GROWTH. 343 



at least towards their free surface, become thickened and 

 cuticularised ; the cells forming the vascular tissue and the 

 sclerenchyma-strands become thickened and lignified, and in 

 some instances (vessels) they lose their protoplasmic contents. 

 The result is that the epidermal, vascular, and sclerenchy- 

 matous tissues become more and more inextensible and rigid. 

 The cell-walls of the parenchymatous ground-tissue of which 

 the pith constitutes the principal mass have not, on the other 

 hand, undergone these changes ; they remain, for a time at 

 least, thin and extensible. Inasmuch as these different tissues 

 are firmly coherent ; and since the epidermal, fibrous, and 

 vascular tissues are relatively rigid, whereas the thin-walled 

 parenchymatous tissue is relatively highly extensible and 

 tends to expand in consequence of its turgidity; tensions 

 are set up between them the nature of which we shall proceed 

 to study. 



It has been found that the tensions thus arising are dis- 

 tributed in two directions, longitudinally and transversely. 

 The longitudinal tension in growing stem-structures (shoots, 

 peduncles, petioles) can be readily demonstrated by splitting 

 the organ from above downwards, by two longitudinal cuts 

 at right angles to each other, into four segments : the segments 

 diverge and bend concavely outwards. This curvature is 

 due to the elongation of the pith and to the shortening of 

 the external tissues. Whilst the organ was still entire, the 

 turgid pith was prevented from elongating to its full 

 extent by the rigidity of the other tissues ; it was in a state 

 of active or positive tension : the external tissues, and es- 

 pecially the epidermis, were stretched by the pith ; they were 

 in a state of passive or negative tension. This is made clear 

 by the following measurements which are due to Sachs. 



A thick longitudinal slice was cut out of a growing internode of 

 Silphium perjoliatum : it was then laid flat, and the pith was divided by 

 a longitudinal cut ; the two halves diverged. 



Shortening of the concave Lengthening of the convex 



outer (epidermal) side. inner (pith) side. 



Right half 2*4 per cent. 9.3 per cent, of the length of the 



entire internode. 

 Left half 2'8 9-3 



