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. 
Inthe 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 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. 
