288 



BOTANY 



are very similar to the habit of the common Ferns. The apical growth 

 of the stem and leaves, and their structure, are also very much like 

 those of the true Ferns. 



The prostrate stem is divided into nodes, which bear the lateral 

 organs, leaves, roots, and branches, and between the nodes are slender, 

 naked internodes (Fig. 252). The leaves are provided with a four- 

 divided lamina in Marsilia. but in Pilularia are slender, pointed 

 structures, without any evident lamina. In Marsilia, the lamina is 

 traversed by numerous dichotomously branched veins. The stem is 

 monostelic, and the leaf-petiole has a single vascular bundle. The 

 roots are produced freely from the ventral side of the nodes, and in 

 their structure and development are not essentially different from 

 those of the other Ferns, 



The Sporocarp 



The sporangia in the Marsiliacese are borne in peculiarly modified leaf-seg- 

 ments or sporocarps (Fig. 252, B, C), which are very different from those of the 



Salviniacese. According to the 



A ^g^^^air^ B recent studies of Johnson (21, 



22), these are marginal in origin. 

 Perhaps they may be most aptly 

 compared to the modified sporan- 

 gial leaf-segments of Aneimia, or 

 Lygodium, with which the Mar- 

 siliaceae show some evidences of 

 affinity. The young sporocarp 

 grows from a two-sided apical 

 cell, and ultimately forms a glob- 

 ular (Pilularia) or bean-shaped 

 (Marsilia) body. According to 

 Johnson, the sporangia arise from 

 marginal cells, as they do in the 

 Schizseacese, but they are very 

 early enclosed by the excessive 

 growth of the outer tissue of the 

 young sporocarp. There are 

 four sori in the sporocarp of 

 Pilularia, which is divided into 

 four parts, perhaps correspond- 

 ing to as many leaf-segments. 

 In Marsilia the number is larger, 

 and there are two series of sori, the ripe sporocarp splitting longitudinally. 



Macrosporangia and microsporangia occur together, and in their earlier stages 

 are alike. There are usually eight sporogenous cells, all of which give rise to 

 microspores ; but in the macrosporangium, as in the Salviniacese, but one spore 

 reaches maturity. The epispore is thick, and composed of closely set prismatic 

 elements. The outer episporic layers, as well as the sporangium-wall, and the 

 tissues of the indusium and placenta, become mucilaginous, and swell up exces- 

 sively when water is applied. The wall of the sporocarp is composed in Pilularia 

 (Fig. 257) of three layers of cells, of which the middle one is extremely hard. If 



FIG. 233. Marsilia vestita. A, longitudinal 

 section of germinated microspore, showing 

 two sterile cells, x, y, and the antheridium 

 with two groups of sperm-cells (X 400). B, 

 transverse section of antheridium. C, two 

 free spermatozoids ( X 800) . 



