of leaves is, therefore, not the result of 

 frost, as is commonly supposed, for leaves begin to languish 

 and change color (as happens with the red maple, especially,) 

 and even fall, often before the autumnal frosts make their 

 appearance, and when vegetation is destroyed by frost the 

 leaves blacken and wither but remain attached to the stem ;; 

 but the death and fall of the leaf is produced by a regular 

 vital process, which commences with the first formation of this 

 organ, and is completed only when it is no longer useful. 

 There is no denying, however, that the frosts of autumn, by 

 suddenly contracting the tissues at the base of the petiole, 

 accelerate the fall of leaves. All must have noticed, on a 

 frosty morning in autumn, that the slightest breath of air 

 moving amongst the decayed and dying leaves, will bring them 

 in complete showers from the trees to the ground. 



In general, we may say, that the duration of life in leaves is 

 inversely as the force of the evaporation which takes place 

 from their surface. For we find that the leaves of herbaceous 

 plants, or of trees which evaporate a great deal, fall before the 

 end of the year, whilst the leaves of succulent plants, or of 

 evergreens, which latter are of a hard and leathery texture, and 

 evaporate but little, often last for several years. In pines, 

 firs, and evergreen trees and shrubs, there is an annual fall of 

 leaves in the spring of the year whilst the growth of the 

 season is taking place ; but as this leaf-fall is only partial, con- 

 sisting of one-half or one-third at a time, there is always a 

 sufficient number left on such trees to keep them clothed with 



