GROWTH. 345 



surfaces are convex, between the apex and the point up to 

 which the cut reaches. This curvature is due to the more 

 active growth of the cortical ground-tissue which lies between 

 the vascular tissue and the epidermis, and which constitutes 

 the principal mass of the parenchymatous ground-tissue of 

 the root. It indicates the existence in the root of a longi- 

 tudinal tension, though a very slight one, of this kind, that 

 the cortical ground-tissue is in a state of active or positive 

 tension, and that the vascular and the epidermal tissues are 

 in a state of passive or negative tension. 



The transverse tension in growing stem-structures can be 

 readily demonstrated by taking a rather thick transverse sec- 

 tion of the growing region and dividing it into two halves by 

 a diametrical cut, when the two cut edges will become convex 

 to each other: or again, by taking off a ring of cortical tissue 

 and attempting to replace it, when it will be found impossible 

 to make the two ends meet. The curvature, in the first case, is 

 due to the expansion of the pith, and to the shortening of the 

 external tissues : the increase in bulk, in the second case, is 

 due to the expansion of the pith consequent upon the removal 

 of the resisting cortical tissue. The transverse tension is 

 due to the fact that the thin-walled turgid parenchymatous 

 tissue, and especially the pith, tends to expand, not in length 

 only, but in all directions. Moreover the epidermis tends to 

 become narrow in consequence of being stretched by the 

 longitudinal tension, and this contributes materially to in- 

 crease the transverse tension. The general distribution of 

 the transverse tension is much the same as that of the longi- 

 tudinal tension. 



In roots the transverse tension, like the longitudinal, is 

 scarcely appreciable. If a transverse section of the growing 

 region of a root be taken and divided in the manner described 

 above, no curvature will be at first seen, but, if the halves be 

 left in water, they will gradually become concave to each other, 

 owing to the expansion of the cortical parenchymatous tissue. 



In some instances (internodes of Grasses and Equfsetum, 

 peduncles of Dandelion, etc.) the growth in circumference of 

 the organ is so great that the pith not only exercises no 



