668 PROFESSOR F. O. BOWER ON 



the right-hand pinna there shown the successive veins alternate throughout its 

 length. In the left-hand pinna they alternate at the base, but in the upper part the 

 lateral veins appear to be inserted opposite to one another. In reality, however, as 

 Hofmeister noted, they alternate still. But the distance between the successive 

 dichotomies is so abbreviated that the appearance is given of paired veins, arising 

 on opposite sides of a continuous midrib. This appearance, which is very common 

 in Ferns, as it is also in the leaves of Angiosperms, is thus simply the result of a 

 slight modification of the sympodial dichotomy. The steps of that modification are 

 well illustrated in the juvenile leaves of Todea barhara (see below). 



The leaf-base of the Osmundacese shows developments which appear only in the 

 older leaves, and have no relation to the distal branchings. They rank with similar 



Fig. 3. — Two pinnules of an adult leaf of Osmunda regalis, showing 

 how their venation is essentially alternate, but may in places be 

 apparently opposite, (x 4.) 



growths in a number of primitive types. In Osmunda these consist in wide lateral 

 flaps which form a broad sheathing base to the leaf-stalk. This development, which 

 is biologically important for protection, is of the nature of an extension of those 

 lateral wings which extend the whole length of the leaf in varying prominence, and 

 merge finally into the marginal growth at the distal end. # 



Most Ferns are liable to occasional variations from the normal ; these may 

 illuminate their morphology. The " forma furcata " is such a variant, seen in 

 Polypodium vulgare, Scolo'pendriwm vulgare, Athyrium, Nephrolepis, etc., etc.t 

 The forking is frequently a very perfect dichotomy, and may affect only the 

 distal ends of the pinnoe, or the apex of the leaf, or it may extend downwards, 

 so that in extreme cases it may result in apparently two complete leaves seated 

 on a common stalk. 



In a specimen of 0. regalis from India the distal ends of the pinnae only were 

 affected, and this gave an opportunity for examining the relation of the forking to 

 the venation, and so to the regular architecture of the leaf. Three examples are 



* See Phil. Trans., 1884, p. 577. t See the writings of Moore, Lowe, Luerssen, etc. 



