V] OF FLUTED OR PLEATED CELLS 261 



In Mr Worthington's experiment, there appears to be something 

 of the nature of a viscous drag in the surface-pellicle ; but whatever 

 be the actual cause of variation of tension, it is not difficult to 

 see that there must be in general a tendency towards longitudinal 

 puckering or "fluting" in the case of a thin- walled cylindrical or 

 other elongated body, rather than a tendency towards transverse 

 puckering, or "pleating." For let us suppose that some change 

 takes place involving an increase of surface-tension in some small 

 area of the curved wall, and leading therefore to an increase of 

 pressure : that is to say let T become T + t, and P become P + f. 

 Our new equation of equilibrium, then, in place of P = Tjr + T/r' 

 becomes 



P^ T+tT+t 



P + P = \ 7— 



and by subtraction, 



]) = t/r + t/r'. 



Now if r < r', t/r > t/r'. 



Therefore, in order to produce the small increment of pressure f, 

 it is easier to do so by increasing t/r than t/r' ; that is to say, the 

 easier way is to alter, or diminish r. And the same will hold good 

 if the tension and pressure be diminished instead of increased. 



This is as much as to say that, when corrugation or "rippling" 

 of the walls takes place owing to small changes of surface-tension, 

 and consequently of pressure, such corrugation is more likely to 

 take place in the plane of r, — that is to say, in the plane of greatest 

 curvature. And it follows that in such a figure as an ellipsoid, 

 wrinkling will be most likely to take place not only in a longitudinal 

 direction but near the extremities of the figure, that is to say again 

 in the region of greatest curvature. 



The longitudinal wrinkhng of the flask-shaped bodies of our 

 Lagenae, and of the more or less cylindrical cells of many other 

 Foraminifera (Fig. 87), is in complete accord with the above theo- 

 retical considerations ; but nevertheless, we soon find that our result 

 is not a general one, but is defined by certain limiting conditions, 

 and is accordingly subject to what are, at first sight, important 

 exceptions. For instance, when we turn to the narrow neck of 

 the Lagena we see at once that our theory no longer holds ; for 



