I- 1 LIC ALES 





leaf-stalk of most modern Ferns (Fig. 374, D). The several strand 

 Dryopteris can be traced into the pinnae and pinnules where they fork 

 freely and end blindly (Fig. 373). But in broad-leaved Ferns such 

 as the Adder's Tongue, and particularly in those of more advan 



Fig. 376 a. 



Dictyostele of the Male Shield Fern dissected out, showing the overlapping 

 leaf-gaps which allow communication between cortex and pith. (After Reinke.) 



type, such as Onoclea and Woodwardia, a network of veins may arise 

 by their lateral fusion, after the manner of Dicotyledons (Fig. 46). 

 Both types serve the same purpose, that of supply within the 

 flattened blade ; but the netted venation is functionally the more 

 efficient. Moreover, comparison shows that it is a derivative state, 

 based on an originally forked venation. 



It thus appears that the vascular system in the leaves of Ferns resembles 

 that of Flowering Plants more nearly than does that of their adult stems. 

 The diversity in vascular structure between the stems of Ferns and those of 

 Seed Plants arises from an essential difference in their way of solving a 

 fundamental problem of support and of supply (see Chapter XXXVI). In the 

 Bracken and Shield Fern the conducting tracts are all of primary origin. They 

 may be traced continuously to their source immediately below the growing 

 point itself, and their outline there corresponds to that which they show when 

 mature. As will be seen in Figs. 376-378, there is no cambium in their make-up. 

 Ferns by their stelar elaborations make the best of this primitive scheme of 

 vascular construction. On the other hand, cambial activity, as described fur 

 Dicotyledons and Gymnosperms in Chapter IV, is an automatic means of 

 meeting the growing demands of increasing size. But it is a morphological 

 afterthought : the tissues it produces being of secondary origin. 



