HISTOLOGY OF THE FERN 115 



of them have undergone modifications. The epidermis cells 

 of the rhizome are lifeless but here they are living and elaborated 

 into flattened epidermal cells with curious wavy outlines, 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 which the term guard cells is 

 usually given them. On the upper surface of the leaf the epi- 

 dermal 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. They 

 are 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 mesophyll. The chambers are 

 in communication with the outer air by means of the stomata. 

 The term mesophyll or sometimes chlorenchyma, is applied to 

 these cells because of the universal presence of chloroplastids 

 colored green by chlorophyll (Fig. 47). 



The fibrovascular 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 products 

 of cellular activity in the form of minute spherical or tabloid 

 granules termed chromoplastids or chloroplastids. They are 

 only a modified form of protoplasm and have the power to 

 reproduce themselves by division, hence they are living ele- 

 ments 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, 



