RHIZOGABPE^. 



381 



are cosmopolitan. All told, there are probably not mora than eigliteen 

 or twenty distinct species, of which we have six within the limits of 

 the United States. 



A species of OpMoglosi,um has been discovered in the Tertiary strata. 

 Order Khizocarpese, the Pepperworts. The prothallia of the 

 Rhizocarps are dicEcious, and are developed 

 from two kinds of spores (the mao ospores and 

 microspores, to be more particularly described 

 below). The antheridia are simple, and con- 

 sist of small, few-celled outgrowths from the 

 germinating microspore (in Salvinia, and Azol- 

 la), or of the transformed contents of the mi- 

 crospore (in Mardlia and PUularia, Fig. 271, 

 B). The spermatozoids are spirally coiled, and 

 in the two last-named genera are produced in 

 definite numbers (tliiny-two) in each antherid- 

 ium. The prothallia which produce archego- 

 nia are small, and barely attain a size large 

 enough to protrude through the ruptured 

 wall of the macrospore (p, p, Fig. 271, A). 

 Tlie archegonia resemble those of true Ferns, 

 but are more sunken in the tissues of the pro- 

 thallia (Fig. 371, A and G). After fertilii^atioa 

 the germ-cell undergoeb division, and give* 

 rise directly to a leafy stemmed plant, ths 

 asexual generation, provided with roots (ex- 

 cept in JSalvin- 

 ia). The stem 

 is horizontal, 

 and floats upon 

 the water or 

 rung through 

 the mud at the 

 bottom of shal- 

 low water. The 

 leaves are cir- 

 cinately devel- 

 oped, and are 

 simple or quad- 

 rifid (Fig. 373). 

 The stem and 

 develop 

 an apical 

 which is 



Fig. 272. 

 . 272.— Plant of Marailla salvatria. 



Fig. 273. 



7r, apex of the etem ; 

 from ihe petioles at x. 



*, S, leaves ; /, /, f, the fruits spriiigmj 

 One half nat. size.— After Sachs. 



Fig. 27.3. — Longitudinal section through three fruits (the fer- 

 tile apices of a water-leaf) of f-alvlnia natans. i, i, two fruits 

 containing microsporangia ; a, one with macrosporangia. 

 —After Sachs. 



XIO. 



root 

 from 

 cell. 



two or three-sided in the stem, and triangular in the root. 



The sporangia, which are usually of two kinds, are produced in 

 "fruits" or receptacles, which are modified parts of leaves. These 



