7o8 MECHANICAL LAWS OF GROWTH. 



would not be at all altered were it found, in harmony with what was said in para- 

 graph b, that the concave side was also sometimes slightly lengthened, since it is 

 stretched by the recoil of the oscillations ; and this elongation is not always entirely 

 neutralised. Pnllieux has compared this curvature to that of a lead-wire fixed to 

 an elastic support, when the support was struck ; he was unable however to see 

 the reason why the older and younger parts of the shoot did not exhibit the phe- 

 nomenon. In the older parts this depends on their more perfect elasticity, in the 

 younger on their smaller flexibility, and on the circumstance that they are not 

 strongly bent, but are only thrown backwards and forwards by the oscillations of 

 the lower and more flexible parts. 



The subsequent neutralisation of the curvature by growth must depend first of 

 all on the increase of turgidity in the concave and its diminution in the convex 

 side, and on the growth being consequently promoted in the former. This may 

 be assisted also by the secondary effect of elasticity, in consequence of which the 

 stretched epidermis of the convex side contracts, while the compressed tissues of 

 the concave side distend. 



Sect. 14. — Causes of the condition of Tension in Plants. The elasticity of 

 the organised parts of plants results in tension chiefly from the operation of three 

 causes; viz. (i) the turgidity, in other words the hydrostatic pressure of die contents 

 of the cell on the cell-wall ; (2) the s\velling and contraction of the cell-walls when 

 they imbibe or lose water; and (3) the changes in volume and form caused by the 

 growth of the cells. 



I. Turgidity. The force by which water is drawn by endosmotic attraction 

 to the cell from the parts that surround it, is not merely sufficient to fill the space 

 enclosed by the cell-wall, but also to enlarge it, the increasing amount of sap dis- 

 tending the cell-wall until its elasticity is brought into equilibrium with the endos- 

 motic absorption. In this condition the cell-\vall is stretched to its full capacity, 

 or the cell is turgid. If the cell loses a portion of its water by transpiration or by 

 neighbouring cells withdrawing it, the tension of the cell-wall is decreased and the 

 volume of the cell diminished. The hydrostatic pressure produced by the endosmotic 

 action of the cell- wall acts from within and is the same at all points within the 

 small cell-cavity ; but this does not prevent diff'erent points of the cell-wall stretch- 

 ing and contracting in different degrees as the turgidity increases, in consequence of 

 local variations in extensibility. Hence not only may the volume but also the form 

 of the cell be changed by turgidity. The greater the tension between the cell- 

 wall and its contents, in other words the greater its turgidity, the greater is the 

 resistance oflfered by the cell to external forces which tend to alter its form by 

 pressure, but the more readily does it burst in consequence. If the cell loses so 

 much water that the space enclosed by the flaccid cell-wall is no longer filled, it may 

 become folded inwards by the external pressure of the air or of the surrounding 

 water, and in this case the cell is said to collapse ; if the cell-wall is thick, firm, and 

 inflexible, a tension of an opposite character to turgidity takes place in the cell. 

 Since turgidity is nothing but the mutual tension of the cell-waU and contents, or 

 a state of equilibrium between endosmotic absorption and the elasticity of the 

 cell-wall, it is evident that only closed cells, i. e. such as have no orifices, can be 



