GROUPS OF PLANTS. 



63 



(3) There is also a group of ferns known as Water Ferns 

 which are aquatic in habit, that is, they live in marshy places or 

 float on water. As representatives of this group may be men- 

 tioned Marsilia,, which has a slender rhizome that is buried in the 

 muddy bottom of streams, and 4-parted, clover-like leaves that 

 float on the water ; and Salvinia ( Fig. 40) which is a small float- 

 ing plant that develops two kinds of leaves, one which floats on 

 the surface of the water and are more or less oblojig, and another 

 which are filiform, branching, root-like and submerged. The 

 water ferns are further distinguished by the production of mega- 

 spores and microspores. 



(4) The Adder's Tongue Family, to which Ophioglossum 

 and Botrychium belong, develops a subterranean prothallus 

 which is destitute of chlorophyll. The prothallus is in some cases 



Fig. 39. Some fern spores. A, B, C, different views of the bilateral spores of the 

 common polypody (Polypodium vulgare) showing outer wall (ep), middle wall (ex), inner 

 wall (end) and line of dehiscence -^dl) ; D, a tetrahedral spore of the royal fern {Osmunda 

 regalis); E, P, spores of Ceratopteris thalictroides seen in two views. — A-D, after Sadebeck; 

 E-P, after Kny. 



tuberous, and the sporophyte produces two kinds of leaves, 

 namely, foliage leaves, and fertile leaves or those which bear the 

 sporangia. The sporangia occur on lateral branches of the sporo- 

 phyll and open at maturity by means of a horizontal slit. 



Ferns Used in Medicine and as Foods. — Many of the ferns 

 contain tannin, a brownish coloring principle apd in addition 

 an anthelmintic principle. They may also contain ethereal oils, 

 starch, coumarin, aconitic acid and other principles. A large 

 number have been used in medicine, of which the following may 

 be mentioned : Aspidium (Dryopteris or Nephrodium) marginalis 

 and A. Filix mas, yielding the official Aspidium (Fig. 277). A 

 number of other species of Aspidium, as well as species of Adian- 

 tum, Asplenium and Polypodium are also used in various parts of 

 the world. The rhizomes of some of the ferns contain considerable 



