loo LEAF-ARCHITECTURE OF FERNS [ch. 



reason for regarding any of these organs as forming a category apart from 

 other segments of the leaf-blade, but as merely specialised examples of them. 

 The basal position often taken by such pinnae is due to the way in which 

 the intercalary growth is distributed in the phyllopodium. Many of the 

 characteristic features of the different types of mature leaves can be thus 

 explained. But the most frequent, as it is also the most marked, result of 

 intercalary growth is the long stipe or petiole, which carries up far above 

 the leaf-base that distal branch-system that constitutes collectively the blade 

 or lamina. Sometimes, however, intercalary growth may be localised between 

 the pinnae, separating them in successive apparent pairs, as in the Bracken, 

 or the Oak Fern, or the large Cyatheoids. Sometimes the growth may be 

 intermittent as in the Gleichenias or the Bramble Ferns. It is only a step 

 from such states to those cases where some of the pinnae appear basal, as 

 in the so-called "aphlebiae," owing to the intercalary growth being localised 

 above and not below them. This is the state seen prominently in Heinitelia 

 capensis, and some other Cyatheoids (Fig. 95). 



Stipular growths 

 This simple explanation will not serve to account for all those growths 

 which are found at the leaf-base. Certain primitive types of Ferns show 

 sheathing structures, often described as " stipular," which, arising from the 

 base of the leaf, protect either its own apex 

 or the whole of the next younger leaf These 

 are seen in the living Ophioglossaceae, Marat- 

 tiaceae, and Osmundaceae, and they are char- 

 acteristic of some very early types, such as 

 A rchaeopteris hibernica. They appear as lateral 

 outgrowths on the leaf-primordium, with or 

 without a commissure connecting them across 

 the face of the" leaf-stalk. The commissure is 

 present in Angiopteris (Fig. 96), Marattia, 

 Christensenia, and Todea, but it is absent in 

 Osmunda, where two distinct lateral flaps are 

 found. There is no sufficient reason to regard l ij; 9'' ^ "un^leuesof -V/^/^y^/t;/!, 



^ . ^, bcfoic the formition of the pinnae 



these lobes as of pmna-nature. i hey were ,;, ^...i ficm above, i.', piesentmg 

 probably in the' first instance basal growths, the adaxial front. «^ = apex of the 



I -' 1 T 1 phyllopodium: j- = stipules. (xio.) 



and distinct in phyletic origin from those distal 



branchings that went to form the leaf-blade as we now see it. They are 

 absent from all the more advanced Filicales, but similar structures are 

 found among the Pteridosperms and the Cycads, Cycas being without, and 

 Stangej'ia with a commissure. 



