452 



BOTANY 



PART II 



simple linear leaves, at the base of which occur the spherical sporocarps, which 

 uri.se singly from the base of each sterile leaf-segment (Fig. 1 1 1 1. 



Salviniaceae. This family 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 pendent, 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 fila- 

 mentous segments (Fig. 415 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 



FIG. 415. Salvinia natans. A, Seen from the side; JB, from above (after HISCHOKK, inluciii) ; f, 

 an embryonic plant ; msp, macrospore ; )>, prothallium ; a, stem ; In, I/.,, !>.., tin- liist tlirci- 

 leaves ; bj, the so-called sctitiform leaf. (After PRIN(;SHEIM, x 15.) 



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. 



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 'an Alga (Anabaena). 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 true 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 first leaf 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 many respects from the Filices. These differ- 

 ences may be best understood on reference to Salvinia natans ( 114 ) as an example. 

 The sporocarps contain either numerous microsporaugia or a smaller number of 



