ORGANOGRAPHY. 45 



latter of which are always called fronds, and are variously traversed 

 by veins, ramifying in a determinate manner in the different geirera. 

 Upon certain definite parts of these veins, generally on the under 

 side of the frond, termed the receptacles, clusters or lines of free 

 one-celled spore-cases (sporangia) are produced, or occasionally 

 many-celled ones (synangia), and in these cases the reproductive 

 spores are contained. The clusters are called sori, 



VERNATION. 



The word vernation, as employed by me, designates the mode of 

 growth of Ferns, or, in other words, the manner in which their 

 fronds are developed and connected with the stem. 



Vernation is either — 



Articulated when the fronds are attached to the stem by a joint, 

 and leave a clean scar when they fall away ; or, 



Adherent when no such joint exists, and the bases are continuous 

 with the stem. 



And it is either — 



Uniserial when the fronds are produced one after the other, in a 

 single lineal series, sometimes close together (contiguous), 

 and at other times far apart (distant) ; or, 



Fasciculate when they surround a central axis, upon the top of 

 which they form a crown. 



STEM. 



In a large number of Ferns the stem is not at first sight very 

 evident ; and even when plainly visible, it is frequently confounded 

 with the root by the unlearned (as, for example, the underground 

 stems of Pteris aquilina) ; but in others, as in Tree-ferns, it 

 is very marked. It is an organ of considerable importance for 

 classifying purposes, and often affords valuable distinctive cha- 

 racters. 



The principal modifications of the stem are the — 



Rhizome, a brittle, fleshy, prostrate stem, producing roots along its 



under side, mostly growing above ground (epigceous), and 



then furnished with scales (squamose), but occasionally 



under ground (hypogoious), and then destitute of scales. It 



