PHYSIOLOGY OF THE LEAF. 55 



variety of interesting phenomena. This state of relaxation and I Bpose seems lo 

 depend on the absence of liglit : with the first rays of the mornin<^ sun, the leaves 

 recommence their chemical labors by drawing in oxygen, the fibers of the roots 

 begin to imbibe sustenance from the earth, and the whole vegetable machinery is 

 again set in motion. It is not solar light alone wliich seems capable of producing 

 its elfeot on plants ; this has been proved by the following experiment, A botanist 

 placed the sensitive plant in a dark cave, and at midnight lighted it up with lamps ; 

 the leaves, which were folded up, suddenly expanded ; and when, at mid-day, the 

 lights were extinguished, they again as suddenly closed. 



63. The period of the falling of the leaf is termed the defo- 

 liation of the plant ; this may be referred to the death of the 

 leaf, and the mtal action of the j^ct'^'is to which it is attached. 

 If a tree be killed by lightning, the leaves will adhere to 

 the dead branches because the latter have not the energy to 

 cast them ofl'. The development of buds, the hardening of the 

 bark, and the formation of vt^ood, accelerate the fall of the 

 leaf. Heat, drought, frosts, wind, and storms, are all agents 

 in their destruction. The decay of the leaf is supposed to be 

 owing to the consolidation of the parenchyma, by the accumu- 

 lation of sqlid matter conveyed to it by the sap, and left by 

 evaporation. The fall of the leaf has been thus explained : — 

 When the leaf and stem are both in a healthy state, the base 

 of the former and the branch that bears it, both increase at the 

 same rate, but after the decay of the leaf, its base can no 

 longer adapt itself to the growing branch, which is constantly 

 increasing in diameter by the formation of new wood. There 

 is, consequently, a rupture of the connecting vessels, the 

 leaf is disjointed at the base, or articulation, and falls off, as a 

 dead part of a living animal is cast off. Endogenous stems do 

 not- increase in diameter, therefore their leaves decay, but do 

 not commonly fall ; they remain on the stem. 



a. About the middle of autumn, the leaves of the sumach and grape -vme begin 

 to look red, those of the walnut, brown, those of the honeysuckle, blue, and those 

 of the poplar, yellow ; but all sooner or later take that uniform and sad hue, called 

 tlie dead-leaf color. The rich autumnal scenery of American forests is regarded by 

 the European traveler with astonishment and delight, as far exceeding any thing 

 of the kind which the old world presents. Painters, who have attempted to imi- 

 tate the splendid hues of our forests, have, by foreigners, been accused of exagger- 

 ation ; but no gorgeous coloring of art can exceed the bright scarlet, the deep crim- 

 son, the rich yellow, and the dark brown, which these scenes present. 



h. The student who has learned something of the anatomy and physiology of 

 leaves, will be induced to pay attention to them in their different stages, from 

 their situation in the bud to their full growth and perfection, — will feel a new inter- 

 est in their change of color, when the philosophy of tliis change is understood ; — 

 even the dry skeletons of leaves, which the blasts of autunm strew around us, may 

 not only afford a direct moral lesson, as emblematical of our own mortality, but, in 

 examining their structure, we are led to admire and adore the Power whicK formed 

 them. 



64. Leaf-like Appendages to Plants. — The ives of those 



63. Defoliation— o. Change of color of leave* in autumn— American forests—*. Reflection.— C4. Ap- 

 pendages. 



