THE ANGIOSPERIMAE : STEMS 



907 



thicker than the other and consisting of difFerently thickened elements. In 

 Conifers the lower wood is reddish, the upper white, the former having 

 much thicker cell walls. The difference is attributed to the mechanical 

 effect of the weight of the branch, the upper wood being called tension-wood 



Fig. 891 . — Driniys zcinteri. Transverse section through 

 the vascular zone of a stem showing homoxylous 

 wood with no vessels. 



and the lower portion compression- wood. Vertical stems subject to con- 

 sistent wind pressure from one side may show the same differences. 



Severe frost may cause upward or downward bending of large branches 

 according to whether the thinner-walled tissue lies on the lower or the upper 

 side of the branch, as is remarked in " Lorna Doone." 



In the contracted stems of rosette plants successive rings of wood may 

 not be formed at all, or if they are, they are related to environmental con- 

 ditions, not to age. A constant relation to annual growth is only maintained 

 in main stems of unlimited growth. In short branches, such as fruit spurs, 

 annual rings are only formed in the first few years ; while in thorns, unless 

 they bear leaves, there is never more than one ring, however old they may be. 



The phloem rarely shows indications of annual rings. The periodicity of 

 cambial growth is less marked on the phloem side. Activity begins later and 

 may, in some cases, continue into and perhaps through the winter, the spring 

 change in the phloem not being the formation of new elements, but the very 

 marked swelling of the old elements through water intake. Towards the end 

 of its life the phloem may, though rarely, become lignified, as may be seen 

 in old stems of the annual Sunflower. 



