RHIZOVABPE^. 



381 



are cosmopolitan. All told, there are probably not more tban eigliteea 

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

 the United States. 

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

 Order Bhizocarpese, the Pepperworts. The prothallia of the 

 Rhizocarps are diceciouri, and are developed 

 from two kinds of spores (the iniic oapores and 

 microspores, to be more j-articularly described 

 belowV The antheridia are simple, and con- 

 sist of small, few-celled nutgrowths from the 

 germinating microspore (in Salmnia and Azol- 

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

 crospore (in Marsilia and Pilularia, Fig. 371, 

 B). The spermatozoids are spirally coiled, and 

 in the two last-named genera are produced in 

 definite numbers (tliiriy-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. 371, A). 

 The archegonia resemble those of true Ferns, 

 but are more sunken in the tissues of the pro- 

 thallia (Fig. 371, A and G). After fertilization 

 the germ-cell undergoes division, and gives 

 rise directly to a leafy stemmed plant, the 

 asexual generation, provided with roots (ex- 

 cept in Salmn- 

 ia). The stem 

 is horizontal, 

 and floats upon 

 the water or 

 runs through 

 the mud at the 

 bottom of shal- 

 low water. Tlie 

 leaves are cir- 

 cinately devel- 

 oped, and are 

 simple OB-quad- 

 rifid (Fig. 373). 

 The stem and 

 root develop 

 from an apical 

 cell, whicli is 

 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 



FiQ. 373. 



Pio. 27.3. 



Fig. 272.— Plant of Marsilia aaivatrUe. 

 b, J, leaves ; f,f 



K, apex of the Ptem ; 

 f, the fruits springing from ihe petioles <sX v. 

 One half nar.'rfze.— After Sachs. , ■ ,,^ , 



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

 tile apices of a water-leaf) of taMnia nutans, i, i, two fruits 

 containing inicrosporangia ; o, one with macrosporangia. X 10. 

 —After Sachs. 



