60 



BOTANY OF THE LIVING PLANT 



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. The petiole appears to be 

 structurally Httle 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 

 probabiHty 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. 43. 



Skeleton of the lamina of Ivy. Natural size. 



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

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

 Dahha, 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. 43). 

 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 



