354 MICROSCOPIC STRUCTURE OF HIGHER CRYPTOGAMIA. 



but they are frequently covered with a delicate membrane, 

 termed the indusium, which may either form a sort of cap upon 

 the summit of each sorus, as in Aspidium (Fig. 143), or a long 

 fold, as in Scolopendrum and Pteris, or a sort of cup, as in De- 

 paria (Fig. 144). Each of these sori, when sufficiently magnified, 

 is found to be made up of a multitude of capsules or thecce (Figs. 

 143, 144), which are sometimes closely attached to the surface of 

 the leaf, but more commonly spring from it by a pedicel or foot- 

 stalk. The wall of the capsule is composed of flattened cells, 

 applied to each other by their edges ; but there is generally one 

 row of these, thicker and larger than the rest, which springs from 

 the pedicel, and is continued over the summit of the capsule, so 

 as to form a projecting ring, which is known as the annulus. 

 This ring has an elasticity superior to that of all the rest of the 

 capsular wall, causing it to split across, when mature, so that the 

 contained spores may escape ; and in many instances carrying 

 the two halves of the capsule widely apart from each other (Fig. 

 141), the fissure extending to such a depth as to separate them 

 completely. It will frequently happen, that specimens of Fern- 

 fructification gathered for the Microscope, will be found to have 

 all the capsules burst and the spores dispersed, whilst in others, 

 less advanced, the capsules may all be closed ; others, however, 

 may often be met with, in which some of the capsules are closed 

 and others are open ; and if these be watched with sufficient at- 

 tention, the rupture of some of the thecse and the dispersion of 

 the spores may be observed to take place, whilst the specimen is 

 Tinder observation in the field of the microscope. In sori whose 

 capsules have all burst, the annual connecting their two halves 

 are the most conspicuous objects, looking, when a strong light is 

 thrown upon them, like strongly banded worms of a bright 

 brown hue. This is particularly the case in Scolopendrum, 



Fig. 143. 



Fig. 14A. 



Soras and iiidasium of Asjtidlum. 



Sorus and cup-shaped indusium of Depariaprolifera. 



whose elongated sori are remarkably beautiful objects for the 

 microscope in all their stages ; until quite mature, however, they 

 need to be brought into view by turning back the two indusial 

 folds that cover them. The commonest Ferns, indeed, which 

 are found in almost every hedge, furnish objects of no less beauty 



