GROWTH. 347 



the pith but are stretched by it; in the third stage the ten- 

 sions are less considerable, and in still older internodes in 

 which the pith-cells have ceased to be extensible and even to 

 be turgid the tensions disappear altogether. 



In many plants, those parts of the stems and roots which 

 have ceased to grow in length, continue to grow in thickness 

 by means of a zone of meristematic cells. The most common 

 mode (Dicotyledons, Conifers) of this growth in thickness is 

 that new cells are produced by division from this zone, which 

 is termed the cambium-layer, and that these then grow to 

 form, on the outside of the cambium-layer, bast or phloem- 

 tissue, and on the inside, wood or xylem-tissue. Constant 

 additions are thus made to the vascular tissue on each side of 

 the cambium-layer. The result is that the tissues external to 

 the ring of vascular tissue, namely the older bast, the cortical, 

 and the epidermal tissues, are subjected to a transverse pres- 

 sure from within outwards, and that the tissues internal to the 

 cambium-layer, namely, the older wood and the pith, are 

 subjected to a transverse pressure from without inwards. And 

 conversely, the tissues outside the cambium-layer exercise a 

 pressure upon it from without inwards, and the tissues inside 

 the cambium-layer exercise a pressure upon it from within 

 outwards. A considerable tension is thus set up between the 

 internal and the external tissues, the effect of which is to 

 stretch the latter in the peripheral direction, that is, in a 

 direction parallel to the surface of a transverse section of the 

 organ, and to compress the former, though owing to the 

 rigidity of the older wood the effect of the compression 

 is slight. The relation between the various external layers 

 of tissue is this, that each is stretched by the layer next 

 inside it and compressed by the layer next outside it. 

 This great and increasing pressure soon produces a visible 

 effect upon the external tissues: they rupture, and longitudi- 

 nal fissures appear on the surface. The rupture does not, 

 however, produce open wounds, for in the meantime a forma- 

 tion of cork has been taking place beneath the epidermis. 

 The cork, in its turn, is ruptured and shed in scales or in rings 

 which are replaced by fresh formations. 



