PTERIDOPHYTA. 21 



green, finely papillose on the upper surface. [Name in honor of Antonio Maria 

 Salvini, 1633-1729, Italian scientist.] About 13 species of wide distribution. 



I. Salvinia natans (L.) Hoffm. (I. Y. f. 75.) Leaves oblong, rather thick, 

 obtuse or emarginate at the apex, rounded or cordate at the base, entire, 15-30 

 cm. long, pinnately veined, bright green and papillose above, the lower surface 

 densely matted with brown pellucid hairs; sporocarps 4-8 in a cluster, the upper 

 ones containing about 10 macrosporanges, each containing a few macrospores, the 

 remainder containing numerous microsporanges each with numerous microspores; 

 macrosporcs marked with 3 obtuse lobes which meet at the apex. Bois Brul 

 Bottoms, Perry Co., Mo., and near Minneapolis, Minn. Introduced into ponds 

 in S. E. N. Y. Reported by Pursh in 1814 from Cent. N. Y., but his exact station 

 is unknown. Also in Europe and Asia. 



a. AZOLLA Lam. 



Minute moss-like reddish or green floating plants, with pinnately branched 

 stems covered with minute imbricated 2-lobed leaves, and emitting rootlets beneath. 

 Sporocarps of two kinds borne in the axils of the leaves, the smaller ovoid or acorn- 

 shaped, containing a single macrospore at the base and a few corpuscles above it 

 whose character is not fully known, the larger globose, producing many pedicelled 

 sporanges, each containing several masses of microspores which are often beset 

 with a series of anchor-like processes of unknown function. [Greek, signifying 

 killed by drought.] About 5 species of wide geographic distribution. 



I. Azolla Caroltniana Willd. (I. F. f. 76.) Plants greenish or reddish, 2-4 

 mm. each way, deltoid or triangular-ovate, pinnately branching, sometimes covering 

 large surfaces of water. Macrospores minutely granulate, with three accessory 

 corpuscles ; masses of microspores armed with rigid septate processes ; leaves with 

 ovate lobes, their color varying somewhat with the amount of direct sunlight, the 

 lower usually reddish, the upper green with a reddish border. Floating on still 

 water, Ont. and W. N. Y. to Br. Col., south to Fla., Ariz, and Mex. 



Order 3. EQUISETALES. 



Rush-like perennial plants, with mostly hollow jointed simple or 

 often much-branched grooved stems, provided with a double series of 

 cavities and usually with a large central one, the branches verticillate, 

 the nodes provided with diaphragms. Rootstocks subterranean. Leaves 

 reduced to sheaths at the joints, the sheaths toothed. Sporanges i- 

 celled, clustered underneath the scales of terminal cone-like spikes. 

 Spores all of the same size and shape, furnished with 2 narrow strap-like 

 appendages attached at the middle, coiling around the spore when moist 

 and spreading, when dry and mature, in the form of a cross (elaters). 

 Epidermis impregnated with silica, rough. Prothallium on the surface 

 of the ground, green, usually dioecious. 



Family i. EQUISETACEAE Michx. 



Horsetail Family. 

 The family consists of the following genus : 



i. EQUISETUM L. HORSETAILS. SCOURING RUSHES. 



Characters of the order. [Name ancient, signifying horsetail, in allusion to 

 the copious branching of several species.] About 25 species, of very wide geo- 

 graphic distribution. 

 Stems annual ; stomata scattered. 



Stems of two kinds, the fertile appearing in early spring before the sterile. 



Fertile stems simple, soon withering; sheaths of branches of sterile stems 



4-toothed. i. E. arvense. 



Fertile stems branched when old, only the apex withering. 



Branches of the stem simple, their sheaths 3-toothed. 2. E. pratens^ 

 Branches compound 3 . E. sylvattcum. 



