STRUCTURE.] PILICALS. 93 



that part of the sporule indicated by the three radiating lines 

 which appear to have been produced by the pressure of the 

 three other sporules that originally helped to constitute the 

 quaternary union ; and as the spores of all the other tribes 

 appear, according to Mohl, to be developed in similar unions, 

 it is most probable that similar lines indicating a valvular 

 dehiscence also exist on them. This is certainly the case in 

 some Mosses, for instance, in (Edipodium, and in Isoetes, 

 Lycopodium, and Osmunda regalis ; and in those instances 

 where such a structure is not visible, it is probably owing to 

 a thickening of the membrane, or a deposition of opaque 

 matter on its surface, as in Pilularia." (Linnaan Transactions, 

 xviii.) 



But it is time to proceed to particulars. 



1. THE PILICAL ALLIANCE.* 

 (Ferns, Dan&ads, Adders Tongues.} 



FERNS are plants consisting of a number of leaves, or fronds 

 as they used to be called, attached to a stem which is either 

 subterraneous or lengthened above the ground, sometimes 

 rising like a trunk to a considerable height. Some of them 

 are the largest of known vegetables in which no organs of 

 fructification analogous to those of phsenogamous plants have 

 been discovered. Their stems often acquire the height of as 

 much as fifty or sixty feet, or even more, and in that case are 

 usually unbranched, of the same thickness at the upper and 

 lower ends, and grow exclusively at the point. The surface 

 of the stem is often hairy or shaggy, sometimes spiny, and in 

 all cases is more or less copiously furnished with callous 

 points, which render it rough like shagreen leather, or co- 

 vered with roots, sometimes entangled into a compact layer 

 much thicker than the trunk itself, and appearing to be the 

 extension of the callous points. 



The anatomy of tree ferns has been skilfully elucidated 

 by Mohl, to whose treatise upon the subject (Martins, Plant. 

 Crypt. Bras. p. 40.) the reader is referred for the details of 

 their curious organisation. I must content myself with a 



* See Vegetable Kingdom," p. 74. 



