156 PLANT-LIFE 



saprophytic and inhabited by the hyphse of a Fungus, 

 which evidently contributes to the sustenance of the 

 plant. 



Hydropterides — Water Ferns. 



These are more or less aquatic plants, allied to the 

 Ferns, occurring either in water or in marshy places. 

 They are heterosporous ; the megaspores are borne singly 

 in megasporangia, and the microspores are produced 

 in quantities in microsporangia. The sporangia are 

 not found on the under surfaces of fronds, but occur at 

 their bases in special receptacles. The megaspores de- 

 velop into female prothalli, whereas the microspores 

 produce tiny, few-celled male prothalli from which 

 spermatozoids are liberated. There are two families — 

 the Marsiliacece, with two genera, Marsilia and Pilu- 

 laria ; and the Salviniacece, also with two genera, Sal- 

 vinia and Azolla. The Salviniaceae embrace nine species ; 

 Salvina occurs in Southern Europe; it has a horizontal 

 stem, which lies on the surface of water, and to which 

 are appended floating and submerged leaves; the latter 

 are filamentous, and hang down in the water; they 

 absorb nutrient substances in solution, and are substi- 

 tutes for roots. The sporangia lie on the submerged 

 leaves, at their bases. Azolla is mostly tropical. Of the 

 Marsiliacese there are thirty-five species, thirty-two 

 being embraced by the genus Marsilia. The genus Pilu- 

 laria is represented in Britain by one species, P. globu- 

 lifera, the Pillwort, or Pepper-Grass. It is a small 

 creeping plant with grassy leaves, occurring in marshy 

 ground and in the shallow margins of lakes and pools. 

 The stem is slender, almost threadlike; it sends down 



