STRUCTUKE OF FEBNS 



6 7S 



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

 chymatous sheaths are a very conspicuous feature in a transverse 

 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 Polypodium (fig. 516), and in Aspidium (fig. 

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



FIG. 516. Leaflet of Poly- 

 podium, with sori. 



FIG. 517. Portion of frond of Hcemionitis, 

 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 Hcemionitis (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 ' on the under surface of the fronds ; 

 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. 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 sporanges, 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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