56 THE STRUCTURE OF FLOWERS. 



been not so great, consequently the supernumerary cord («) 

 has not been developed. 



Such slight differences are ^gnificant, because they show- 

 how readily an organ can respond to different degrees of 

 force brought to bear upon it by different insect visitors ; 

 and the cords are invariably placed just where the strains are 

 greatest. 



The number of ribs to the calyx has been • adopted by 

 systematists as generic characters in some of the Labiatce, as 

 well as the tubular or campanulate shape of it. Now, it will 

 fee found that the shape corresponds with the requirements 

 of the corolla ; so that if the tube of the latter be compara- 

 tively short and slender, the calyx completely encloses it, and 

 has its surface strengthened by a variable number of ribs 

 according to the genus ; though they are not always constant 

 on the same plant. As examples, may be mentioned, Mentha 

 and Melittis, which have a broad campanulate calyx, and a 

 broad tube to the corolla. Stachys has 5-10 ribs surrounding 

 the cylindrical corolla-tube. Galeopsis versicolor has 10 

 prominent ribs, and 10 othei-s which reach from the base of 

 the calyx-tube to about half-way up. Melissa has a very 

 narrow elongated calyx, which fits the slender tube of the 

 corolla exactly, and has 13 or 14 ribs.* Similarly Nepeta 

 Cataria and N. Glechoma support the contracted slender basal 

 •part of the corolla-tube, and have 15 ribs to the calyx. 



Teucrium Scorodonia has only 5 dorsal ribs and 2 (posterior) 

 marginal. The calyx is very broad compared with the slender 

 corolla-tube, and scarcely, if at all, supports it. This flower, 

 is visited both by bees, and nocturnal Lepidoptera which 

 suck without throwing any weight upon the flower. 



Cohesion of Petals, oe Gamopetalous Coeolla. — As 



• This difference in the nnmber of ribs depends npon the lateral and 

 marginal being single or double. 



