J. SMITH ON THE GENERA OF FERNS. 41 



gradations of structure, and consequently due allowance must 

 be made for all seeming misapplications of terms. 



DEFINITIONS. 



Ferns are flowerless plants, consisting of leafy fronds which 

 are produced from a rhizoma^ unfolding in a spiral manner, 

 and traversed by veins, which, from definite parts on the under 

 surface, produce unilocular, rarely multilocular cases (js/jo- 

 rangia), containing reproductive sporules. 



The rhizoma is a rootlike creeping or csespitose caudex, 

 emitting descending fibrous radicles, and ascending fronds, 

 often becoming an arboreous, usually hollow, cylindrical trunk. 



The fronds are of various forms, dimensions, and textures, 

 sessile or stipitate, and either attached to the rhizoma, by a 

 distinct lateral articulation, or decurrent, persistent, and ter- 

 minal ; their united bases forming the progressing rhizoma. 

 They vary from linear-lanceolate to deltoid, and from less 

 than an inch to 15 or 20 feet in length, and from simple, 

 entire, to once or many times pinnate and multifid ; each 

 ultimate pinna, or segment, being analogous in its parts to a 

 simple frond. Their surface is either smooth, or furnished 

 with different kinds of glands, hairs, or membraneous scales, 

 the fertile or sporangiferous fronds or segments often contract- 

 ed and differing in aspect from the sterile. 



The veins are either produced equally from both sides of 

 a midrib (costa) {Jig. 1, 2, 3, 4, &c.), or radiate from the 

 base or axis of development, the midrib being evanescent ifg' 

 3.); or from one side of an eccentric, or unilateral costa {Jig. 

 16.) They are either simple {Jig. 1.), or once or repeatedly, 

 dichotomously branched (forked) {Jig. 2, 3, 5, 6, 7, 8.); or 

 the primary veins are costaform and pinnate, {fig. 4, 8, 9, 10, 

 11.), the branches (fcnw/es) either simple or forked {fig. 4. a. 6.) 

 Their apices are either free and terminating at or within the 

 margin {direct) {fig. \. a. 2, 3, 4, 5.), or the whole combined 

 by a continuous anastomosing vein which runs parallel with, 

 and close to the margin {transverse marginal) {fig. 1. b. 6.); 

 or, the apices of the venules of each fascicle anastomose with 



Vol. IV No. 25. F 



