SECT. VIII. 



PTERIDE&. 



357 



Fig. 65. Blechnum 

 Spicant. 



part of Great Britain, and extends on the Continent 

 from Swedish Lapland to the borders of the Mediter- 

 ranean. 



The species of Lomaria are marked by having the 

 fertile fronds contracted, so that the 

 sorus is quite marginal ; but there are 

 many species of Blechnum in which 

 the elongated sorus, placed parallel 

 to the midrib, is quite distant from 

 the margin, much more so than in 

 Blechnum Spicant. 



The genus Pteris is the type of 

 another group, the Pteridese, in which 

 the sori form a continuous marginal 

 line covered by the attenuated edge 

 of the frond folded over it, and form- 



in- an indusium. This is the structure in most of 



t 



the species of Pteris, a large family abounding in the 

 tropics, and very widely distributed in almost every part 

 of the globe. In the Pteris aquilina, or Bracken (fig. 66), 

 however, in which the lateral veins of the leaf are divided 

 two or three times before they reach the margin, and the 

 extremities of the branches anastomose and form a 

 vein at the exterior or extreme margin of the leaf, the 

 sporangia are produced on the upper- 

 surface of the marginal vein, and are 

 enclosed by an extension of the skin 

 from both surfaces of the leaf; so 

 that the fructification, which is folded 

 back on the under-surface of the leaf, 

 is in the earliest stage of its develop- 

 ment enclosed between two thin mem- 

 branes, both of which have their 

 margins ciliated with jointed hairs, 

 while, under the microscope, their cellular structure 

 will be found to differ in accordance with that of the 



Fig. 66. Pteris aquilina. 



