PHYSIOLOGY OF THE LEAP 
51 
These two are the only plants known which have cancellated 
leaves. 
Some of the uses of Leaves. 
Leaves perform a very important office, in sheltering and protect- 
ing the flowers and fruit; the fact of their inhahng or absorbing air, 
is thought to have been proved, by placing a plant under an exhaust- 
ed receiver, permitting the leaves only to receive the influence of air ; 
the plant remained thrifty in this situation for a length of time ; but 
as soon as the whole plant was placed under the receiver, it wither- 
ed and died,"^ 
The upper surface of leaves is usually of a deeper green, and sup- 
posed to perform a more important part in respiration, than the un- 
der surface. The upper surface also repels moisture ; you may per- 
ceive upon a cabbage-leaf after a shower, or heavy dew, tljat the 
moisture is collected in drops, but has no appearance of being ab- 
sorbed by the leaf It has been found that the leaves of plants, laid 
with their surface upon water, wither almost as soon as if exposed 
to the air ; although the leaves of the same plants, placed with their 
under surfaces upon water, retain their freshness for some days. But 
few among the vegetable tribes are destitute either of leaves, or 
green stems, which answer as a substitute. The Monotropa, or In- 
dian pipe, is of pure white, resembling v/ax-work. Mushr-ooms are 
also destitute of any green herbage. It is not known in what manner 
the deficiency of leaves is made up to these vegetables. 
The period in which any species of plant unfolds its leaves, is 
termed Frondescence. Linnaeus paid much attention to this subject ; 
he stated, as the result of his investigations, that the opening of the 
leaf-buds of the Birch-tree, was the most proper time for the sowing 
of barley. The Indians of our country had an opinion, that the best 
time for planting Indian corn was v/hen the leaves of the White-Oak 
first made their appearance ; or according to their expression, are 
of the size of a squirrel's ears. 
One of the most remarkable phenomena of leaves, is their irrita- 
bility^ or power of contraction upon coming in contact with other 
substances. Compound leaves possess this property in the greatest 
degree ; as the sensitive plant, (mimosa sensitiva,) and the American 
sensitive plant, (cassia nictrtuns ;) these plants, when the hand is 
brought near them, seem agitated as if with fear ; but as plants are 
destitute of intelligence, we must attribute this phenomenon to some 
physical cause ; perhaps the warmth of the hand, which produces the 
contractions and dilatations of the leaves. 
The effect of light upon leaves is very apparent, plants being al- 
most uniformly found to present their upper surfaces to the side on 
which the greatest quantity of light is to be found. It has already 
been observed, that plants throw off oxygen gas ; but for this pur- 
pose they require the agency of light. 
Carbonic acid gas is the food of plants ; this consists of carbon and 
oxygen, and is decomposed by the agency of light; the carbon be- 
comes incorporated with the vegetable, forming the basis of its sub- 
stance, while the oxygen is exhaled, or thrown oJffinto the atmosphere. 
Many plants close their leaves at a certain period of the day, and 
* I give this experiment on the authority of Barton; but although the respiration of 
leaves seems not to be doubted, this experiment may not be thought a fair one ; for it 
would seem very difficult, to place a plant under a receiver, with the leaves exposed to 
the air, without, at the same time, admitting any air into the receiver. 
Few plants are destitute of leaves— Frondescence— Irritability— Effect of light— 
What effect has light upon the carbonic acid gas imbibed by plants 1 
6 
