RHTZOCARPE.E. 



399 



The structure of the sporocarp of Sal'vinia natans was sufficiently explained by 

 Fig. 294, B, C. The sporangia are set free by the decay of the whole plant during 

 winter ; the macrospores fill up the sporangium, and do not become detached from it 

 even during germination. The structure of the sporocarp of the Marsileae (the species 

 mentioned are perennial) determines at the same time a remarkable mode of dehiscence 

 and dispersion of the spores, which must be briefly explained. Beneath the epidermis 

 of the wall of the sporocarp, which is at first very hairy and provided with stomata, 

 lie two or three layers of thickened and lignified cells elongated radially, which form a 

 very hard sporocarp-wall, in the case of Marsilea scarcely permeable by water (com- 

 pare Figs. 295 and 300). Beneath lie layers of 

 parenchymatous cells, into which run the fibro- 

 vascular bundles of the sporocarp ; in Piluloria 

 globiilifera there are twelve in all, ascending as 

 meridian lines from the base to the apex, one 

 lying beneath each placenta, and a pair right and 

 left of each of the four partition-walls of the 

 sporocarp. In Marsilea the bundle belonging to 

 the pedicel runs along the dorsal edge of the 

 sporocarp, and sends out right and left transverse 

 anastomosing branches corresponding to the pla- 

 centae, as far as the ventral edge ; they lie in 

 the parenchyma beneath the hard external testa. 

 The apparent median partition of the sporocarp 

 of Marsilea is not an independent structure, but 

 consists of the walls of the compartments of the 

 sporocarp which meet on the median plane ; 

 these are themselves composed of large cells, the 

 outer layer of whose cell-wall is thin and solid, 

 while the inner become converted into mucilage 

 and are capable of swelling. The compartments 

 of the sporocarp are small longish sacs lying trans- 

 versely in it in two rows one over another, and 

 attached by their anterior and posterior ends 

 to a cushion of tissue which fills up the angle 

 at the dorsal and ventral edges of the sporocarp, 

 and runs round its cavity. The inner layers of 

 the cell-walls of this cushion have also great 

 capacity for swelling. If now by any injury to 

 the hard testa of the sporocarp, the entrance 

 of water becomes possible, the cushion and the 

 walls of the compartments begin in a short time 

 (about 10 minutes) to swell so violently that first 

 of all the testa is split along the ventral edge 

 into two valves, the annular cushion detaching 

 itself from the inner surface of the testa, en- 

 larging in consequence of the swelling, and protruding from the slit in the form of a 

 ring (Fig. 300 B). The compartments of the sporocarp are attached at both ends to the 

 inner periphery of this ring ; but they are all torn off on one side by its gradual expan- 

 sion ; it now stretches to a great size, increasing at the same time in thickness, and 

 usually subsequently breaks, as shown in Fig. 300 C, unrolling into a straight or vermi- 

 form linear body, and bearing the alternating compartments of the sporocarp in con- 

 tiguous pairs. Each of these compartments contains on its outer side the projecting 

 ridge on which the sporangia grow {D, E); both the macrospores and microspores 

 escape from the sporangia and from the sac of the sporocarp which envelopes them ; 



Fir.. 300. — Marsilea salvatrjx; A a sporocarp 

 (natural size), st the upper part of its pedicel ; B a 

 sporocarp which has burst in water and is protruding' 

 its gelatinous ring (after Hanstein) ; C the gelatinous 

 ring g- ruptured and extended, sr compartments of 

 the sporocarp ; sch testa of the sporocarp ; D a com- 

 partment of an unripe sporocarp with its sporangia ; 

 E one from a ripe sporocarp; ■>ni microsporangia, 

 ma macrosporangia. 



