OEGANOGEAPHY. 53 



defined, but in many cases the receptacles are so very close to- 

 gether that one sorus runs into another (confluent}, or sometimes 

 the receptacles themselves are joined and form a more or less 

 perfectly united simple sorus, or when not perfectly joined (as in 

 Cryptogramme and Platyloma) a compound linear sorus. 



4. Indusium. 



As stated above, the sori of some ferns are naked while those of 

 others are furnished with a kind of cover, to which the name 

 indusium is given by some authors, and involucre by others. 



The indusia present many well-marked forms, and often afford 

 valuable characters for distinguishing genera, though they are by 

 no means constant. Three kinds are distinguishable : special, 

 accessory, and universal. 



True or special indusia are of a cellular membranous nature, and 

 are produced from the receptacles to which they are attached in 

 iifferent ways. In some cases they are in the form of an orbicular 

 iisk, and then rise from the centres of the receptacles to which 

 they are attached by their own centres, their edges being free all 

 round ; this form is called peltate or central (plate I. fig. 7). More 

 frequently, however, the indusia are more or less elongated, and 

 ire then attached to the sides of the receptacles (lateral) (plate I. 

 ig. 8). In this case their attachment is either on the side next the 

 josta (interior), or on that next to or at the margin (exterior), and 

 s either by a point or sinus on their side, in which case their form 

 varies from reniform to oval and oblong, or it is by the entire length 

 )f one side, when they are linear (110). Their surface is flat (plane), 

 irched (vaulted), or hood-like (cucullate), and their edges are either 

 entire or variously laciniated or fringed. 



Besides these two modes of attachment, there is a third kind 

 >vhere the indusia are attached all round the base of the receptacle, 

 md they are at first globose and entire, but ultimately their apex 

 >pens, and then they assume a cup-like (calyciform) form with the 

 nargin more or less entire (plate I. fig. 9) ; sometimes the attachment 

 s only half round the receptacle (semi-calyciform). 



Accessory indusia, sometimes in addition to the true indusia, 

 x>rtions of the margin of the frond are changed in texture and 

 'orm, what are here termed accessory indusia, and which resemble 

 ;he true indusia in appearance. These connive more or less with 



