INTRODUCTION. IX 



junction of the next superior pair of venules; or all the 

 venations are nearly uniform and combined, forming equal or 

 unequal sided areoles, with all the veins joined together, when 

 they are termed reticulated; or they are irregularly joined, 

 whilst at the same time the areoles have produced from their 

 sides, excurrent or recurrent, simple branched or brachiate, 

 straight or uncinate, free or conniving veins, which terminate 

 in the unequal sided areoles, and are called compound anas- 

 tomosing. When the veinlets are superficial and distinct, plane 

 or elevated, they are termed external, but when buried in the 

 frond itself, and then frequently indistinct, internal. 



THE REPRODUCTIVE ORGANS. 



The sporangiferous receptacle is a thickened point or length- 

 ened portion of the ultimate venules; when it is formed on their 

 apex, it is called terminal; when between their base and apex, 

 medial; when on the point of the forking, axillary; when on 

 the angular crossings, or points of confluence of two or more 

 venules, compital; when a portion or the whole of the disc of 

 the frond is changed in texture, and occupied closely by the 

 sporangia, amorphous. Mostly superficial, sometimes immersed, 

 (internal,) or considerably elevated and then globose or columnar. 



The sporangia are globose, oval, or pyriform, transparent 

 unilocular cases, and each is surrounded by an elastic articu- 

 lated ring, or without a ring, then sometimes oblong opaque, and 

 multilocular. They are mostly pedicellate, very rarely sessile; 

 are produced in crowded masses termed sori, on each recep- 

 tacle, very seldom solitary or few; sometimes pilose, and a 

 number frequently abortive and deformed. 



The sori are collections of sporangia, and have the same 

 form, position, and dimension as the receptacles, consequently 

 are round, elongated, or amorphous, and are placed on or close 

 to the margin of the frond or segment, (called marginal,) or 

 between the margin and midrib, (intramarginal,) or close to 

 the midrib, (costal;) when elongated, are either oblong or linear, 

 making a continuous or broken line, forming an angle with 

 the midrib or margin, (transverse.) 'ihe sori are each furnished 

 Avith a membranaceous covering, called an indusium, which 

 rises from the receptacle, and is of various forms; or the sori 

 are naked, that is without an indusium. 



