456 



BOTANY 



PART II 



Pihdaria also grows in bogs and marshes. P. glolulifcra is found in 

 Britain. It differs from MarsiUa in its simple linear leaves, at the base of which 

 occur the spherical sporocarps, wliich arise singly from the base of each sterile 

 leaf-segment ; the sporocarp corresponds to a segment of the leaf (Fig. 402, B). 



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

 genera Salvinia and A-olla. In S'alvinia nalans, 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 filamentous segments 

 (Fig. 403, J). The sporangia are produced within the sporocarp from a column-like 



A 



Fin. 403.— Salvinia. ■nataiis. A, Seen from the side; B, from above (after Bischoff, reduced). (', 



An embryonic plant ; nif'p, macrospore ; /', prothalliiim ; a, stem ; bj, 6._. 

 leaves ; &l, the so-called sciitiform leaf. (After Prinosheim, X 15.) 



').■), the first three 



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. 



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 oi Xostoc. From tlie fact that hairs grow out of 

 tlie walls of the cavity between the algal filaments, the existence of a .symbiotic 

 relation between tlie 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 sj)herical, and produced usually in pairs on the under 

 side of the first leaf of some of the lateral branches. 



In the structure of the sjiorangia and spore.s, and in the development of the pro- 

 thallia, the Ilydropterideae dilfer in some resjjects from the Filices. These ditter- 



