PTERIDOPHYTES 



145 



occurrence of the leaves in cycles (whorls) 

 instead of in the scattered or spiral 

 arrangement observed in the first two 

 groups. In Sphenophyllales the same 

 cyclic arrangement of leaves occurs, and 

 this disposition of the leaves is associated 

 with very distinct differentiation of the 

 stem into nodes and internodes. Such a 

 differentiation means a localization of the 

 power of producing lateral members, 

 which is not generally distributed, but is 

 restricted to the nodes. It is from the 

 nodes, therefore, that the leaves arise, 

 and from the axils of the leaves that the 

 branches arise. The aerial branches may 



be all alike, or they may be dimorphic, (one beneath each ridge) enclosing 

 . , '■ the pith (which is breaking down). 



as m E. arvense, in which case special 



strobilus-bearing branches mature in the spring, and later the green 



vegetative branches develop (figs. 332, 333). 



Stem structure. — The structure of the stem is remarkabl y specj aliaed- 



FlG. 339. — Cross section of 

 stem of Eguisetum: outer zone is 

 cortex containing large air pas- 

 sages (one beneath each furrow) ; 

 inner region (bounded by dotted 

 line) is the stele, containing a ring 

 (in section) of vascular bundles 



Fig. 340. — Segment of cross section of stem of Equisctum in detail, showing epi- 

 dermis (with stomata), zone of fibrous cells beneath the epidermis, the deeper zone of 

 chlorophyll tissue (penetrating the fibrous zone under the stomata), the large air 

 passages of the cortex, the layer of cortical cells bounding the stele (endodermis), the 

 collateral vascular bundles (each showing phloem, but with xylem replaced by an air 

 passage), and the central pith. 



