THE GREAT GROUPS OF PTERIDOPHYTES 
337 
" four-leaved clover." The dichotomous venation and cir- 
cinate vernation at once suggest the fern alliance. From 
near the base of the petiole another 
leaf branch arises, in which the blade 
is modified as a sporophyll. In this 
case the sporophyll incloses the spo- 
rangia and becomes hard and nut- 
like. Another common form is the 
FIG. 303. A water-fern (Marsilia), 
showing horizontal stem, with 
descending roots, and ascend- 
ing leaves ; a, a young leaf 
showing circinate vernation ; 
s,s. sporophyll branches ("spo- 
rocarps "). After BISCHOFF. 
FIG. 304. One of the floating water-ferns (Sal- 
vinid), showing side view (A) and view from 
above (B). The dangling root-like processes 
are the modified submerged leaves. In A, 
near the top of the cluster of submerged 
leaves, some sporophyll branches ("sporo- 
carps ") may be seen. After BISCHOFF. 
floating Salvinia (Fig. 304). The chief interest lies in the 
fact that the water-ferns are heterosporous. As they are 
leptosporangiate they are thought to have been derived from 
the ordinary leptosporangiate Ferns, which are homosporous. 
EQUISETALES (Horsetails or Scouring rushes) 
219. General characters. The twenty-five forms now rep- 
resenting this great group belong to a single genus (Equise- 
