420 FERNS LESS. 



once a cell in the apical meristem, that every vessel has 

 arisen by the concrescence of a number of such cells, and 

 that the meristem cells themselves are all derived, by the 

 ordinary process of binary fission, from the apical cell. In 

 this way the concurrent processes of cell-division, cell- 

 differentiation, and cell-fusion result in the production of 

 the various and complex tissues of the fully-formed stem. 



The leaves vary greatly in form in the numerous genera 

 and species of ferns : they may consist of an unbranched 

 stalk bearing a single expanded green blade: or the stalk 

 may be more or less branched, its ramifications bearing the 

 numerous subdivisions of the blade, or pinnules. 



The anatomy of the leaf, like that of the stem, can be 

 readily made out by a rough dissection. The leaf-stalk and 

 its branches have the same general structure as the stem, 

 consisting of parenchyma coated externally with epidermis 

 and strengthened internally by vascular bundles, which are 

 continuous with those of the stem. But the blade, or, in the 

 case of a compound leaf, the pinna, has a different and quite 

 peculiar structure. It is invested by a layer of epidermis 

 which can be readily stripped off as an extremely thin, colour 

 less membrane, exposing a soft, green substance, the leaf- 

 parenchyma or mesophyll. The leaf is marked externally by 

 a network of delicate ridges, the veins ; these are shown by 

 dissection to be due to the presence of fine white threads 

 which ramify through the mesophyll, and can be proved by 

 tracing them into the leaf-stalk to spring from its vascular 

 bundles, of which they are in effect the greatly branched 

 distal ends. 



Microscopic examination shows the epidermis of the leaf 

 (Fig. no, K, ep and L) to consist of flattened, colourless cells 

 of very irregular outline and fitting closely to one another like 



