GEOWTH 823 



clearly compressed by the external portions, and when 

 these are removed it undergoes an extension which is the 

 expression of the amount of such compression. Similarly 

 the external parts are stretched longitudinally by the 

 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 accom- 

 panied by the same changes of dimension. A similar ex- 

 periment 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 is 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 



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