158 



PLANT STRUCTURES 



trycMum) (Fig. 129) and adder's tongue (Opldoglossum) 

 (Fig. 130). It is important to note, however, that the 

 Horsetails and Club-mosses are Eusporangiates, as well as 

 all the Seed-plants. 



Another small but interesting group of Ferns includes 

 the "Water-ferns," floating forms or sometimes on muddy 

 flats. The common Marsilia may be taken as a type (Fig. 



133). The slender creeping stem 

 sends down numerous roots into 

 the mucky soil, and at intervals 

 gives rise to a comparatively large 

 leaf. This leaf has a long erect 

 petiole and a blade of four spread- 





FIG. 133. A water-fern (jfarsilia). 

 showing horizontal stem, with 

 descending roots, and ascend- 

 ing leaves ; a, a young leaf 

 showing circinate vernation : 

 ,*,sporophyll branches ("spo- 

 rocarps "). After BISCHOFF. 



FIG. 134. One of the floating water-ferns (Sal- 

 vinia), 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. Aft er BISCHOFF. 



ing wedge-shaped leaflets like a " four-leaved clover. " The 

 dichotomous venation and circinate vernation at once sug- 

 gest the fern alliance. From near the base of the petiole 



