OEGANOGEAPnY. 47 



Fully developed fronds vary in size from less than an inch to 15 

 or 20 feet in length, and from a line, or even less, to 10 or 15 feet 

 in breadth. They also vary in form, in circumsdription, and in 

 texture ; and they are either furnished with a leaf-stalk (stipes) 

 or are leafy to the base {sessile). 



In describing the form, circuinscription, texture, and surface of 

 ihe fronds of Ferns, the same terms are employed as in the case of 

 the leaves of flowering plants. They vary from simple entire to 

 decompound-multifid. In compound fronds the primary divisions 

 are termed pinnce, and when more than once divided, the ultimate 

 ones pinnules ; and the terms applied to simple fronds are equally 

 applicable to these divisions. The . divisions or branches of their 

 stipes aI?o are termed the rachis. 



Their texture is very different in different species. Some being 

 thin, membranous, and even pellucid, while others are thick and 

 coriaceous, or fleshy, rigid or flaccid. 



The surfaces of the fronds are either quite smooth, or furnished 

 with different kinds of hairs, glands, or scales (the latter have 

 received the name of samenta, and are generally membranous and 

 deciduous), or they are covered, particularly the under surface, 

 with white or yellow farina. 



The plants called Fern Allies differ entirely in habit and mode 

 of growth from true Ferns ; that the word fronds is not applicable ; 

 but as the genus Selaginella is called " fern-like plants," I there- 

 fore apply the term "frondules " to the species with distinct stems, 

 and to the main branches of the surculose species. 



VEINS. 



In Ferns the mode in which the veins are disposed in the sub- 

 stance of the fronds, or the venation, as it is termed, is of more 

 importance than in flowering plants, the characters relied upon for 

 distinguishing the genera depending more or less upon it, and 

 there are numerous terms applied to it. 



The midrib of simple fronds, or of the pinnse or pinnules of 

 compound fronds, is called the eosta, and is in the former a con- 

 tinuation of the stipes, gradually decreasing in thickness towards 

 the apex, or altogether disappearing {evanescent), and in the latter 



