100 COMPOUND OROANS OF PLANTS. 



growth of the previous year, which it re-dissolves and carries 

 along with it ; so that the sap which circulates in the superior 

 parts of a plant offers a composition more rich in organic prin- 

 ciples. 



It is, however, principally in the leaves that the sap under- 

 goes those changes which render it subservient to the growth 

 and nutrition of the plant. In the leaves, the sap is exposed 

 to the influences of the light and air, and is thickened and 

 condensed by the evaporation of the useless water. Under the 

 influence of light, the oxygen of the carbonic acid is given off 

 from the leaves into the atmosphere, and the carbon is fixed, 

 chlorophyl being formed in the cells. The sap is distributed 

 to all parts of the leaf by means of the veins in the leaf, 

 which are immediately connected with the alburnum or sap- 

 wood of the stem. The mechanism of the leaf and the action 

 of the pores has been already explained. Not only the leaves, 

 but the young branches, scales, and all the herbaceous or green 

 parts of the plant, act on the atmosphere in a similar manner 

 to the leaves. 



After having been elaborated in the leaves, the sap, which is 

 now called the proper juice, re-descends from the leaves towards 

 the root. 



The vascular and cellular system of the leaf not only offers 

 the same composition as the stem, but it preserves the same 

 relative situation in the leaf as in the stem; those vessels 

 which occupied the' interior of the stem next to the pith, 

 becoming superior in the leaf whilst the more external vessels 

 become inferior, and all retaining the same relative parallelism 

 in the petiole and lamina. 



Now the fibro-vascular tissue which thus issues from the 

 stem into the petiole, consists of two layers of vessels, an 



