RHIZOCARPEM. 



395 



elevated on a tooth of the submerged leaf simultaneously all round, which grows up, 

 envelopes the tooth, and closes it; the enclosed end of the tooth swells into a 

 globular form, and the sporangia arise as trichomes from it. Dr. Pfeffer, who 

 confirms in this respect the statements of Griffith and Mettenius (as stated in a letter 

 received from him) compares, as A. Braun also did, the envelope of the sporocarp 

 of Salvinia to the integument of an ovule. I consider, however, that a closer and 

 better comparison may be drawn with the indusium of the Hymenophyllaceae. 

 If a resemblance can thus be traced between the sporocarp of Salvinia and the 

 indusiate sorus of this family of Ferns, Braun shows, on the other hand, that the 

 much more complicated sporocarps of INIarsilea and Pilularia must be considered 

 as metamorphosed leaves with united pinnae and bearing the sporangia on their 

 upper sides in a definite relation to the course of the veins or vascular bundles, in 

 the same manner as among the Polypodiaccee. It appears also from the history of 

 development given by Russow (otherwise not very clear) that in Marsilea at an 

 early period the compartments of 

 the sorus open outwards by narrow 

 apertures. 



The sporangia arise, as has al- 

 ready been mentioned, from some 

 of the superficial cells of tlic placenta 

 or part to which the sorus is at- 

 tached. According to my observa- 

 tions, which are not yet entirely 

 completed, the order of development 

 of the cells in the young sporangium 

 of Pilularia is very similar to that in 

 Polypodiacece. After the formation 

 of the pedicel and mother-cell of the 

 capsule, two circular series of oblique 

 divisions arise in them, by which a 

 double parietal layer of cells and a 

 tetrahedral central cell are formed. 

 While the former are broken up by 



radial walls into a number of cells, and the size of the capsule increases, the 

 central cell first divides into two, and then, by successive bipartitions, into eight 

 spore-mother-cells, which become isolated in the cavity of the sporangium which is 

 filled with granular fluid, and assume a round form. The inner parietal layer 

 remains in the condition of a delicate epithelium till the time of the formation of 

 the spores, but disappears when they are ripe ; so that here also the wall of the 

 sporangium finally consists of only one layer. In Marsilea and Pilularia, where the 

 envelope of the sporocarp is very hard, it remains thin and colourless, the sporangia 

 forming small hyaline sacks ; in Salvinia, where the \vall of the sporocarp is thin 

 and delicate, the cells of the wall of the sporangium assume, at the period of ripe- 

 ness, greater consistency and a bro\vn colour, as in Ferns. Until the commence- 

 ment of the division of the spore-mother-cells into four, the development of 

 all the sporangia proceeds similarly; the diff"erentiation into macrosporangia and 



FlC. 295. — Transverse section of the sporocarp oi Pilularia 

 i^lohiili/era below the middle, where the macrosporang^ia and 

 microsporanjjia ma and 7ni are intermingled ; g the fibro- 

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



