ANATOMY 27 



The formation of rings of secondary wood takes 

 place also in roots, so that when they are very old, 

 and the inner tissues are crushed, it is not easy to dis- 

 guish them from stems. 



The primary structures, however, are easily dis- 

 tinguished, and when there is any doubt from the ex- 

 ternal morphology alone as to whether any organ is a 

 root or a stem, a section showing the internal tissues 

 will establish its nature. 



The leaf, with its flat expanded surface, differs from 

 the stem and root in having a bilateral and not a radial 

 symmetry. In a typical dicotyledonous leaf the single 

 vascular strand which runs out from the stem into its 

 petiole branches in one plane to form a complete net- 

 work like a fan. Each finer branch of the vascular 

 strand in this is like the one from which it arose, and 

 is composed of a single group of wood cells and a group 

 of bast cells side by side. Between the meshes of this 

 fan, webbing the whole together, is the soft-celled 

 parenchyma. In most cases the upper layers are more 

 closely packed and composed of more regular cells 

 than those on the lower side, and generally all of them 

 contain numerous green granules of chlorophyll. En- 

 closing and protecting this web of tissue on both sides 

 is an epidermis. In many cases, particularly in the 

 tough leaves of plants which grow in hard conditions, 

 there are strengthening bands and props of scleren- 

 chymatous tissue arranged to great mechanical ad- 

 vantage. 



To the theoretically minded anatomist, and him who 

 concerns himself with the phylogeny of plant structures, 

 the greatest interest lies in the woody tissue. Not only 

 is this easier to recognise and stain in living plants, but 

 it is better preserved in the fossils than the softer cells, 



