EVOLUTION OF THE LAND PLANT 105 



described. There are the three systems of tissue, 

 dermatogen, periblem, and plerome, but they are not so 

 clearly distinguishable. They originate by divisions in 

 a single large pyramid-shaped cell at the apex the so- 

 called apical cell, which cuts off segments of itself by 

 walls parallel in turn to each of its sides except the 

 outside one. These segments divide later to make up 

 the mass of cells at the end, in which the differentiation 

 into the three regions spoken of takes place. 



The plerome does not give rise to a single solid or 

 hollow cylinder, but to one in which the conducting 

 strands form a cylindrical network. The strands to 

 the leaves leave the network at the margins of the 

 meshes. The strands are composed of wood and bast 

 as in other cases, but in their arrangement the bast 

 usually surrounds the wood completely. There is no 

 cambium and no increase in thickness occurs. 



The leaves are something like the leaves of a dicoty- 

 ledon in structure, but most of the internal tissue re- 

 sembles the spongy tissue rather than the palissade. 

 The epidermis contains chloroplasts. 



The structure of the root resembles that of a dicoty- 

 ledon very closely, but there is no provision for any 

 increase in thickness. The pericycle is several layers of 

 cells in thickness. 



The fern bears its spores in little cases known as 

 sporangia. In our common ferns these are grouped to- 

 gether on the under sides of the leaves in little patches called 

 sori (Fig. 42). Each sporangium contains a number of 

 spores. Each spore on germination produces a prothallus. 



We found it impossible to say how such a plant as 

 this has come to be formed in the place of the sporocarp 

 of the mosses. We find almost as great difficulty in 

 tracing the formation of the flowering plants from 

 plants having the degree of development of the ferns. 



We may well realise that with the gradual increase of 



