118 THE STRUCTURE OF FLOWERS. 



when visiting tlie flowers, hang downwards upon the corolla, 

 as the hp and adjoining lobes are in one vertical plane, and 

 give no thrust upon the posterior side. All weight, therefore, 

 is thrown upon the front, just as it is on the stamens of 

 Epilohium angiistifolium, described above. Their weight has 

 consequently, so to say, "split" the hood in twain, and the 

 stamens now stand erect in the cleft. 



The peculiar form of the corolla, with the whole of the 

 limb dependent in a vertical direction, must throw the weight 

 of the insect so much to the front, that the leverage will be 

 at a considerable disadvantage — much more so than when 

 the insect stands more directly over the tube of a corolla; 

 which latter, in that case, is often strengthened by that of 

 the calyx. To meet this difiBculty the pedicel is curved over 

 at the top, as may be readily seen in our common Wood- 

 sage, and forms a spring; while hypertrophy has attacked the 

 posterior side of the calyx, in that it now carries two extra 

 7 marginal ribs, one on either side of the pos- 



m m terior dorsal one, as shown in the accompany- 

 d d ing diagram. This is exactly the reverse of 



^' d what occurs in Salvia, and others which are 

 much more strengthened on the anterior side, when the 

 insect stands more directly over the centre of the flower. 



Additional aid is also gained by the tube of the corolla of 

 Teucrium being resilient ; the anterior pair of stamens form 

 two thick ridges, much aiding it in this respect ; the posterior 

 pair, however, are, so to say, " sunk " into the tissue of the 

 corolla as to be invisible in a transverse section. 



Transitional Forms. — We may sometimes, as it were, 

 catch the formation of irregular and zygomorphic flowers in 

 the process of formation ; for it not infrequently happens 

 that one genus will be irregular amongst its allied regular 

 ones. Thus Verhascum and Petunia are transitional genera, 



