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 



