THE PRINCIPLE OF COHESION. 55 



otliers, wtile supernumerary cords can be formed, which 



appear to have for their function to strengthen the calyx to 



meet the strain upon it when an insect alights upon the 



flower. 



In the calyx of some species of Salvia, which is strongly 



bi-Iobed, though retaining its five teeth, three dorsal (d) 



are posterior and two are anterior. There 



are two single marginal (m) cords between 



the three posterior and dorsal, which corre- ^ ,; 



spond to the mid-inbs of three sepals. The m m 



two lateral and marginal cords are each in m 



double ; while a supernumerary cord (s) lies "' * 



beneath the lip of the corolla between the ^'* "^ 



. s 



two anterior marginals. The accompanying 



diagram of the sepaline cords of 8. Verbenaca will illustrate 



the arrangement. 



The ai'raugement of the cords (vi and s) shows that the 

 strain being greater on the anterior side, the calyx has, as 

 it were, stretched in that direction, the two marginals having 

 separated so widely in front, as to require an extra cord (s). 

 The two lateral ones have not separated to so great an extent, 

 while on the posterior side, where little or no strain is felt, 

 the marginal cords have remained single. 



As the cord (s) shows how Nature can add a fibro-vascular 

 cord if required, so one or more can be subtracted by atrophy 

 where no stress occurs. Thus the petals of the Compositce 

 have no dorsal or median cords, the five sepaline only being 

 present below, but pass up the margins of the petals. Con- 

 versely, in the Primrose, the calyx, giving no support to the 

 corolla, has no marginal cords. 



The above diagram will represent the distribution of the 

 sepaline cords of S. glutinosa and other species, as well as 

 S. Verhe7iaca, but in S. ])ratensis the strain has apparently 



