62 
PHYSIOLOGY OF THE LEAR 
open them at another ; almost ever}^ garden contains some plants in 
which this phenomenon may be observed ; it is particularly remark- 
able in. the sensitive plant, and the tamarind-tree. This folding up 
of the leaves at particular periods, has been termed the sleep of plants ; 
a celebrated botanist,* remarks, " this may be as useful to the vege^i 
table constitution, as real sleep 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 on looking for them at night, they, 
were no longer visible. This excited his attention, and he began 
to watch their unfolding. He was thus led to investigate the appear- 
ance of other plants at the same time, and to observe their different 
manner of sleeping. He found, as darkness approached, that some 
folded their leaves together, others threw them back upon their pe- 
tioles, or closed their corollas, thus exhibiting a variety of interest- 
ing phenomena. This state of relaxation and repose seems to depend 
on the absence of light; with the first rays of the morning sun, the 
leaves recommence their chemical labours by drawing in oxygen, 
the fibres of the roots begin to imbibe sustenance from the earth, and 
the whole vegetable machinery is again set in motion. It is not so- 
lar light alone which seems capable of producing its effect 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 midday, the Ughts were extinguished, they again as sud- 
denly closed. 
Falling of the Leaf. 
The period at which leaves fall is termed the Defociation] of the 
plant. The " fall 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 at- 
tached. If a whole tree be killed by hghtning, or any sudden cause, 
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, wind, and storms, are all agents in 
their destruction. 
About the middle of autumn, the leaves of the Sumac and Grape- 
vine 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 the dead-leaf colour. 
The rich autumnal scenery of American forests is regarded by the 
European traveller with astonishment and delight, as far exceeding 
any thing of the kind which the old world presents. Painters, who 
have attempted to imitate the splendid hues of our forests, have, by 
foreigners, been accused of exaggeration ; but no gorgeous colour- 
ing of art can exceed the bright scarlet, the deep crimson, the rich 
yellow, and the dark brown, which these scenes present. 
After what you have now learned of the anatomy and physiology 
of leaves, you will probably be induced to pay attention to them in 
their different stages ; from their situation in the bud, to their full 
grovv'th and perfection ; you will feel a new interest in their change 
of colour, now that you understand something of the philosophy of 
this change ; — even the dry skeletons of leaves, which the blasts of 
autumn strew around you, may not only afford a direct moral lesson, 
* Sir J. E. Smith. 
t From de, signifying to deprive of, and folium, leaf. 
fSlecp of plants— Cause of the sleep of plants— Defoliation— Fall of the leaf. 
