294 



PLANT MORPHOLOGY 



ops from a tetrahedral apical cell that cuts off three longitudinal rows of 

 segments. The leaves arise from the two dorsal rows, the roots from 

 the ventral row. The rhizome of Marsilea is an amphiphloic siphonostele 

 without secondary thickening (Fig. 249). The rhizome of Pilularia is 

 similar except that the vascular tissues are reduced. 



Fig. 249. Cross section of the rhizome of Marsilea quadrifolia, an amphiphloic sipho- 

 nostele, X50. 



Sporocarp. The sporangia of the Marsileaceae are borne in special 

 structures, called sporocarps, which occur on long or short stalks arising 

 adaxially from the petiole of the leaf. They are usually borne singly, but 

 in some species of Marsilea several or even many may be borne together. 

 The sporocarp is a specialized leaf segment enclosing a group of sori. In 

 Marsilea it is an ovoid or bean-shaped structure with a hard outer cover- 

 ing (Fig. 251). It contains 14 to 20 sori. In Pilularia the sporocarp is 

 spherical and contains 2 to 4 sori. In both genera the sori are arranged 

 in two rows. Each is surrounded by an indusium and contains both 

 microsporangia and megasporangia. The sporangium wall is only one 

 layer of cells in thickness, as in the Filicales, but is without an annulus. 



The sporangia are leptosporangiate in development. Four cavities 



