PHYSIOLOGY OF THE LEAP. 
55 
rtiriety of interesting phenomena. This state of relaxation and i epose seems tv: 
depend on the absence of hght : with the first rays of the morning 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 whicli seems capable of producing 
its effect on plants ; this has been proved by the following experiment. A botanis'^ 
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 vital action of the parts 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 off. The development of buds, the hardening of the 
bark, and the formation of wood, accelerate the fall of the 
leaf. Heat, drought, frosts, Tvind, 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 solid 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 oft'. 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 begm 
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 
the 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. 
6. 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 this 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 which formed 
them. 
64. Leaf-like Appendages to Plants. — ^The uses of these 
63. Defoliation — a. Change of color of leaves in autumn — American forests — b. Reflection. — 64 Ap" 
r«nda^es. 
