PHYSIOLOGY OF THE LEAF. 
75 
the leaves of the same plant, placed with their under surfaces 
upon the water, retain their freshness for some days. But few 
among the vegetable tribes are destitute of leaves, or green stems, 
which answer as a substitute. The Monotropa, or Indian pipe, 
is of a pure white, as if made of wax ; the mushrooms are also 
destitute of any green herbage. It is not known in what man- 
ner the deficiency of leaves is made up to these vegetables. 
The period in which any species of plant unfolds its leaves, is 
termed its Frondescence. Linnseus 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, (Betula,) 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 when the leaves of the White Oak, (Quercus alba,) 
first make 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 great- 
est degree ; as the sensitive plant mimosa sensitiva : and the 
American sensitive plant, cassia nictitans : these plants on hold- 
ing the hand near them, exhibit agitation, yet as they are desti- 
tute of intelligence, we must attribute this phenomenon to a 
physical cause ; this cause is probably the warmth of the hand, 
which in some manner not understood by us, produces the con- 
tractions, and dilatations of the leaves. 
The effect of light upon leaves is very apparent, as plants are 
almost 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 purpose they require the agency of light. Oxygen exists in 
plants united with carbon, forming carbonic acid gas ; by the 
agency of light, the oxygen is separated from the carbon, the 
latter is incorporated into the substance of the vegetable, while 
the former is exhaled by the leaves. 
Many plants close their leaves at a certain period of the day, 
and open them at another regular period ; almost every gar- 
den contains some plants, in which this phenomenon may 
be observed ; it is particularly remarkable in the sensitive plant, 
and the tamarind tree. This folding up of the leaves at partic- 
ular periods, has been termed the sleep of plants ; it may seem a 
singular term to apply to plants ; but a celebrated botanist* re- 
marks, “ this folding up of the leaves may be as useful to the 
* Sir J. E. Smith. 
Few plants destitute of leaves — Frondescence — Irritability — Effect of 
light — Sleep of plants. 
