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, 



i . , . j. ., , , FIG. 339. Cross section of 



Which IS not generally distributed, but IS stem O f Equisctum: outer zone is 



restricted to the nodes. It is from the cortex containing large air pas- 



nodes, therefore, that the leaves arise, sages (one beneath each furrow); 

 . f .. . , inner region (bounded by dotted 



and from the axils of the leaves that the Kne) is the stele> containing a ring 

 branches arise. The aerial branches may (in section) of vascular bundles 



beneath each ridge) enclosing 



the pith (which is breaking down). 



be all alike, or they may be dimorphic, 

 .... . 



as in h. 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 remarkably specialized 



FIG. 340. Segment of cross section of stem of Equisetum 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. 



C. B. & C. BOTANY IO 



