MICROSCOPIC STRUCTURE OF HIGHER CRYPTOGAMIA. 



345- 



resemblance of the regular markings on their walls to the rungs of a 

 ladder. 



341. What is usually considered i\iQ fructification of the Ferns affords 

 a most beautiful and readily-prepared class of opaque objects for the 

 lower powers of the Microscope; nothing more being necessary than to 

 lay a fragment of the frond that bears it upon the glass Stage-plate, or 

 to hold it in the Stage-forceps, and to throw an adequate light upon it 

 by the Side-condenser. It usually presents itself in the form of isolated 

 spots on the under surface of the frond, termed sori, as in the common 

 Polypodium (Fig. 227), and in the Aspidium (Fig. 229): but sometimes 

 these ' sori ' are elongated into bands, as in the common Scolopendrum 

 (hart's tongue); and these may coalesce with each other, so as almost to 

 cover the surface of the frond with a network, as in Hcemionitis (Fig. 

 228); or they may form merely a single band along its borders, as in the 

 common Pteris '(brake-fern). The sori are sometimes 'naked' on the 

 under surface of the fronds; but they are frequently covered with a deli- 

 cate membrane termed the indusiwn, which may either form a sort of 

 cap upon the summit of each sorus, as in Aspidium (Fig. 229), or a long: 

 fold, as in Scolopendrum and Pteris; or a sort of cup, as in Deparia (Fig. 



FIG. 229 



EIG. 230. 



Sorus and Indusium of Aspidium. 



Sorus and cup-shaped Indusium of 

 Deparia prolifera. 



230). Each of these sori, when sufficiently magnified, is found to be- 

 made up of a multitude of thecm or spore-capsules (Figs. 229, 230), 

 which are sometimes closely attached to the surface of the frond, but more 

 commonly spring from it by a pedicle or footstalk. The wall of the 

 thcca 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 pedicle, and is continued over the summit of the 

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

 (Fig. 230). 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 the two halves of 

 the capsule are carried widely apart from each other, the fissures extend- 

 ing to such a depth as to separate them completely. In Osmunda (the 

 so-called ' flowering-fern ') and Ophioglossum (adder's tongue), the thecae 

 have no annulus. 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 



