STRUCTURE OF FERNS 



675 



have become very hard and of a deep brown colour. These scleren- 

 chymatous sheaths are a very conspicuous feature in a transver-M 

 section of the stem or rhizome of most ferns, and are the principal 

 agent in giving it strength and solidity. 



What" is usually termed the fructification of the fern 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 Poli/podium (fig. 516), and in A*i>i<liinii (fig. 

 5 IS); but sometimes these sori are elongated into bands, as in 



FIG. 516. Leaflet of Poly- FIG. 517. Portion of frond of Hcemionitis, 



podium, with sori. with sori. 



the common Scolopendrium (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 Hmnionitis (fig. 517) ; or they may form merely a 

 single band along its borders, as in the common Pteris (brake-fern). 

 The sori are sometimes ' naked ' 011 the under surface of the fronds ; 

 but they are frequently covered with a delicate membrane termed 

 the indusmm, which may either form a sort of cap upon the summit 

 of each sorus, as in A*/>i<Iinm (fig. 518), or a long fold, as in Scolo- 

 pendrium and Pteris, or a sort of cup, as in Deparia (fig. 519). 

 Each of these sori, when sufficiently magnified, is found to be made 

 up of a multitude of sporaityes, or spore-capsules (figs. 518, 519). 

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

 but more commonly spring from it by a pedicel or footstalk. The 



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