ORGANOGRAPHY. 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 

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

 disk, 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 

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

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

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

 is 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 

 of oneside, when they are linear (110). Their surface is flat (plane), 

 arched (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 

 where the indusia are attached all round the base of the receptacle, 

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

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

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

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



Accessory indusia, sometimes in addition to the true indusia, 

 portions of the margin of the frond are changed in texture and 

 form, what are here termed accessory indusia, and which resemble 

 the true indusia in appearance. These connive more or less with 



