408 



BOTANY 



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 that 

 of the Marsiliaceae ; they are spherical, and are borne in small groups 

 on the submerged leaves at the base of the filamentous segments 

 (Fig. 342). 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, 



A 



Wt , ..; 



P I'* 



H 



Fig. 342.— Salvinia natans. A, Seen from the side ; B, from above (after Bischoffj reduced) ; V, 

 an embryonic plant ; msp, macrospore ; p, prothallium ; a, stem ; b 1( b-2, &3, the first three 

 leaves ; b ly the so-called scutiform leaf. (After Pringsheim, x 15.) 



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. 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 rela- 

 tion between the two plants would seem to be indicated. Unlike 

 Salvinia, Azolla 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. 



