INTRODUCTION. 



The indusium is a plane, or vaulted, or cup-shaped membrane, 

 produced from the receptacle of each sorus, called special, and 

 generally falls off (deciduous,) as it becomes replicate. When 

 produced from the centre of the receptacle, it is central; 

 when on one side, lateral; and when its base is attached all 

 round the base or margin, calyciform. When central it is usually 

 orbicular, with its margin depressed and free all round, or it 

 is inflated. AVhen lateral, it is either reniform, oval, or oblong, 

 and attached by a point or base of one side, or linear, and its 

 whole length fastened on the one side of an elongated recep- 

 tacle, the other side being free. When calyciform, it is entire 

 and globose; but after a time it opens with an entire or laci- 

 niated margin, forming a cup which contains the sporangia. 

 Frequently all the margin or lobules of the frond are changed. 

 in texture, forming an accessary indusium, with which the 

 interior lateral attached special indusium more or less connives; 

 united, they form a vertical or reflexed, continuous or urceolate 

 calyciform, or bilabiate marginal cyst, which contains the spo- 

 rangia, and opens exteriorly. Sometimes the whole of the sori 

 of each segment are inclosed within a universal indusium, 

 which is formed by the revolute margin of fertile contracted 

 fronds. 



E. J. LOWE. 



Ohservatory, Beeston, May 16th., 1855. 



