112 BOTANY 



In the stems of Ferns, strands or plates of selerenchymatous fibres are dispersed 

 throughout the fundamental tissue, which belongs to the primary cortex. These 

 plates of sclerenchyma, particularly noticeable from the brown colour of the walls of 

 their fibres, surround and accompany the schizosteles. In Selaginella, on the other 

 hand, the schizosteles are suspended within intercellular passages by means of cell 

 filaments. In the case of Lycopodium the gamosteles are protected, and the rigidity 

 of the stem secured by a strongly thickened inner zone of the cortex (Fig. 128, to). 



In Roots, the division between primary cortex and central cylinder 

 is sharply marked by the endodermis, into which the innermost layer 

 of the primary cortex is usually transformed (Figs. 120, 126, e). The 

 central cylinder becomes completely shut off from the primary cortex 



Fig. 12S. — Transverse section of stem of Lycopodium eomplanatum. ep, Epidermis ; vc } ui, pp, outer, 

 inner, and innermost parts of the primary cortex, surrounding the gamostele ; sc, scalariform 

 traeheids ; sp t annular and spiral tracheids ; v, phloem, (x 26.) 



by the suberisation of the lateral walls of the endodermal cells, 

 and by their close and uninterrupted contact. While, by this means, 

 the passage of gases from the intercellular spaces of the cortex into 

 the central cylinder, and the consequent obstruction of the water- 

 channels, are prevented, the passage of water from the cortex to the 

 central cylinder can, at the same time, go on unhindered through the 

 unsuberised inner and outer walls of the endodermal cells. In this 

 manner it is possible for the water, absorbed from the soil by the root 

 hairs or by the surface of the roots, to be transferred to the tissues of 

 the central cylinder. In the older parts of the roots, which no longer 

 absorb water from the soil, the cells of the endodermis become greatly 

 thickened, but generally on one side only. Should the thickenino- 

 occur at an early stage, then special endodermal cells, directly external to 

 the xylem strands, remain unthickened and serve as transfusion cells 



