THE GEEAT GROUPS OF PTEEIDOPHYTES 



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 



Fis. 303. A -water-fern (MarsUla), 

 showing horizontal stem, with 

 descending roots, and ascend- 

 ing leaves ; a. a young leaf 

 showing circinate vernation ; 

 s, s. sporophyll branches (" spo- 

 rocarps ").— After Bischofp. 



Fig. 304. One of the floating water-ferns (Sal- 

 vinia), showing side view (A) and view from 

 above iB). 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- 



