318 VEGETABLE PHYSIOLOGY 



central region, and when they are freed from it, the recoil 

 is accompanied by a diminution of their length. There is 

 thus a longitudinal tension in the petiole, due to the 

 greater turgescence of the central part, which stretches the 

 outer portions, and is itself compressed by their greater 

 rigidity resisting the hydrostatic extension. This tension 

 is not due to greater growth, but to more pronounced 

 turgidity, for if such a petiole is soaked for a time in salt 

 solution till the water is in great part removed from its 

 interior, and it has become flaccid, removal of the cortex is 

 not accompanied by the same changes of dimension. A 

 similar experiment may be performed on the hollow flower- 

 stalk of a Dandelion. If it is slit into two halves by a 

 vertical cut, the two parts curl outwards from each other, 

 showing a similar tension in the internal regions. 



Transverse tensions in young growing axes can also be 

 demonstrated. The cortex is found to be strained outwards 

 by the central tissues, so that if a ring of it is cut out of 

 such an axis and split longitudinally, it shortens. If the 

 split ring is again put back in its original position, it will 

 not completely surround the stem. The central tissues 

 are in a state of compression, and the cortex in one of 

 extension, laterally as well as longitudinally, as in the 

 other case already quoted. 



Transverse tensions of a similar kind are set up in the 

 course of the thickening of stems and roots by the activity 

 of the cambium layer, by the division of whose cells new 

 bast is formed behind, and new wood in front of it. The 

 bast and cortex are thus compressed outwards, and the 

 wood and pith inwards, on account of the formation of the 

 new material. The phellogens which form rings of cork at 

 various depths in the cortex, give rise to similar strains. 

 Sheaths of new cells are intercalated in the substance of 

 the delicate tissue, which thus becomes greatly thickened. 

 These tensions are due to growth, and not, like the others, 

 to turgidity of the tissues. They cannot consequently be 

 removed by treatment with salt solutions. 



