146 
MORPHOLOGY 
(figs. 339, 340). The outer walls cf the epidermal cells are so impreg- 
nated by a deposit of silica as to give the characteristic rough feeling to 
the stem. The stem is fluted, and within the ridges strands of fibrous 
ceUs are developed; while in the furrows the chlorophyll tissue reaches 
the epidermis. It follows that the stomata (see p. 250) are in the furrows 
(usually along the slopes) rather than on the ridges. Deeper within 
the cortex a zone of large air passages occurs, each one lying beneath a 
furrow. The central cylinder or stele is remarkably reduced, the vas- 
cular bundles being very feebly developed. They are arranged (usually 
one beneath each ridge) so as to outline a hollow cylinder enclosing a 
pith, which disappears early; but instead of being concentric bundles, 
characteristic of most pteridophytes, they are collateral; that is, the 
xylem and phloem strands lie side by side on the same radius, with 
the xylem towards the center of the stem. In fact, however, in Equi- 
setum the xylem is hardly at all developed, its position being occupied 
by a small air passage (fig. 340). 
Sporangium. The sporangia occur in a very distinct strobilus 
(fig. 332). The structure usually called a sporophyll has a stalklike 
base and a peltate top, beneath which five to ten sporangia are borne 
(figs. 334, 335). 
To understand this structure it is necessary to be familiar with certain of the 
extinct Equisetales. A series can be arranged, beginning with Sphenophyllum, 
passing on to Calamites (an extinct group of Equisetales), and ending with Equisc- 
tum, which indicates that in the strobilus of Equisetum the sporophylls have been 
suppressed and that the structures bearing spo- 
rangia are sporangiophores. In Calamites the 
strobilus is made up of alternating sets of sporo- 
phylls and sporangiophores, and the latter are 
just such structures as appear in the strobilus of 
Equiselum, without the alternating sets of spo- 
rophylls. In Selaginella there are sporophylls 
that do not bear sporangia; and in Equisetum 
there is apparently a strobilus that does not 
consist of sporophylls. 
342 
FtGS. 341, 342. Sporangium 
of Equisetum: 341, early stage, 
three cells having arisen from the 
superficial initial, the innermost 
(shaded) being sporogenous, the 
two outer being wall cells; 
342, further development of wall 
layers and sporogenous tissue. 
--Alter 
The sporangium arises from a single 
superficial cell, and not from a transverse 
row as among Lycopodiales. There is 
the usual periclinal wall, setting apart an 
outer wall cell from an inner sporogenous 
cell, as in all eusporangiates (fig. 341). 
