GROUP III. PTERIDOPHYTA : FILICIXJE. 



375 



the ventral surface of the petioles of the foliage-leaves (compare 

 Ophioglossacese), 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. polycarpd) a series of 10- 

 20 branches, one above the other, each bearing a single sporocarp. 

 In Pilularia the fertile leaf-branches ap- 

 pear to be also developed from the ventral 

 surface of the foliage-leaves : each leaf has 

 at its base a single almost sessile sporo- 

 carp. 



The sporocarp of Marsilea (Fig. 225) 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 developed 

 in tubular cavities, extending from the 

 ventral margin of the sporocarp for some 

 distance towards the dorsal margin, which 

 are disposed in two longitudinal 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 of macrosporangia and a double row of microsporangia 



FIG. 225. Stem of Xarsilea 

 Salvatrix with leaves (reduced 

 one-half). K Terminal bud ; 

 b b leaves ; / / sporocarps 

 borne on petioles. 



