286 BOTANY PART i 



the cell sap must become more dilute. This does not actually occur owing to the 

 power of the growing cell to regulate the osmotic pressure of the cell sap. The 

 pressure can be increased by the transformation of sugar into organic salts ; thus, 

 for example, by a change of glucose into oxalic acid the osmotic pressure can be 

 trebled. On the other hand, the pressure can be lessened, e.g. by complete 

 combustion of sugar in respiration. 



Besides the expansion in the longitudinal direction, expansion in 

 a transverse plane (growth in thickness) has to be considered. The 

 diameter of the mature root or stem is often considerably greater than 

 that immediately behind the growing point. As has been seen on 

 p. 140, a distinction is drawn between primary and secondary growth 

 in thickness. Only the primary growth in thickness is a phenomenon 

 of the kind that is here being considered. In secondary growth new 

 meristematic cells are formed from an intercalary meristem or cambium, 

 and only later pass into a phase of expansion. 



TISSUE TENSIONS. The expansion of the cells in length and 

 breadth does not always take place uniformly and simultaneously in 

 the whole cross -section of an organ. It is usual to find that, in 

 growing stems for instance, the pith strives to expand more strongly 

 than the peripheral tissues. Since no breach of continuity between 

 the two regions is possible, a state of tension (tissue tension) results. 

 The pith expands the cortical tissues and these compress the pith ; 

 the actual length of the organ is the resultant of these antagonistic 

 tendencies. If the tissues are artificially separated, each assumes its 

 own specific length ; the pith elongates and the cortex contracts and 

 the tension disappears. 



The tissue tensions which occur generally in growing organs may be demon- 

 strated in this way. In a sunflower shoot the pith is separated for some 

 distance from its connections to neighbouring tissues by means of a cork-borer. 

 On withdrawing the cork-borer the cylinder of pith projects for some distance from 

 the cut surface of the stem (Fig. 259, 1). If a similar shoot is split longitudinally 

 the two halves curve outwards owing to the elongation of the pith and the 

 contraction of the epidermis. Even in the case of hollow shoots such as the stalk 

 of the inflorescence of the Dandelion (Taraxacum} a tension exists between the 

 outer and inner tissues which is expressed by curvatures when the stalk is split 

 longitudinally (Fig. 259, 2a). If the stalk after this treatment is placed in water 

 the curvature increases considerably (Fig. 259, 2V). 



Tissue tensions also occur in leaves and roots. The tensions need not be in the 

 longitudinal direction alone ; there are also transverse tensions. Thus, for example, 

 the rind of trees which increase in thickness by secondary growth is considerably 

 stretched in the tangential direction. On being separated from the wood it there- 

 fore contracts. 



The tissue tensions gradually arise at some distance from the 

 growing point when the expansion of cells is commencing, and as a 

 rule they again disappear in the fully-grown zone, though they persist 

 in the case of some organs. They are of great importance for the 

 rigidity of growing tissues ; they increase the rigidity given by the 



