FILICINF.^. 



455 



out beyond it into a pencil of delicate filaments ; three (or three times three) large 

 masses of it are attached at this point. The frothy mucilage with its cavities full 

 of air thus forms a float for the macrospore, and bears on its surface the upper 

 part of the ruptured sporangium. 



The sporocarps of the Marsiliacese are much more complex and more firmly 

 constructed than those of the preceding family. The sporocarp of Pilularia is 

 a shortly-stalked spheroidal capsule, standing apparently by the side of a leaf 

 towards its ventral aspect. Its morphological significance is not yet fully under- 

 stood. The capsule has a very thick hard 

 wall, consisting of several layers of cells, 

 and contains, besides a considerable quan- 

 tity of soft succulent parenchyma, hollow 

 chambers which extend from base to apex. 

 Pilularia globuUfera (Fig. 320) has four 

 such chambers, P. minuta two, and P. 

 americana three. The external wall of 

 each chamber bears a thickening along its 

 median line of the inner surface throughout 

 its whole length, and in this thickening 

 runs a fibrovascular bundle. On this 

 receptacle (placenta) the stalked sporangia 

 are borne, forming a sorus which consists 

 at its lower part mostly of macrosporangia, 

 at its upper of microsporangia. Probably 

 each chamber has, at an early stage of 

 development, an aperture at its apex, but 



it is not clear here, any more than in Alarsilia, how far a comparison of the 

 delicate tissue which invests the sorus to an indusium is justifiable, a comparison 

 which is made by many botanists. 



The sporocarps of the various species of Marsilia are usually somewhat 

 bean-shaped capsules with very hard walls, borne on long or short stalks, which 

 arise either upon the ventral aspect of the petioles of ordinary foliage -leaves (Fig. 

 317) or at their bases. Their stalks may be simple, bearing but one sporocarp, or 

 forked, bearing many sporocarps. From a petiole several usually arise together. 

 The stalk is continued along the posterior edge of the sporocarp (Fig. 325), giving 

 off" lateral branches right and left, which dichotomise and run towards the ventral 

 ^^^<^. The ripe capsule is bilaterally symmetrical and contains two rows of chambers 

 each of which extends from the ventral to the dorsal margin (Fig. 321, A^ B), In 

 the very young fruit, according to Russow, each chamber communicates with the 

 exterior by means of a narrow canal opening on the ventral aspect. A thickening 

 (placenta) runs along the external wall of each chamber, bearing the macrosporangia 

 on its central ridge, and the microsporangia on its flanks ; thus each chamber 

 contains a sorus consisting of two kinds of sporangia. When the sporocarp bursts 

 it becomes evident (Fig. 325) that the soft internal tissue forms, as in Pilularia, 

 a completely closed investment for each sorus. When mature the microsporangia 

 contain numerous (4x16) spores, the macrosporangia but a single one. 



Fig. 320. — Transverse section of the sporocarp oi Pilularia 

 globuli/era below the middle, where the macrosporangia and 

 microsporangia ma and mi are intermingled ; g: the fibro- 

 vascular bundles, h hairs, e epidermis of the outer surface. 



I 



