112 PHYSIOLOGY OF VEGETABLES. 



oil, on which depends the aroma or peculiar perfume of 

 flowers, are made from it. By far the greater portion 

 of sap is carried into the leaves, where it is subjected 

 to important changes. In these organs the sap is ex- 

 posed to the action of light and air, two powerful 

 agents by which it is enabled to form various secre- 

 tions, which give peculiar flavours and qualities to the 

 leaves themselves, at the same time that much super- 

 fluous matter passes off by perspiration. 



The sap, thus modified in the leaves, is returned by 

 another set of vessels into the new layer of bark. 

 A More will be said of the? descent of the sap and 

 growth of the plant after we have given an account of 



S. The Structure and Functions of the Leaves. 



The Leaves are very aptly compared to the lungs of 

 animals. They are the organs of respiration, perspi- 

 ration, and absorption. 



The alburnum or sap-wood is continued into the 

 leaf-stalk, which branches out into the leaf, form- 

 ing what is called the skeleton of the leaf, which is 

 frequently seen after the worms have devoured the 

 other parts. The spiral tubes attend the alburnum 

 throughout this skeleton ; they are conspicuous in the 

 leaf of the Rose, Lilac, Lilies, and particularly so in 

 the Common Eel-grass ; when these are torn and gent- 

 ly separated, they are seen unrolling themselves from 

 the broken ends of the nerves and veins. 



This skeleton is clothed on both sides by a membra- 

 nous extension of the cuticle ; between these two 

 membanes is the fleshy substance of the leaf called 

 parenchyma, which consists of vessels and cells filled 

 with the juices of the plant. 



The upper surface, which is exposed to the sun, is 

 darker than the under, its epidermis is thicker but 

 transparent, allowing a free passage of light. 



On the under surface, the epidermis is thin and full 

 of cavities, and, it is probable, altogether by this sur- 



