118 



The^e sections, taken from the curved lateral liranehes. were 

 all cut from the upj^er portion of the curve, so that tlie red wood 

 vessels were subjected to compresision. and the white wood vessels 

 were subject to tension. 



Concerning the thickness of the walls, the ratio of the thickness 

 of the white to the red ti-acheides is not very different from their 

 ratio when they were not subjected to any special pressure, and 

 so, presumably, the thickness of the w^alls does not to :iny appre- 

 ciable extent depend on pressure effects of the intensity JjJ'o- 

 duced by forcibly bendinp- a stem into circular form or laymc; 

 a vertical stem in a horizontal poi>ition. Also in those .sections, 

 cut from the parts of the branches covered with tinfoil, in prac- 

 tically every case, it was found that the thickness nf the walls of 

 red wood vessels and white wood vesseli? was the same, which 

 indicates that photomorphic stimuli take an important part in 

 the regulation of the thickness of the walls. In this respect my 

 experiments; appear to agree with those of KiiiL'ht,^ who found, 

 for instance, that roots freed from soil a-nd exposed to light 

 formed firmer wood. 



The C'ryptomeria, Callitris and Pinus had some of their lateral 

 branches curved and tied round in the manner described pre- 

 viously, the upper part of the curve being covered over with 

 tinfoil. Ttiey were placed upright in the glass-house nn 16th 

 August, 1907. 



Measurements of the diameters of the red and white wood 

 vessels, and of their Avails, were taken on 4th November, 1907. 



IPfeffer's Ph.vsiolojr.v of Plants, Riij^lish Translation. Vol. ii.. page 88. 



