324 VEGETABLE PHYSIOLOGY 



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 direction 

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

 of them may continually show the movement of circum- 

 nutation, 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 

 Rectipetality. 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. 



