PTERIDOPHYTA 141 
round the nodes or joints of the stem. These reduced 
leaves are practically useless as assimilative organs, and 
their office is assumed by the internodes of the stem 
and branches, where the green tissue is largely devel- 
oped, and connected with the outside by numerous 
stomata in the epidermis. The leaves serve as protec- 
tive organs only, forming a thick covering over the 
apex of the young shoot, and also covering the buds 
from which spring the lateral branches. 
In studying the development of the tissues of the 
sporophyte, one is struck by the almost mathematical 
regularity in the divisions of the cells at the stem-apex, 
as well as in the roots. The shoot in all species termi- 
nates in a single apical cell (Fig. 86, C), having the 
form of an inverted three-sided pyramid from whose 
lateral faces segments are cut off in regular succession, 
and the tissues of the mature stem bear a definite 
relation to the early divisions in these segments. A 
similar regularity exists in the early divisions of the 
cells at the apex of the root. The stem is traversed by 
a regular system of lacune, or air-passages (Fig. 386, 
B, 2), and the vascular bundles are arranged in a circle, 
recalling the arrangement in the stem of the typical 
Dicotyledons. In the arrangement of the woody tissue 
and bast, they recall the flowering plants rather than 
the ferns, although among the latter the Ophioglossacez 
show a somewhat similar type of vascular bundle, — the 
“collateral” form,—and also other structural resem- 
blances. 
The sporangia of Equisetum occur upon peculiar um- 
brella-shaped sporophylls which are arranged in whorls 
about the apex of certain shoots, and crowded together so 
