THE LEAF 



71 



as it curves out from the stem is directed upwards (ad-axial), and 

 the phloem downwards (ab-axial), and these relative positions are 

 regularly maintained throughout the leaf (Figs. 20, 47). The petiole 

 appears to be structurally little more than a means of junction between 

 the axis and the blade. Its presence brings two advantages : that 

 the blade is carried some distance outwards from the stem, and 

 thus the probability of one leaf overshadowing another is avoided ; 

 and secondly, the narrow petiole allows the blade to yield to the 

 pressure of wind, instead of rigidly resisting it. 



Fig. 46. 



Skeleton of the lamina of Ivy. Natural size. 



It is the blade, or lamina, which is the distinctive feature of the 

 foliage leaf. As shown in such common types as the Sunflower, 

 Dahlia, Cabbage, or Sycamore, the blade consists of a skeleton or 

 framework of thickened and mechanically firm ribs, which support 

 an expanse of relatively thin and delicate tissues. A prominent 

 mid-rib runs up continuously from the leaf-stalk to the tip of the 

 blade, and branch-ribs of successively smaller size pass off from it 

 towards the margin. On removing the softer tissues by reagents, 

 the vascular system can be demonstrated as a leaf-skeleton (Fig. 46). 

 It is then seen that vascular strands of the midrib and of 

 the stronger lateral ribs, give off thinner lateral branches ; that 

 smaller branch-veins pass off from these, and with further ramifica- 

 tions and many fusions form a fine network traversing the thinner 

 areas of the blade. Such a reticulate venation is characteristic of 



