172 



PHYSIOLOGICAL BOTANY. 



green tissue. (See Structural Botany, Chapters XX.- 

 XXIII. inclusive.) 



426. An important function of the leaf is to expose 

 a large surface ; consequently, the blade is thin and so 

 formed as to present the largest number of cells to 

 the air and sunlight. 



The layer on the upper side of the blade is made 

 up of oblong cells, closely packed with their ends next 

 to the surface. The lower layers are made up of 

 smaller, more irregular and more loosely arranged 

 cells, and have their 

 longer diameters in the 

 direction of the surface 

 of the blade. The deep 

 green color of the up- 

 per surface of leaves is 

 largely due to the com- 

 pactness of the green 

 cells in the upper layer, 

 while the paler color of 

 the under side is the 

 consequence of the 

 loose arrangement of 8how its opening into the intercellular " assa e es - 

 those in the lower strata. The epidermal covering of 

 the leaf, as before described, is a thin membrane made 

 up of one, two, or three layers of empty thick-walled 

 cells (Figs. 489-524 inclusive). 



427. Respiration is the act of drawing air into the 

 lungs and casting it out again. (From the Latin re, 

 again, and spirare, to blow or breathe.) The air while 

 in the lungs is known to part with some of its oxygen, 

 and what is breathed out is charged with substances 

 which it did not possess when taken in ; therefore the 



526, Magnified portion of the leaf of Viola tricolor in 

 perspective; a, cells of the epidermis, sometimes called 

 plate cells; b, compact layer of green cells next to the 

 upper surface; c, loose cells below; d, epidermal cells of 

 the lower surface, with stomata, one of which is cut to 



