CHAPTER IX 



PTERIDOPHYTA (Concluded) 



ORDER HYDROPTERIDINE^E 



THERE are two families of heterosporous Ferns, which although 

 not closely related to each other, are evidently allied to the other 

 leptosporangiate Ferns. These have been put together in the order 

 Hydropteridinese, or Water-ferns, as they are all aquatics. 



They agree in the general characters of the sporangia, and in pro- 

 ducing a single very large macrospore in each macrosporangium. 

 The Hydropteridineae fall into the two very natural families, Salvi- 

 niaceae and Marsiliaceae. 



Family i. Salviniaceae 



The Salviniacese are small floating plants which show very little 

 superficial resemblance to the Filices, from which they have been 

 derived. Their inner structure, however, and the development of 

 the sporangium are very much like those of the typical Ferns. The 

 character of the sporangium and its position suggest the Hymeno- 

 phyllaceee, to which the Salviniacese may possibly be remotely 

 related. 



There are two genera, Salvinia and Azolla. The former is repre- 

 sented in a few places in the United States by the European species, 

 S. natans (Fig. 249, D, E), but there is some question whether it is 

 really indigenous. Azolla is represented in the Atlantic states by 

 A. Caroliniana, and on the Pacific coast by the larger A. filiculoides 

 (Fig. 249), both species extending into South America. A third 

 species, A. pinnata, has been introduced in some places, with the 

 Japanese Lotus. The sporophyte in both genera has a slender 

 horizontal stem, floating upon the surface of quiet water. Two or 

 four rows of dorsal leaves quite conceal the stem. The dorsal leaves 

 in Salvinia are oval ; in Azolla, each leaf has two lobes, dorsal and 

 ventral. Salvinia, which is quite destitute of roots, produces upon 

 the ventral side of the stem two rows of leaves which are divided 

 into many slender, rootlike segments, functionally replacing the 

 true roots (Fig. 249, D, I). In Azolla roots are developed. In 

 both genera more or less conspicuous hairs are found upon the leaves. 



Lateral branches are freely produced, and by the detachment of 



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