THE PTERIDOPHYTA : FILICALES, THE FERNS 



487 



off to the right and the left, which enter the pinnules and form the veins 

 of their laminae. 



The upper surface of the lamina is formed by an epidermis, beneath which 

 is a mesophyll, composed of irregular parenchymatous cells with large 

 intercellular spaces. Not only do the mesophyll cells contain chloroplasts 

 but the epidermis does also. This is a character which the Ferns share 

 only with shade-loving and with submerged aquatic Angiosperms. The 

 presence of chloroplasts in the epidermis indicates that this layer differs 

 markedly in its physiological importance from the epidermis of higher plants. 

 In many Ferns the epidermis cells have lobed inner walls, simulating the 



Guard cell 



Epidermal cell with 

 chloroplasts 



Stomatal Pore 



Fig. 475. — Dryopteris filix-mas. Lower epider- 

 mis of pinnule showing stomata and 

 irregularly shaped epidermal cells con- 

 taining chloroplasts. 



arm-palisade of some Angiosperms, and the epidermis is not only strongly 

 photosynthetic but is also the principal channel of translocation. 



Below the mesophyll lies the lower epidermis, in which are the stomata 

 (Fig. 475). These stomata consist of a pair of specialized curved cells, 

 which are termed the guard cells, and these surround a narrow slit, which 

 is referred to as the stomatal pore. By variation in the shape of the guard 

 cells this pore may be opened or closed, and it is by this means that the 

 interchange of gases between the atmosphere and the leaf is regulated. We 

 shall consider later (Volume III) the mechanism of the opening and closing 

 of stomata. In common with the other organs, the Fern leaf also grows 

 from an apical cell, in this case two-sided, from which two marginal series of 

 cells are derived, which in turn produce the cells of the lamina. The growth 

 of the young leaf is thus marginal as well as apical. 



