THE PRFNCIPLK OF COHESION. 53 



We may see, as it were, Nature's first attempt to form a 

 tubular process in the Cruciferce. Here it is obtained by 

 simple approximation of the slender claws of the petals, 

 which are supported by the erect and closely imbricated 

 sepals. A step further is gained in Dianthus, in which the 

 sepals cohere but the petals are stiU free. The third and 

 last stage is arrived at when both calyx and corolla are 

 tubular. 



Subsequent to this state of cohesion many additional 

 structures may arise as they are required in the formation 

 of ribs, etc., as already explained ; while the very form of 

 the tube may change from a purely straight cylinder to a 

 curved or expanded funnel, etc., according as special strains 

 have to be met, which the original form was not well calcu- 

 lated to sustain. 



These changes of Form will be more fully discussed when 

 I treat of that principle of Variation. 



