LESSON 20.] 



xni-: PLANT IN ACTKJN. 



i:>7 



the same space of the n])|)<>r surface. iMon^ commonly (here arc few 

 or none on the upper side ; direct sunshine eviih-ntly being unfavoi'- 

 able to their operation. Tlieir immense numbers make up for their 

 minuteness. They are said to vary from less than 1,000 to 170,('00 

 to tiie square incij of surface. In the A|)ple-tree, where tliey are 

 under the average as to number, there are about 24,000 to the 

 sfpiare incii of the lower surface ; so that each leaf has not far from 

 100,000 of these openings or mouths. 



LESSON XXVI. 



THE PLANT IN ACTION, DOING Till: WORK OF VEGETATION. 



445. Being now ac<iuainted with the machinery of the plant, we 

 naturally proceed to inipiire what the use of it is, and how it works. 



44G. It has already been stated, in the first of these Lessons (7), 

 that the great work of plants is to cltaiKjc inorganic info on/anic 

 matter ; that is, to take portions of earlh and air, — of mineral mat- 

 ter, — upon which animals cannot live at all, and to ct)nvert them 



FIO. 357. Portion of .1 Wliito Lily leaf, cut throiich .tikI tii.iBnificd, «1io\vinu .i sprtion of 

 the thickness, ami also a part of iho skin of ilio lower siilc, with M)ini> liroalliiiiu-Doroa 

 11 



