18 ON GENERA AND SPECIE?. 



INDUSIUM. 



The sori are either naked or furnished with a special 

 covering called the indusium, or involucrum by some, which 

 presents many well marked different forms, and is highly 

 important for distinguishing genera ; it consists of three 

 kinds special, accessory, and universal. True or special 

 indusia are more or less membranous, or thick and firm 

 in texture, and rise from the receptacles to which they are 

 attached in different ways. Some rise in the form of an 

 orbicular disc, covering the sorus in the form of a shield 

 (peltate) ; more frequently they are attached on the interior 

 or exterior side of the receptacle ; such are termed lateral, 

 which open like a lid (operculiform) when attached on the 

 side next the costa, interior, and when on the side next or 

 on the margin exterior. 



Lateral indusia partake of the form of the sori, being 

 round, oblong, or linear ; in punctiform sori they are 

 attached by a point, or more or less by the base or sides, 

 and are of various forms, being reniform, subrotund, oval, 

 or oblong, sometimes very small (squamiform). Their 

 surface is flat> vaulted, or when attached by their sides 

 cucculate, their free margin being entire or variously 

 lacinated or fringed. 



Besides these two modes of attachment there is a third 

 kind, in which the indusium is attached all round the recep- 

 tacle it is at first globose, and includes the sporangia, 

 but ultimately their apex opens and widens, thus assuming 

 the form of a cup (calyciform), the margin being more or less 

 entire or deeply laciniated, even to be like a fringe ; some- 

 times the attachment is only half round the receptacle 

 (semicalyciform) , 



