XVII] THE COMPARISON OF RELATED FORMS 731 



to cases where there exists (either within or without the figure) 

 some part which is supposed to suffer no deformation ; a simple 

 illustration is afforded by the diagrams which illustrate the 

 flexure of a beam (Fig. 358). In biology these co-ordinates will 



Fig. 856. 



Fig. 357. 



Fig. 358. 



be especially applicable in cases where the growing structure 

 includes a "node," or point where growth is absent or at a 

 minimum; and about which node the rate of growth may be 

 assumed to increase symmetrically. Precisely such a case is 

 furnished us in a leaf of an ordinary dicotyledon. The leaf of a 



