HISTOLOGY OF THE FERN 117 



lines, which form the outer covering for both the upper and the 

 under sides of the leaf. The epidermis cells* are colorless for 

 the most part, but here and there among them bright green 

 chlorophyll-bearing cells may be seen in pairs (Fig. 46, s.t.). 

 These cells are bean-shaped and surround a minute pore termed 

 the stoma, which connects the inner air spaces of the leaf and the 

 surrounding atmosphere. They further have the function of 

 swelling or of decreasing in size with the humidity of the air, and 

 thus regulate the openings of the stomata; for this reason, the 

 term guard cells is usually given them. On the upper surface of 

 the leaf the epidermal cells are continuous, and guard cells are 

 absent but they are widely distributed on the lower surface. 



The fundamental parenchyma of the rhizome is continued 

 into the leaf, but a new function is there undertaken by the 

 generalized cells. They become large cuboidal cells, closely 

 packed together on the upper side forming a palisade mesophyll 

 layer, while on the under side they are loosely arranged with 

 relatively great gaps or chambers, forming the spongy meso- 

 phyll. The chambers are in communication with the outer 

 air by means of the stomata. The term mesophyll, or some- 

 times chlorenchyma, is applied to these cells because of the 

 universal presence of chloroplastids colored green by chloro- 

 phyll (Fig. 47). 



The vascular bundles break up in the leaves into a series 

 of fine tubes which are differentiated for collecting food sub- 

 stances, and for conducting fluids, while the stereome is re- 

 duced to a minimum. 



Throughout the protoplasm of the mesophyll cells are prod- 

 ucts of cellular activity in the form of minute spherical or 

 tabloid granules, termed chloroplastids. They are only a 

 modified form of protoplasm and have the power to reproduce 

 themselves by division; hence they are living elements of 

 plant protoplasm and are often colorless, especially in the 

 dark. In the light these chloroplastids have the power of 

 forming an oily fluid substance of green color, the chlorophyll, 

 which disappears after some time in the dark, but can be re- 

 formed in the light. Its chemical composition is very complex 

 and is of the nature of protein, its formula, as given by Willstat- 



