PTEBIDOPHYTA— HYDEOPTEEIDE^ 



143 



Azolla bears numerous adventitious roots ; Salvinia is altogether 

 rootless. 



In Salvinia the phyllotaxis is whorled, three leaves being 

 produced at each node. Of these, two are broad, somewhat 

 rugose, entire floating leaves, placed opposite each other. The 

 third is divided mto a number of filamentous branches which 

 hang down freely into the water (fig. 900) and function as roots. 

 In AzoUa the leaves are alternate and are arranged in two 

 rows on the upper surface of the rhizome. Each leaf is two- 

 lobed, one lobe floating while the other is submerged. Pilularia 

 and Marsilea have perennial rhizomes and are attached to the 

 substratum. They are found in bogs or marshes. From the 



Fio. 900. 



Fig. 900. Part of a plant of Salviuia. ?, Ploating leaves, w. Submerged 

 leaves, s. Sori. k. Apex of stem. B. Longitudinal section tbroughthree 

 sori of Salvinia. i, i. Two "with microsporangia. a. One with macro- 

 sporangia. After Sachs. 



rhizome the leaves grow vertically upwards, being arranged 

 alternately on the stem. In the former genus the leaf is un- 

 branched and somewhat cylindrical ; in the latter it is compound, 

 bearing four leaflets at the end of a long petiole [figs. 901, 903). 



Both PUularia and Marsilea produce adventitious roots from 

 the rhizome. 



In Salvinia and AzoUa the stemismonostelic, the stele being of 

 very small dimensions. It is not furnished with a pericycle, but 

 is surrounded by a two-layered endodermis. In the other genera 

 the stem is at first polystelic, but eventually becomes gamostelic 

 from the fusion of the separate steles to foim a ring [fig. 902). 

 This ring encloses some fundamental tissue which simulates a 



