PHYSIOLOGY OF THE LEAF 61 



These two are the only plants known which have cancellated 

 leaves. 



Some of the uses of Leaves. 



Leaves perform a very important otfice, in sheltering and protect- 

 ing- the flowers and fruit; the fact of their inhaling or absorbing air, 

 is thought to have been proved, by placing a plant under an exhaust- 

 ed receiver, permitting the leaves 07ihj 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 w^as 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, that 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 wax-work. Mushrooms 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 die 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 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. 



One of the most remarkable phenomena of leaves, is their inita- 

 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 nictitans ;) these plants, when the hand is 

 brought near them, seem agitated as if with fear ; but as plants are 

 destitute of inteUigence, 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 

 Deen 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 off into 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 aUhough the respiration of 

 leaves seems not to be doubted, this experiment may not be thought a fair one; for it 

 would seem very diificult, to place a plant under a receiver, witii tne 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 



