52 The Living Plant 



blade.- The tiniest veins are embedded within the green tissue, 

 where they end in polygonal areas, as one can see with a lens in 

 some leaves by holding them up to the light (for example in Rose, 

 Cabbage, and Wild Ginger), and as shown in the accompanying 

 cut (figure 9) ; but the larger veins stand out from the surface, 

 though always from the undermost side where they are out of 

 the way of the light. The veins have a double function, the 

 conduction of water from the stem to the green tissue, and the 



FIG. 8. An oak tree, showing an approximation to the theoretical form of figure 7. 

 (Copied from Blanchan's American Garden.) 



conduction of the photosynthetic sugar back to the stem; and 

 they have also a secondary use in helping a little to support the 

 soft tissue, though the rigid but elastic stiffness of the healthy 

 green leaf is due for the most part to osmotic turgescence, of 

 which I shall speak in the suitable place. In addition to the blade, 

 most leaves possess a leaf-stalk, or petiole, stem-like in appear- 

 ance and function and varied in length, which carries the blade 

 out into the light and aids to adjust it therein, as we shall later 



