82 PHYSIOLOGY OF THE LEAF. 



system and cellular texture from injury, and is the medium, by 

 which the leaf performs the important functions of absorbing 

 nourishment, and throwing off such substances as are useless 

 or hurtful. The cuticle is sometimes covered with downy or 

 hairy glands, which seem to afford security against changes of 

 weather ; such plants are capable of enduring a greater degree 

 of heat than others. In some cases the cuticle is covered with 

 a transparent varnish, which preserves the plant from injury 

 by too much moisture, and adds to the beauty of the leaves. 

 The trees of Abyssinia and some other countries, which are sub- 

 ject to long rains, and continued moisture, are thus shielded 

 from the injurious effects of the weather. 



Some of the uses of Leaves. 



Leaves perform a very important office, in sheltering and 

 protecting the flowers and fruit ; the fact of their inhaling or 

 absorbing air, is thought to have been proved, by placing a plant 

 under an exhausted receiver, permitting the leaves only to re- 

 ceive 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 withered and died.* 



The upper surface of leaves is usually of a deeper green, and 

 supposed to perform a more important part in respiration, than 

 the under surface. The upper surface repels moisture ; you 

 may perceive upon a cabbage leaf after a shower, or heavy 

 dew, that the moisture is collected in drops, but has no appear- 

 ance of being absorbed by the leaf. It has been found that the 

 leaves of plants, laid with their upper surface upon the water, 

 wither almost as soon as if exposed to the air ; although the 

 leaves of the same plants, placed with their under surfaces upon 

 the 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 Indian pipe, 

 is of 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. Linnaeus paid much attention to this 



skin ; the true skin being not the outer covering, but a cellular substance be- 

 neath; thus the thin skin upon the hand which so easily becomes rough, is the 

 cuticle, or epidermis (sometimes called the scarf-skin), while the real skin is 

 below. 



* I give this experiment on the authority of Barton; but although the respi- 

 ration 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. 



Important office of the leaf Few plants destitute of leaves Frondescence. 



