PHENOMENA DUE TO THE TENSION OF TISSUES. 805 



water indicated by the weight ; it is probably rather caused by the inner cells of the 

 pith withdrawing water from the outer cells, and thus lengthening; but the outer 

 cells would become shorter if they were not stretched by the inner ones. That this 

 is actually the case is shown by the rigidity of the pith under these circumstances, 

 caused by the tension that subsists between the dry outer layer and the moister 

 inner mass. If the prism of pith is divided lengthwise, the parts curve outwards ; 

 and sometimes the outer surface becomes even strongly concave. If the inner cells 

 of the pith are able to withdraw water from the outer ones, it may be inferred that 

 the outer cells are also able to withdraw it from the surrounding wood and especially 

 from the peripheral tissues, preventing these from becoming strongly turgid ; their 

 growth being thus retarded in favour of that of the pith, by which they are now 

 placed in a state of passive tension. It is noteworthy that the medullary cells which 

 contain a minimum quantity of dissolved substances nevertheless absorb water suffi- 

 ciently powerfully to abstract it from the surrounding tissues which must evidently 

 contain a much greater quantity of dissolved substances. 



It is now clear from the observations which have been described, why portions 

 of shoots cut lengthwise in half or in four and placed in water curve outward to 

 such a remarkable extent ; and why a curvature, which may be small but continues 

 to increase for some time, takes place when such pieces are placed in a closed glass 

 tube in dry air. 



(2) Transverse tension caused by subsequent growth in thickness of the wood'^. It 

 has already been shown that transverse tensions also arise during growth which are 

 caused by the longitudinal tension; a more exact knowledge of these is still a 

 desideratum. With the commencement of the increase in thickness of the stem by 

 means of the cambium-ring, a new cause of tension arises, acting in both a radial 

 and peripheral direction ; and this transverse tension generally continues as long as 

 the cambium-ring remains active. The layers of tissue formed from the cambium- 

 ring have at first a tendency to expand in the tangential direction to an extent 

 greater than the space enclosed by the epidermis and the primary cortex permits. 

 These outer tissues therefore become stretched in the peripheral direction; and, 

 since they are elastic and have a tendency to contract, they exert a pressure in the 

 radial direction on the cambium and the tissue formed from it. It happens however 

 also that the rings of wood produced on the inside of the cambium grow more 

 strongly in the tangential direction than the phloem produced on the outside, which 

 is therefore passively stretched. A tension is hence set up in the transverse diameter 

 of the stem during its increase in thickness of such a kind that each layer is stretched 

 peripherally on its outside and compressed radially on its inside ; in other words, is 

 in a state of negative tension on its outside, of positive tension on its inside. If the 

 separate layers of a transverse segment — epidermis, primary cortex, phloem, and 

 xylem — are separated, and their peripheral length compared, we get the following 

 expression for the transverse tension : — 



E < C < Ph < X. 

 As the increase in thickness proceeds the transverse tension increases, as is shown 



* [See Detlefsen, Ueb. DiGkenwachsthum cylindrischer Organe, Arb. d. bot. Inst, in Wurzburg, 

 II. I, 1878.] 



