Red Wood in Conifers. 123 



Callitpis Gunnii. 



Plant grown vertically (lateral branches curved). 

 White wood on the illuminated side. 



Thickness of Walls Intkrnal Diameter 



Red Wood White Wood Ked Wood White Wood 



.008 mm. 008 mm. 



.008 ,, .008 ,, 



Average Axerajje Average Average 



.0020 mm. .0030 mm. .0080 mm. .0080 mm. 



Lateral branches curved. Both sides equally darkened. 



Thickness of Walls Internal Diameter 



Red Wood Wliite Wood Red Wood White Wood 



.0080 mm. .OOGO mm. .0080 mm. .0050 mm. 



Conclusion. 



So far as my results dealing with this matter go, they point to 

 the conclusion that the formation of red wood is primarily due 

 to a gravitational stimulus, while the lesser thickness shown by 

 the wall of the red wood tracheides, as compared with that of 

 the white wood tracheides. appears to be largely the result of a 

 photomorphic stimulus, the response being somewhat akin to 

 etiolation in character. 



The preceding averages all agree in showing that the thickness 

 of the tracheide walls on the more strongly illuminated side ex- 

 ceeded that of the tracheide walls where the illumination was 

 less intense. The same uniformity, under similar conditions, 

 does not apparently prevajil in the size of the internal cavities 

 of the tracheides ; thus as a general rule- along a single radial 

 row of tracheides, isolated cases occurred in which the internal 

 cavities were of abnormal size in either direction, while such 

 aibrupt variations did not appear to occur to any marked extent 

 in the thickness of the tracheide walls of either kind of wood. 



Considering the cases in which both sides of the branches were 

 equally darkened, the assumption is :-trengthened by the fact 

 that in eveiy case tested except one. the thickness of the 

 tracheide walls in both red and white wood tallied exactly. 



