Structure and Functions of the Leaves. 



65 



cells of chlorophyll ; I are air spaces ; u, minute masses of chloro- 

 phyll, thus brought in near contact; and sp, " stomata," or breathing 

 pores, through which the air enters. These pores are chiefly 



49. Vertical Section of a Beech Leaf, very greatly enlarged. 



on the under side of the leaf, and vary in number from 1,000 to 

 170,000 to a square inch. In coniferous leaves there is no network 

 of ribs, but a longitudinal and sometimes a diverging system of 

 fibers. A' cross-section shows a symmetrical arrangement of cells, 

 some filled with grains of chlorophyll, others with air, and others 

 with resinous matter. The arrangement of these cells is constant 

 within genera, and to some extent in species, afibrding character- 

 istics upon which classifications have been formed. The stomata 

 upon coniferous leaves arc more abundant upon the under and lighter 

 colored side of the leaves, but iu some cases where both sides are 

 colored alike, they are found equally on both sides. The ginkgo 

 (SaMiAaria adiantifolia)), one of the Japanese species that is found 

 to thrive in the Middle and Southern States, is a conifer with very 

 exceptional form of leaf. It spreads out flat like a fan, and is de- 

 ciduous. 



237. The moisture of the soil, is brought up in the form of sap 

 from the roots to the leaves, bearing in solution certain mineral and 

 organic materials. These are then exposed to the air, and to the 

 carbonic acid in the air, and under the action of the light the latter 

 is decomposed. Its carbon is retained and oxygen given out. A' 

 vast amount of evaporation also takes place from the leaves, so that 



