THE FERN ALLIES. 



39 



E. THE RHIZOCARPS. 



1O2. General Characters. The Rhizocarps, so-called 

 from the fruit being produced at or near the roots in some 

 genera, include aquatic plants of very diverse characters. 

 Some, like Marsilia, root in mud and produce long-petioled 

 leaves resembling " four-leaf clovers" which sometimes float on 

 the surface of water ; more often the plant will be found grow- 

 ing in wet or marshy places, with more rigid stipes. Others, 

 like Pilularia, resemble the sterile conditions of EUocharis or 

 other sedges with a habit similar to Marsilia. Others, like 

 Azolla or Salvinia, float on the surface of water, and consist of 

 a leafy axis, sending numerous roots deep into the water. In 

 Azolla the leaves are minute and lobed ; in Salvinia (Fig. 30) 

 they are larger and simple. Marsilia and Pilulana have cir- 

 cinate vernation. 



1 O3. Fructification. The fruit of Marsilia consists of a 

 hollow-stalked receptacle known as 

 the sporocarp, which is oblong or 

 rarely globose, and bears the spo- 

 rangia in sori on the inner walls of 

 its two valves. The spores are of 

 two kinds, as in all rhizocarps. The 

 numerous microspores are con- 

 tained in microsporangia, while the 

 macrospores are solitary in the few 

 macrosporangia. 



1 04. The sporocarp of Pilu- 

 Iraia is globose, containing from 

 two to four cells, which produce 

 microsporangia in the upper portion 

 and macrosporangia below ; the 

 microspores are numerous, while a 

 single macrospore is found in each 

 sporangium. 



1 O5. In Azolla the sporocarps 

 are of two kinds, borne in the axils 

 of the leaves ; the larger are glo- 

 bose, and contain numerous microspores, which are aggregated 



FIG. 30. Salvinia natans 

 Hoffm., natural size. (Re- 

 drawn from Thome".) 



