CUTICLE AND EPIDERMIS. 23 



in allusion to the function they are supposed to perform — that 

 of mouths through which the plant respires, Fig. 21. These 

 stomata are curiously constructed, generally consisting of two 

 oblong cells, placed parallel to each other on opposite sides of 

 the aperture, as seen in Fig. 21, and have the power of shutting 

 the orifice, and at other times of opening it ; thus the respiration 

 and evaporation of the plant is controlled by these little cells. 

 Of these we shall speak more particularly when describing the 

 functions of the leaves. Stomata occur on the green parts of 

 plants, and not usually on other parts, not even on blanched 

 portions of a plant. 



The number of stomata is very various on different plants, 

 and even on the same plant, as on the upper and under sur- 

 faces of leaves, being much the most numerous usually on the 

 under surface. On the leaf of the Misletoe, the number of 

 stomata on a square inch is only 200, the same number being 

 on each surface; on the Vine-leaf 13,600 to the square inch on 

 the under surface, none on the upper; Holly, 03,600 on the 

 under surface, none on the upper; Lilac, 160,000 on the under 

 surface, few on the upper. With regard to the origin of 

 stomata, considerable discussion has been carried on, but no 

 very satisfactory conclusion has been arrived at. Schleiden and 

 Link are supporters of different opinions. The former supposes 

 that the stomata result from the limit of development of cyto- 

 blasts ; that two internal cells are developed, and by the ab- 

 sorption of the parent cells, the space between them becomes 

 the stomata, and that the cells forming the stomatic sphincter 

 differ in no respect from the other cells. The latter believes 

 the stomata are secreting glands, and not mere openings in the 

 cuticle for the transmission of air and gases. 



38. The epidermis gives rise to various little organs, which 

 are classed under the heads of Hairs, Glandular Hairs, Stings, 

 Prickles, Scurf, and Lenticels. 



39. Hairs are formed of one or more cells proceeding from 

 the epidermis, and are covered with the cuticle. To examine 

 their structure, a good microscope is absolutely necessary. In 

 the Spider-wort (Tradescantia) the hair is composed of cells 

 placed end to end, and has the appearance of the antennae of 

 insects, and in these cells a circulation is distinctly visible. The 

 sides of these cells are double, although the wall of a cell under 

 common circumstances, appears of simple membrane. That 

 this is not the fact, is proved by permitting the cell to dry on 



Of what do they consist? Where do they occur ? "What is said of their 

 number ?— 38. To what does the epidermis give rise ?— 39. How are hairs 

 formed ? 



