Ch. X. | LEAVES.— APPENDAGES. 5% 
to the side on which the greatest quantity of light 1s to be 
found. It has already been observed, that plants throw off oxy- 
gen gas; but for this purpose they require the agency of light. 
233. Carbonic acid gas is a necessary food of plants; this 
consists of carbon and oxygen, and is decomposed by the agency 
of light; the carbon becomes ineorporated with the vegetable, 
forming the basis of its substance, while the oxygen is exhaled 
or thrown off into the atmosphere. 
234. 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 the tamarind 
tree. The folding up of leaves at particular periods, has been 
termed the sleep of plants; this may seem a singular term to 
apply to plants ; but a celebrated botanist remarks, “this folding 
up of the leaves may be as useful to the vegetable constitution, 
as real sleep is to the animal.” 
235. Linnzus was led to observe the appearance of plants in 
the night, from the following 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 at- 
tention, and he began to watch the plants through the night, in 
order to observe the period of their unfolding. He was thus 
led to investigate the appearance of other plants in the night, 
and to observe their different manner of sleep.—He found that 
some folded their leaves together, some threw them back upon 
their stems, or exhibited other curious appearances.—This phe- 
nomenon has been attributed to the absence of light. 
236. The following experiment was once made by a botanist; 
he placed the sensitive plant in a dark cave at midnight, and 
then lighted up the cave with lamps; the leaves which were be- 
fore folded up suddenly expanded, and when on the following 
day the lights were extinguished, the leaves again closed. 
237. The period at which the leaves fall off is termed the 
Defoliation* of the plant. About the middle of Autumn, the 
leaves of all annual, and of many perennial plants, begin to 
_ lose their vigour, change their colour, and at length fall from 
their stems. 
* From. de, signifying to deprive of, and folium, leaf. 
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233. What is a necessary food of plants ? 
234. What is meant by the sleep of plants? 
235. How was Linnzeus led to observe the appearance of plants in 
the night 2 
236. What experiment was once made with the sensitive plant? 
237. What is the deiotiation of plants ? 
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