116 THE STRUCTURE OF FLOWERS. 



CHAPTER XII. 



THE ORIGIN OF " ZYGOMORPHISM." 



Bilateral Symmetry. — A feature abundantly illustrated 

 through the flowering world, in the construction of irregular 

 flowers which are highly specialized for insect agency, and 

 of which the Labiates and Scrophularinece, for example, fur- 

 nish many instances, is the hypertrophy of the corolla in 

 the direction of an antero-posterior plane, giving rise to a 

 bilateral structure. 



On the one hand, the lips of various kinds, as also the 

 keel, and often the wing petals too, where they help to sup- 

 port the insects in papilionaceous flowers, are accounted for 

 by the weight of the insects bringing about a responsive action 

 in the protoplasm, thus determining a flow of nutriment to 

 the parts demanding it, which now grow into the forms re- 

 quired. On the other hand, the opposite or posterior side is 

 often influenced as well, so that, as in Lamium, the lobes of 

 the two posterior petals have grown into the enlarged hood. 

 The cause of this I take to be the powerful tJirust which 

 insects exert against the posterior side while their weight is 

 expended on the anterior. If a humble-bee be watched, as 

 represented in Fig. 31 (p. 107), it will be seen how eagerly 

 and determinedly it forces its way into a corolla -tube if it 

 expand upwards, as in Duvernoia or Lamium. All the pres- 

 sure is exerted along the median plane, like an oblong wedge 



