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. 
, . . FIG. 339. Cross section of 
which is not generally distributed, but is stem O f Equisetum: outer zone is 
restricted to the nodes. It is from the cortex containing large air pas- 
nodes, therefore, that the leaves arise, fges (one beneath each furrow); 
inner region (bounded by dotted 
and from the axils of the leaves that the Kne) i s the stele, containing a ring 
branches arise. The aerial branches may (in section) of vascular bundles 
be all alike, or they may be dimorphic, (one beneath each ridge) enclosing 
, . . the pith (which is breaking down). 
as in 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 remarkably specialized 
FIG. 340. Segment of cross section of stem of Equisetum in detafl, 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. 
