54 



Practical Plant Biology. 



soft green fundamental tissue are the thread-like conducting 

 tracts given off from the meshes of the network in the stem. A 

 similar arrangement of tissues is found in the mid-ribs of the 

 pinnae and pinnules. In the upper parts of the rachis the 

 sclerenchymatous walls are reduced and finally disappear. The 

 photo-synthetic tissue forms a wing on each side of the mid-ribs, in 

 which the ultimate branches of the conducting tracts bifurcate. 



The green tissue of the pinnules is about 075 mm. thick. 

 Their upper and under surface is formed by a single uniform layer 

 of cells the upper and the under epidermis. These cells are 

 flat and their outlines, formed by their vertical walls, are sinuous 

 or wavy. They fit together closely and interlock like pieces of a 

 jig-saw puzzle. Their walls are thin and in their protoplasmic 



FIG. -^.Aspidium filix-mas, leaf, transverse section, x 200. a, cuticle ; 

 b, upper epidermis; c, cells of palisade layer ; d, cells of spongy layer ; 

 e, lower epidermis ; /, wood (xylem) ; g, bast (phloem) of conducting 

 tract ; //, air (intercellular) spaces. 



lining are seen many chloroplasts and often a single nucleus. 

 Their outer horizontal wall is waterproofed by the presence of a 

 substance called cutin in the cellulose. The outer walls of the 

 adjacent cells form a continuous membrane over the whole leaf, 

 called its cuticle. The cells of the upper epidermis fit together 

 without leaving gaps. In the under epidermis, however, minute 

 slits are found the stomata. They are about 0-02 mm. in length 

 and may open about 0-004 mm. wide. There are about thirty or 

 fifty of these slits to the square millimetre. The stomata are limited 

 on each side by a guard-cell. The guard-cells are crescentic and 

 bend round the stomata. They are much richer in chloroplasts 

 than the other epidermal cells. 



Between the upper and lower epidermis is the mesophyll the 

 photosynthetic tissue proper of the leaf. It is composed of five or 



