710 



THE SUBDIVISIONS OF THE VEGETABLE KINGDOM. 



big oval macrospore; the microspores are produced in numbers in the microspor- 

 angia. The macrospore, in addition to a hard wall, possesses an external gelatinous 

 layer with stratified structure. On germination, the macrospore develops its 

 (female) prothallium at one end, and, on the bursting of the spore-wall at the apex, 

 tills prothallium and the archegonia developed on its surface, are exposed. Of the 

 contents of the spore, only a portion forms the prothallium, the rest remaining as a 

 reserve mass. The microspores, which are usually embedded in mucilage, undergo 

 several divisions (forming antheridia), and liberate spermatozoids, which fertilize 



Fig. 402.— Hj-dropterides. 



' Salvinia natans, showing the floating and submerged leaves. ~ A portion of the same seen from the side, and showing the 

 sporocarps at the base of the submerged leaves, s a section through two sporocarps of Salvinia. nalajis; that to the left 

 contains macrosporangia only, that to the right microsporangia. * Pilularia globtUifera; one or two sporocarps are 

 shown at the base of the needle-like leaves. * Marsilia quadn/olia, showing sporocarps. » x 4 ; the rest, natural Bize. 

 (After Luersseo.) 



the archegonia. From one of the archegonia the young sporophyte arises, and 

 gradually develops into the adult form. 



Salviniacece. — Include two genera, Salvinia and AzoUa, both of which are 

 floating plants. The former occurs in Southern Europe, the latter, though hardy 

 in Europe, is a native of America, Australia, &c. Salvinia (figs. 402 ^ and 402 -) 

 has a stem which lies horizontally on the water, and develops both floating and 

 submerged leaves. The latter divide into numerous filaments, which hang down 

 like tassels in the water (c/. fig. 402 -). They are the absorptive organs of the 

 plant, and play the part of roots. True roots, however, are wanting, even from the 

 embryo. The sporangia are borne on these submerged leaves close to the point 

 of attachment to the horizontal stem, generally in three groups or sori, each of 

 which is inclosed in a cup-like upgrowth arising from the submerged leaf (c/. fig. 



