VIU I]SITRODUCTION. 



traversed by veins. From the under surface of these veins are 

 produced, mostly unilocular, although occasionally multilocular, 

 cases, termed sporangia, which contain the reproductive sporules, 

 or seeds. 



The rhizoma is a root-like creeping, or coespitose caudex, 

 from the base of which, root fibres descend and fronds ascend. 



The fronds are very various in form, texture, and dimensions. 

 They are sessile or stipitate, being attached to the rhizoma by 

 a lateral articulation; or they are decurrent, persistent, and 

 terminal, the united bases of which produce the progressing 

 rhizoma. 



From the immense number of species, it is not remarkable 

 that so much variation in form and size occurs; being linear- 

 lanceolate to deltoid, and from under an inch to twenty feet in 

 length, and from a simple and entire frond to once or many 

 times pinnate and multifid; each ultimate pinna or segment 

 being similar in its parts to a simple frond. The surface of 

 the fronds have sometimes scattered over them hairs, glands, 

 or membranous scales. Frequently the barren and fruitful 

 fronds differ, the latter being often contracted, and differing in 

 appearance. 



The veins of the fronds differ very considerably in different 

 tribes. First, they are produced equally from each side of a 

 midrib, (costa.) Secondly, they radiate from the axis of devel- 

 opment, the costa being evanescent. Thirdly, they are developed 

 from one side of an eccentric or unilateral costa. They are 

 either as a simple line, or are once or repeatedly branched 

 dichotomously; or the primary or principal veins are costoeform 

 and pinnate. The branches to these principal veins, termed 

 venules, being either simple or branched, and the ends or 

 apices of these venules are either free, and terminate at or 

 within the margin, in which case they are called direct; or all 

 their apices are joined together by a continuous anastomosing 

 vein, which runs parallel with, and close to, the margin, when 

 it is termed transverse marginal; or, the ends of the venules of 

 each fascicle anastomose with the ends of the proximate fascicle, 

 either angularly or arcuated, and produce on the points where they 

 meet, (or exterior side,) one or more outwardly directed (excur- 

 rent,) tertiary veins, (veinlets,) which are either free, termina- 

 ting in the areoles or margin, or anastomose in the angular 



