HORSETAILS AND CLUB-MOSSES 199 
pact group of modified leaves bearing sporangia. Just 
as in some ferns certain leaves are set apart to do chloro- 
phyll work and others 
to bear sporangia, so in 
the Equisetum the same 
division of work oc- 
curs; but the notable 
thing is that the spo- 
rangium-bearing leaves 
are massed together in 
a cluster that is quite 
distinct from the rest of 
the plant. Leaves set 
apart for bearing spo- 
rangia are called spo- 
rophylls, which means 
“spore leaves.’ A 
strobilus, therefore, is 
a group of sporophylls 
that form a more or 
less distinct cluster, dis- 
tinct from the rest of 
the plant. 
In Egquisetum each 
sporophyll consists of 
a stalk-like portion and 
a shield-like top, be- 
neath which the several 
sporangia hang (Fig. 
192, A). The spores 
have a very peculiar 
outer wall. It consists 
of two spiral bands 
ee od oo eee 7 ven t 
5 
Fic. 191.—Later sterile shoots of the species 
shown in Fig. 190, and photographed a 
month later. 
wound about the spore and fastened to it only at the point 
where they intersect (Fig. 192, B). When dry, the bands 
14 
