48 STRUCTURAL BOTANY 



to those of the outer endodermal layer. This outer layer 

 alone has the usual structure of an endodernris (see p. 

 75, Part I.) and becomes thick- walled. 



^. Other Tissues of the Stem 



The great mass of the ground-tissue, in which the 

 steles are embedded, consists of ordinary parenchyma 

 containing abundant starch. The outer cells have 

 thicker walls, and those nearest the epidermis are narrow 

 and fibrous. They serve to give mechanical stiffness to 

 the stem. The epidermis itself has a thick brown outer 

 wall, and otherwise presents no peculiarities, It bears 

 flat chaffy scales, or ramenta, which are very characteristic 

 of the plant, and indeed of almost all Ferns. They are 

 sometimes of large size, reaching half an inch in length, 

 and consist of a plate of tissue one cell thick, attached 

 to the epidermis at one end ; they arise each from the 

 growth and division of a single epidermal cell. 



2. The Leaf 



As we have already seen, each leaf, at least in the 

 mature plant, receives several steles from the stern (see 

 Figs. 19, 20, and 21). The structure of the petiole is 

 simple enough. The steles (see Fig. 22), as seen in 

 transverse section, are arranged in a horseshoe, embedded 

 in ground-tissue, the outer layers of which consist of 

 very thick-walled cells. The basal part of the petiole 

 is densely clothed with chaffy ramenta, which are more 

 scattered higher up on the leaf. 



A bundle enters each pinna of the leaf, branching 

 off from one of the two larger steles which are situ- 

 ated near the upper surface of the leaf-stalk. This 

 bundle gives off branches to the right and left, which 



