318 VEGETABLE PHYSIOLOGY 



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. 



These tensions are capable of demonstration all through 

 the life of such stems and roots as increase in thickness. 

 They give us a partial explanation of the structure of the 

 annual rings of wood which are exhibited by such stems and 

 roots, and of the ruptures that are generally noticeable in 

 the exterior of such parts. 



In the absence of various external stimulating influences, 

 which will be discussed later, young growing members 

 show a tendency to elongate uniformly, so that the direc- 

 tion of their growth is a straight line. Though the apex 

 of any of them may continually 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, usually due to 

 the combined action of a number of external forces acting 

 upon the growing member. The inherent tendency just 

 spoken of can be satisfactorily seen when, by artificially 

 eliminating the action of such forces, the plant is not 

 exposed to their stimulating influences. Such a tendency 

 has been called Bectipetality. It becomes apparent also 

 in the case of a member 'which has become curved, owing 

 to the action of one or other of the stimulating influences 

 referred to. If it is removed from the influence of the 

 stimulus, it becomes straight again. 



