FALL OF LEAF. 79 



capabiKty of plants to bear the action of direct light varies 

 according to their specific nature. One species is organized to 

 suit the atmosphere of a dense wood, into which diffuse light 

 only will penetrate ; another is planted by nature on the 

 exposed face of a sunburnt rock, upon which the rays of a 

 shadeless sun are daily striking : in these cases, the light 

 which is necessary to the one would be destructive of the other. 

 The organic difference of such species seems to consist chiefly 

 in the epidermis, which regulates the amount of perspiration. 

 It is therefore to be remarked, that it is not the greatest 

 quantity of light which can be obtained that is most favourable 

 to the healthiness of plants, but the greatest quantity they wiU 

 bear without injury. If the former were true, the concentrated 

 light of a lens would be better than the strongest ordinary 

 light ; but the effect of the concentrated light of a lens is to 

 burn the surface, and the ordinary solar rays produce the same 

 effect upon many plants, probably by exhausting the tissue of 

 its water faster than it can be supplied from the roots. 



In the course of time, a leaf becomes incapable of performing 

 its functions ; its passages and surface are choked up by the 

 deposit of impurities ; there is no longer a free communication 

 between its parenchyma and that of the rind, or between its 

 veins and the wood and liber ; or the air and its interior. It 

 changes colour, ceases to decompose carbonic acid, absorbs 

 oxygen instead, gets into a morbid condition, and dies : it is 

 then thrown off. This phenomenon, which we call the fall of 

 the leaf, is going on the whole year round, except mid-winter, 

 in some plant or other. Those which lose the whole 

 of their leaves at the approach of winter, and are called 

 deciduous, begin, La fact, to cast their leaves within a 

 few weeks after the commencement of their vernal growth: 

 but the mass of their foliage is not rejected till late in 

 the season. Those, on the other hand, which are named ever- 

 greens, part with their leaves much more slowly ; retain them 

 in health at the time when the leaves of other plants are 

 perishing ; and do not cast them till a new spring has com- 

 menced, when other trees are leafing, or even later. In the 

 latter class, the functions of the leaves are going on during aU 



