LESSON 



THE PLANT IN ACTION. 



157 



the same space of the upper surface. More commonly there are few 

 or none on the upper side ; direct sunshine evidently being unfavor- 

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

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

 to the square inch of surface. In the Apple-tree, where they are 

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

 square inch 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 THE WORK OF VEGETATION. 



445. BEING now acquainted with the machinery of the plant, we 

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



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

 that the great work of plants is to change inorganic into organic 

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

 ter, upon which animals cannot live at all, and to convert them 



FIG. 357. Portion of a White-Lily leaf, cut through and magnified, showing a section ol 

 the thickness, and also a part of the skin of the lower side, with some breathing-porea- 



14 



