no 



OUTLINES OF PLANT LIFE. 



ditional protection against evaporation or for use as a water-storing 

 tissue. (See ^ 342.) Numerous narrow slits, each bounded by a pair 

 of specialized cells called guard cells, are formed in the epidermis. The 

 whole apparatus is called a stoma (figs. 104, 105). The guard cells are 

 crescent-shaped, and are sensitive to various external conditions, espe- 

 cially light, so as to control the size of the slit-like passage between 

 them by becoming straighter or more curved (fig. 105). This passage 



Fig. 105. — ^, perspective view of a stoma from the under epidermis of the beet leaf, 

 showing the sloping sides of the slit, the crescentic guard cells with chloroplasts. 

 B, sections tlirough stomata of beet at right angles to their length. The upper figure 

 shows the stoma open ; the lower closed. The black line represents the primary wall, 

 to which additional material, especially in the guard cells, has been added. These 

 thickenings serve by their elasticity to close the stoma. Opening is due to turgor of 

 the guard cells. The chloroplasts and granular protoplasm are shown. Highly mag- 

 nified. — After Frank. 



is formed by the partial splitting apart of the guard cells and com- 

 municates with extensive spaces between the green cells in the in- 

 terior. 



The stomata are very numerous. In different plants, in the space 



here enclosed, 

 30,000, some- 

 to 70,000 in 



sq. cm. 



the numbers usually vary from 4000 to 

 times, however, reaching as many as 60,000 

 the olive and rape. They are not equally 



distributed on the two sides of the leaf, being usually more numerous on 

 the under side, where there are more internal spaces. They may be 

 wanting on the upper side, as in lilac, begonias, and oleander. There 

 are no stomata on submerged leaves nor on the under side of floating 

 leaves. In some plants they are found in clusters, in others uniformly 

 distributed. 



138. Cortex. — The cortex of leaves is called the mesophyll. It con- 

 sists of thin-walled, active cells, for the most part richly supplied with 

 chloroplasts. In very thick leaves the internal cells are colorless. In 

 some leaves the cells of the mesophyll are nearly uniform, but in most 



