86 LEAF-ARCHITECTURE OF FERNS [CH. 



Dichotomy and its derivatives 



In many Fern-leaves, whether with blades deeply cut, toothed at margin, 

 or entire, dichotomy of the veins is a quite obvious feature. In some of them 

 the dichotomous branch-system is developed with all its branches equal, as in 

 Trichomanes renifonne (Fig. 78), Actiniopteris radiata, or Elaphoglossum 

 {Rhipidopteris) peltattmi (Fig. 79). But more frequently a sympodial develop- 

 ment of the dichotomy is seen, so as to form a dichopodiuni or a number of 



Fig. 78. Trichomanes reniforfne, showing detail of venation and vascular connections 

 downwards to the petiole. ( + ) marks the limit between the right and left halves 

 of the dichotomy. (Drawn by Dr J. M. Thompson.) Natural size. 



dichopodia, which may be followed along lines similar to those already 

 traced for dichotomous stems (Chapter iv). As in them so in leaves the 

 dichopodium is probably a secondary development as regards its evolution : 

 it is produced in relation to rapid growth in length, in the one case of the whole 

 shoot-.system, in the other of the branching leaf Biologically this is specially 

 an advantage in leaves, for the longer the leaf the greater is the radius 

 outwards from the central stem of the area functional for the capture of 

 photo-synthetic rays of light. The ontogenetic facts frequently support 

 this progressive history. The first leaf may be simple, with a single vein 



