HISTOLOGY OF TUB RHIZOME. 117 



MINUTE ANATOMY OF THE RHIZOME OF PTERIS AQUILINA. 



Besides the above-mentioned tissues, the rhizome contains 

 certain other secondary varieties which will be described further 

 on. 



Epidermal System. Epidermis. It is the function of the 

 epidermis (aided in this case by the underlying sclerotic paren- 

 chyma) to protect the inner tissues from contact with the soil 

 and to guard against desiccation of the rhizome during droughts. 

 The cells (Fig. 49) are dead and empty, with enormously thick, 

 hard walls perforated by numerous branching canals. The outer 

 wall is especially thick. 



Fundamental System. The tissues of this system form the 

 main body of the plant, and in the fern have two widely differ- 



