PUYSIOLOGY OF THE LEAF, 
throiTgli tlie stomata ; these are only open in the presence 
of* light, therefore in the dark this process cannot proceed. 
Absorjotion is the peculiar office of the root, but leaves also 
perforin this function, as water sprinkled on the leaves of a 
parched plant is well known to revive it. Hesjpiration is per- 
formed chiefly by the leaves, but sometimes by the root ; it con- 
sists in the absorption of oxygen and giving off carbonic acid. 
Digestion consists in the decomposition of carbonic acid, the 
fixation in the plant of solid carboji, and the evolution of 
oxygen. 
The upper surface of leaves is usually of a deeper green, and performs a more 
impoi tant part in respiration tlian the under surface. The upper surface also repels 
moisture : a cabbage-leaf after a shower or heavy dew, shows the moisture collect- 
ed in drops, but with no appearance of its being absorbed by the leaf. It has been 
found that the leaves of plants laid with their upper surfaces upon water, soon 
wither ; although the leaves of the same plants, placed with their under surfticea 
flpon water, retain their freshness for some days. Few among the vegetable tribes 
tire destitute either of leaves, or green stems, which answer as a substitute. The 
Monotropa, or Indian pipe, is of pure white, resembling wax-work. Mushrooms 
are also destitute of any green herbage. It is not known in what manner the de- 
ficiency of leaves is made up to these vegetables. 
62. The period in which any species of plant unfolds its leaves, was called by 
LinntEus, Frondescence ; according to his observation in the climate of Sweden, the 
opening of the leaf-buds of the birch-tree is the proper time for the sowing of 
barley. The American Indians held that the best time for planting Indian corn 
was when the leaves of the white-oak first made their appearance ; or, according 
to their expression, " are of the size of a squirrel's ears." 
a. One of the phenomena of leaves is their irritability, or power of contraction, 
upon coming in contact with other substances. Compound leaves often possess this 
property ; as the sensitive plant (mimosa sensitiva), and the American sensitive plant 
(cassia nictitans) ; on bringing the hand near them they shrink as if from the touch 
the warmth of tlie hand is supposed to produce the contractions of the leaves. 
b. The effect of light upon leaves is very apparent, plants being almost uniformly 
found to present their upper surfaces to the side on which the greatest quantity of 
light is to be found. 
c. Carbonic acid gas is the food of plants ; this consists of carbon and oxygen, 
and is decomposed by the agency of light ; the carbon becomes incorporated with 
the vegetable, forming the basis of its substance, while the oxygen is exhaled, or 
thrown off into the atmosphere. 
d. Many plants close their leaves at a certain period of the day, and open them at 
another ; almost every garden contains some plants in which this phenomenon may 
be observed ; it is particularly remarkable in the sensitive plant and tlie 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 
vegetable 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 appearance 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 petioles, or closed their corollas, thus exhibiting a 
• Sir J. E. Smith. 
Of the upper and under surface of leaves. — 62. Fronilescence — a. Irritability — J. Effect of li^ht— 
t What effect has light u])ou the carbonic acid gas imbibed by plants ? — d. Sleep of plants 
