120 



Jean White : 



.010 mm. 



PiNUS STROBUS. 



.006 mm. .010 mm. 



.006 mm. 



A large branch of Cupressus sempervirens with sufficient bark 

 and phloem to cut off nearly all light from the cambium was 

 removed from the tree on which it was growing normally. An 

 extremely thick layer of red wood was visible on the under 

 side of the branch as it grew. 



Sections of the red and white wood were examined, and the 

 diameters of the cavities, and the thickness of the walls of the 

 xylem vessels in each kind were measured, -wath the following 

 results : — 



Bed Wood 

 Internal Diameter Thickness of W:ill 



White Wood 

 Internal Diameter Thickness of Wall 



In this case the difiereiice between the intensity of the illum- 

 ination on the upper (white wood) and the lower (red wood) 

 surface must be extremely small, and the average thickness of the 

 white wood walls is little or not at all greater than in the red wood. 



Sum/nary of Preceding:; Experiments. 



Thickness of Walls Internal Diameter 



Red Wood White Wood lied Wood White Wood 



.0160 mm. .0052 mm. .0140 mm. .0045 mm. 



Pinus strobus. 



Plant grown horizontally. 



White wood on illuminated side. 



Thickness of Walls Internal Diameter 



Red Wood White Wood Ked Wood White Wood 



Average 



.0040 mm. 



Average 



.0045 mm. 



Average 

 .0135 mm. 



Average 



0135 mm. 



