CHAPTER IX 



THE VASCULAR SYSTEM OF THE LEAF 



From the studyof certain sporeling-leaves of the Fihcales given in Chapter IV, 

 some idea will have been gained of that increasing complexity of the leaf 

 which is shown in the development of the individual plant. It may be held 

 that an advance similarly carried out in the race, combined with the " web- 

 bing" of the finer segments, has led to that confirmed megaphyllous state 

 which is characteristic of Ferns. Photo-synthesis and Transpiration are the 

 chief functions of the expanded blade. These functions impose a demand 

 for conduction roughly proportional to the area of exposed surface, so that 

 an elaboration of the vascular supply of the leaf-stalk may be expected to 

 follow progressive expansion of the blade. Moreover this effect in the en- 

 larging leaf will need to be harmonised with the stelar structure of the stem, 

 so as to balance the dimensions and the physiological activity of the leaves 

 which it supplies. Comparative study of the stele of the axis shows clear 

 evidence of its adjustment in size and construction to the physiological 

 demands. This also is seen in the ontogeny from the sporeling onwards. 

 Though the phyletic sequences are naturally less clear than the ontogenetic, 

 a like adjustment can be traced in them also. This being so, it will be to 

 the base of the leaf that we shall look for the characters that are archaic, 

 while the upper leaf with its complex branchings, from which the initiative 

 in its evolutionary advance as a photo-synthetic organ has clearly come, may 

 be expected to give evidence of structural innovations. That this expectation 

 is justified by the results of comparative examination of the vascular system 

 of the leaf in Ferns is the very general opinion of anatomists. 



For the purpose of anatomical description the leaf may be divided into 

 four successive regions. They are : First, the region of insertion of the vas- 

 cular supply upon the stele of the stem, and its passage through the cortex 

 into the leaf-base. This is designated the region of the leaf-trace. Secondly, 

 the region of passage outwards through the stipe or petiole, and its continua- 

 tion upwards in monopodial types through the midrib or rachis ; this is the 

 region of thQ phyllopodium. Thirdly, the region of insertion of the supply to 

 the several pinnae upon the upper part of the phyllopodium, each of the 

 branches being designated 2i pinna-trace. Fourthly, the ultimate branchings of 

 the strands in the flattened expansion, which are designated collectively the 

 venation. It wilj be found that the facts relating to each of these have their 



