DIV. I 



PTERIDOPHYTA 



513 



The Salviniaceae contains only free-floating aquatic plants belonging to the two 

 genera Salvinia and Azolla. In Salvinia natans, as representative of the first genus, 

 the sparingly-branched stem gives rise to three leaves at each node. The two 

 upper leaves of each whorl are oval in shape, and developed as floating foliage 

 leaves ; the third, on the other hand, is submerged, and consists of a number of 

 pendant, filamentous segments which are densely covered with hairs, and assume 

 the functions of the missing roots. The sporocarps have an entirely different mode 

 of development from those of the Marsiliaceae ; they are spherical, and are borne 

 in small groups on the submerged leaves at the base of the filamentous segments 

 (Fig. 478 A). The sporangia are produced within the sporocarp from a column-like 

 receptacle, which corresponds in origin to a modified leaf-segment. The envelope 

 of the sporocarp is equivalent to an indusium ; it arises as a new growth in the 

 form of an annular wall, which is at first cup-shaped, but ultimately closes over 

 the receptacle and its sorus of sporangia. 



c 



FIG. 478. Salvinia natans. A, Seen from the side ; B, from above (after BISCHOFF, reduced). C, 

 An embryonic plant ; msp, macrospore ; p, prothallium ; a, stem ; 6j, 6 2 , 63, the first three 

 leaves ; & 1} the so-called scutiform leaf, (x 15. After PRINGSHEIM.) 



The second genus, Azolla, is chiefly tropical, represented by small floating 

 plants, profusely branched, and beset with two-ranked closely crowded leaves. 

 Each leaf consists of two lobes, of which the upper floats on the surface of the 

 water, while the lower is submerged, and assists in the absorption of water. A 

 small cavity enclosed within the upper lobe, with a narrow orifice opening outwards, 

 is always inhabited by filaments of the Blue Green Alga, Anabaena azollae. From 

 the fact that hairs grow out of the walls of the cavity between the algal filaments, 

 the existence of a symbiotic relation between the two plants would seem to be 

 indicated. Azolla, unlike Salvinia, possesses long slender roots developed from 

 the under side of the stem. The sporocarps are nearly spherical, and produced 

 usually in pairs on the under side of the leaves of some of the lateral branches. 



In the structure of the sporangia and spores, and in the development of the pro- 

 thallia, the Hydropterideae differ in some respects from the Filices. These differ- 

 ences may be best understood on reference to Salvinia natans ( 121 ) as an example. 

 The sporocarps contain either numerous microsporangia or a smaller number of 

 macros porangia (Fig. 479 A, ma, mi}. In structure both forms of sporangia 

 resemble the sporangia of the Leptosporangiate Ferns ; they are stalked, and have, 



2L 



