7G 
PHYSIOLOGY OP THE 'LEAF. 
vegetable constitution, as real sleep is to the animal.” Linnaeus 
was led to observe the appearance of plants in the night, from a 
circumstance which occurred in raising the Lotus plant ; he 
found one morning some very thrifty flowers, but at night they 
had disappeared ; this excited his attention, and he began to 
watch the plants through the night, in order to observe the pe- 
riod of their unfolding. He was thus led to investigate the ap- 
pearance of other plants in the night, and to observe their differ- 
ent manner of sleep ; some folding their leaves together, others 
throwing them back upon their stems, and exhibiting a variety 
of appearances. This phenomenon has been attributed to the 
absence of light. A curious experiment has been made by 
another botanist, who placed the sensitive plant in a dark cave, 
at midnight, and then lighted up the cave with lamps; the leaves 
which were before folded up, suddenly expanded, and when on 
the following day the lights were extinguished, the leaves again 
closed. 
The period at which the leaves fall off is termed the Defolia- 
tion* of the plant ; about the middle of autumn, the leaves of all 
annual, and of many perennial plants, gradually lose their vig- 
our, change their colour, and at length fall from their stems. 
The “hill of the leaf” may be referred to two causes ; the 
death of the leaf, and the vital action of the parts to which it is 
attached. If a whole tree is killed by lightning, or any sudden 
cause, the leaves will adhere to the dead branches, the latter not 
having the energy to cast them off. The richness and variety 
of colouring exhibited about the end of autumn, by our groves 
and forests, is splendid beyond the power of the painter to imi- 
tate. Yellow, red, and brown, are the most common colours of 
the dying leaf ; but these colours vary from the brightest scar- 
let, and the deepest crimson, intermixed with every shade of 
yellow, from the deep orange, to the pale straw colour. 
Although we have said considerable upon leaves, yet we have 
merely touched upon the most important circumstances with re- 
spect to them. You will, perhaps, be induced to pay more at- 
tention than formerly to them, in their different stages ; from 
their situation in the bud, to their full growth and perfection; 
and will feel a new interest in their change ofcolour, when youun- 
derstand something of the philosophy of this change; even thedry 
skeletons of leaves, which the blasts of autumn strew around us, 
may not only afford a direct moral lesson, but, inducing you to 
examine their structure, lead you to admire and adore the 
power which formed them. 
* Or depriving of leaves. 
Defoliation— Fall of the leaf— Concluding remark. 
