144 



VEGETABLE LIFE AND WOEK. [SECTION 16. 



of Houseleek. The epidermis is usually composed of a single layer, occa- 

 sionally of two or tliree layers, of empty 



cells, mostly of irregular outline. The sin- I / / u~ Jtssy 



nous lines which traverse it, and may be dis- 



cerned under low powers of the microscope (Kg. 487), are the boundaries 

 of the epidermal cells. 



443. Breathing-pores, or Stomates, Stomata (singular, a Stoma, — 

 literally, a.mouth) are opeuings through the epidermis into the air-chambers 

 or intercellular passages, always between and guarded by a pair of thin- 

 walled guardian cells. Although most abundant in leaves, especially on 

 their lower face (that which is screened from direct sunlight), they are 

 found on most other green parts. They establish a direct communication 

 between the external air and that in the loose interior of the leaf. Their 

 guardian cells or lips, which are soft and delicate, like those of the green 

 pulp within, by their greater or less turgidity open or close the orifice as the 

 moisture or dryness varies. 



444. In the White Lily the stomata are so remarkably large that they 

 may be seen by a simple microscope of moderate power, and may be dis- 

 cerned even by a good hand lens. There are about 60,000 of them to the 

 square inch of the epidermis of the lower face of this Lily-leaf, and only 

 about 3000 to the same space on the upper face. It is computed that an 

 average leaf of an Apple-tree has on its lower face about 100,000 of these 

 mouths. 



§5. PLANT FOOD AND ASSIMILATION. 



445. Only plants are capable of originating organizable matter, or the 

 materials which compose the structure of vegetables and animals. The es- 

 sential and pecuhar work of plants is to take up portions ' of earth and air 

 (water belonging to both) upon which animals cannot live at all, and to 

 convert them into something organizable ; that is, into something thnt, 

 under life, may be built up into vegetable and animal structures. All the 

 food of animals is produced by plants. Animals live upon vegetables, 



Fio. 484. Small portion of epidermis of the lower face of a White-Lily leaf, 

 with stomata. 



Fig. 485. One of these, more magnified, in the closed state. 486. Anothei 

 stoma, open. 



Fig. 487. Small portion of epidermis of the Garden Balsam, highly magiiilied, 

 showing very sinuous-walled cells, and three stomata. 



