PTERIDOPHYTES 
direction. In Tmesipteris a sterile plate divides the large sporangium into two 
chambers so distinct that they are called two sporangia M 
(figs. 326-329). That this partition is sterilized spo- 
rogenous tissue is proved by its development and by 
the fact that in exceptional cases it functions as spo- 
rogenous tissue. In Psilotum the same condition occurs, 
except that the development of two sterile plates results 
in three chambers, or a group of three sporangia (figs. 
330, 331). The other noteworthy feature of Psilotum 
and Tmesipteris is the development of the subarche- 
sporial pad of Lycopodiales into a short stalk, which 
bears the two or three sporangia and is called the spo- 
rangiophore. 
(3) SPHENOPHYLLALES 
This group contains the single large Carboniferous 
genus Sphenophyllum, which illustrates the further de- 
velopment of the sporangiophore. In Splienophyttum 
there is a distinct strobilus, with whorls of linear spo- 
rophylls coalescent at base into spreading funnels. 
From the adaxial surface of these sporophylls sporangio- 
phores arise, which vary from very short to very long, 
simple or branching, and bear one to several sporangia. 
In this genus, therefore, the sporangiophore develop- 
ment is carried much farther than in Psilotum and 
Tmesipteris, resulting in a multiplication of sporangia 
by means of the sporangiophore. 
(4) EQUISETALES 
This great group is represented in our present 
flora by the single genus Equisetum, comprising 
about twenty-five species of horsetails or scouring 
rushes. This is only a remnant of a great group 
that flourished in the Paleozoic along with the 
ancient Lycopodiales. 
Sporophyte. The sporophyte body consists 
of a subterranean, dorsiventral main axis, which 
gives rise to erect (radial) aerial branches, them- 
selves simple or branched (figs. 332, 333). Equi- 
setum is characterized by its remarkably small 
leaves, which for the most part are insignificant 
scales that occur in a whorl at each joint and 
coalesce to form a close sheath. As a conse- 
331 330 
FIGS. 330, 331. Psilo- 
turn: 330, general habit, 
showing the branching 
body bearing much re- 
duced leaves (scales) and 
the characteristic three- 
lobed sporangium (or 
synangium of three spo- 
rangia); 331, the spo 
rangia in greater detail. 
