GROUP III. PTERIDOPHTTA: FILICIN^E ; HYDROPT BRIDES. 



409 



the fertile leaf-branches spring from the ventral surface of the 

 petioles of the foliage-leaves (compare Ophioglossaceee), and each 

 bears a sporocarp at the end of a longer or shorter stalk : the 

 petiole bears a single fertile leaf -branch near its base in some 

 species, or two or more adnate branches springing from the same 

 point ; or it bears (e.g. M. polycarpa) a series of 10-20 branches, 

 one above the other, each bearing a single sporocarp. In Pilularia 

 the fertile leaf-branches appear to be also 

 developed from the ventral surface of the 

 foliage-leaves : each leaf has at its base a 

 single almost sessile sporocarp. 



The sporocarp of Marsilea (Fig. 269) is 

 dorsiventral, somewhat pod-shaped, with 

 its dorsal margin directed upwards ; the 

 stalk is continued along the dorsal margin 

 as a midrib : the sporocarp may, in fact, 

 be regarded as being developed from the 

 laminar portion of the leaf-branch. The 

 sporocarp of Pilularia is globular, though 

 it is slightly pointed at the apex: it may 

 be likewise regarded as being a leaf- 

 branch, four (usually) leaflets or segments 

 being concerned in its construction. In 

 both genera, especially in Marsilea, the 

 wall of the sporocarp is composed of 

 several layers of cells with thick hard 

 walls: vascular bundles, springing from 

 the stalk, are distributed in the wall. 



The number of sori in the sporocarp of 

 Marsilea varies from five to twenty-three 

 in the different species : they are de- 

 veloped in tubular cavities, extending 

 from the ventral margin of the sporocarp 

 for some distance towards the dorsal 

 margin, which are disposed in two longi- 

 tudinal rows, one row on each side of the 

 middle line : when young, these cavities 

 are open at the ventral margin, but the 

 apertures become closed as the sporocarp matures : the external 

 wall of each cavity developes into a projecting ridge of tissue, the 

 placenta, which bears the sorus, consisting of a single median row 



FIG. 269. Stem of Marsilea 

 Salvatrix with leaves (reduced 

 one-half). K Terminal bud; 

 b b leaves ; // sporocarps 

 borna on petioles. 



