TENSIONS IN GROWING ORGANS 477 



will be found to be appreciably shorter than the rest of the 

 petiole. If the petiole be carefully measured, and then deprived 

 of its cortical covering by separation of successive strips, the 

 central part when measured wiU be found to be slightly longer 

 than the original petiole. In such a petiole the central part 

 was clearly compressed by the external portions, and when 

 these w^ere removed it underwent an extension which was the 

 expression of the amount of such compression. Similarly the 

 external parts were stretched longitudinally by the central 

 region, and when they were freed from it, the recoil was ac- 

 companied by a diminution of their length. There was thus a 

 longitudinal tension in the petiole, due to the turgescence of 

 the central part, which stretched the outer portions, and was 

 itself compressed by their greater rigidity resisting the hydro- 

 static extension. This tension is due not to greater growth, 

 but to increased targidity, for if the petiole be soaked for a 

 while 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 be 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 growing stems can also be demon- 

 strated. The cortex is found to be strained outwards by the 

 pith, so that if a rmg of it be cut out of the stem, it will be 

 found to shorten on removal. The pith is in a state of com- 

 pression, and the cortical tissues in one of extension, as in the 

 other case quoted above. Transverse tensions of this kind are 

 set up in the course of the thickening of stems and roots by the 

 activity of the cambium layer, the bast and cortex being com- 

 pressed outwards and the wood compressed inwards on account 

 of the formation of the new material. This gives us a partial 

 explanation of the formation of the annual rings of such stems 

 and roots, and of the ruptures that are generally noticeable 

 on the exterior of such parts. 



In the absence of the external stimulating influences re- 

 ferred to above, growing organs show a tendencA^ to grow in 

 straight lines. Though the apex of any of them may con- 

 tinually show the movement of circumnutation, the mature 

 part generally takes up a fixed position, growing vertically or 

 horizontally as the case may be. This position is, however, 

 usuaUv the resultant of a number of external forces acting 



