GROWTH 323 
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 ig 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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