PTERIDOPHYTA 



259 



layer of green cells, and of course in these, stomata are absent. The 

 development of a stoma (Fig. 224) in the Polypodiaceae is preceded 

 by the formation of a U-shaped wall in a young epidermal cell. 

 Within the cell thus cut off, a second similar cell is cut out, and this 

 becomes the mother-cell of the stoma. Below the stoma is developed 

 an air-space, which communicates with those between the very loose 

 cells of the mesophyll. The guard-cells of the stoma are filled with 

 chlorophyll-granules, which probably bear some relation to the open- 

 ing and closing of the pore between the guard-cells, this being de- 



A. 



FIG. 224. Adiantum emaryinatum. Development of the stomata (X525) ; v, acces- 

 sory cell ; st, mother-cell of stoma. 



pendent upon light. The causes of the movements in the guard-cells 

 are changes in their turgor, which are supposed to be due to the 

 development of certain soluble substances in these cells under the 

 influence of light. Beneath the upper epidermis the green cells are 

 often closely set, and form the so-called " Palisade-parenchyma." 



Vascular Bundles. The vascular bundles of the stipe and larger 

 divisions of the leaf closely resemble those of the stem, with which 

 they are joined. The small bundles in the finer veins are usually 

 collateral, the xylem lying upon the upper side. 



The ground-tissue of the stipe often shows a large development of 



