SECT. II 



PHYSIOLOGY 



261 



Tho 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 neiglihouring 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. 211, 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. 211, 2a.) If the stalk after this treatment is placed in 

 water the curvature increases considerably. (Fig. 

 211, 2h.) 



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 in- 

 crease in thickness by secondary growth is con- 

 siderably stretched in the tangential direction. 

 On being separated from tlie wood it therefore 

 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 turgescence of the individual 

 cells. The tissue tension presents a certain 

 resemblance to the turgescence of the cell ; 

 this is most evident in the typical stem. 

 Just as the cell-sap distends the cell-wall 

 by its osmotic pressure, the expanding 

 pith stretches the cortical tissues. In- 

 creased resistance to deformation and 



increased rigidity result from the stretching of the cortex, just as they 

 do in the cell from the stretching of the wall. 



The tissue tension ceases as all the cells attain the permanent 

 mean length dictated by the size of the organ. Sometimes, however, 

 certain cells after attaining their greatest length exhibit a consider- 

 able contraction associated with an alteration in shape. This occurs 

 often in roots (^*) when the tissues of the cortex and of the central 

 portion are thrown into folds by the contraction of the tissue that 

 lies between them. The significance of this contraction of roots, 

 which may lead to a shortening of the fully-grown structure by 

 10-70 per cent, is very great. Thus, it is due to it that the leaves 

 of many " rosette plants," in spite of the continued growth in length 



Fic 211.— 1. Shoot of Helianthus 

 annuus wiUi the leaves removed 

 and the pith separated from the 

 periplieral tissues by means of a 

 cork - borer. 2. Stalk of the in- 

 florescence of Taraxacum, split 

 longitudinally by two incisions at 

 right angles to one another, u, 

 Just after splitting, h, After im- 

 mersion in water. 



