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. It 
it is removed from the influence of the stimulus, it becomes 
straight again. 
