102 THE STRUCTURE OF FLOWERS. 



insect more than another. It is only when the flower is 

 situated laterally and projects horizontally, or approximately 

 so, with its limb or border in a vertical plane, and, moreover, 

 is more or less closely applied to the axis, that an insect is 

 compelled to alight upon it on one side only, when approach- 

 ing it directly from the fi'ont. It then throws all its weight 

 upon the organs on the lower or anterior side of the flower, 

 as is the case with the keel petals of papilionaceous flowers, 

 with the lips of Labiates, etc. ; or else its weight is sustained 

 by the stamens or style, or by both together, as in Epilobium 

 angustifolium, Circcea, Veronica, Larkspur, and Monkshood; 

 and whenever the stamens are declinate, as in Horse-chestnut, 

 Dictamnus^ Echium, Amaryllis, etc. 



Flowers which have irregular corollas mostly show various 

 degrees of " bilateral " form in their different whorls, and, 

 have been called " zygomorphic." Such flowers, as a rule, do 

 not receive the visits from so many differeut species of 

 insects as regular flowers. These latter, not being charac- 

 terized by the possession of any very definite contrivances for 

 securing special insect agency, are accordingly visited by a 

 much greater number and variety than those flowers which 

 have become markedly adapted, and consequently restricted 

 to particular visitors. 



It must not be forgotten, however, that regular flowers, 

 if the tube leading to the honey be very contracted and more 

 or less elongated, may become almost as much exclusive as 

 very irregular ones ; for such flowers are mainly restricted to 

 Lepidoptera. 



The following examples may suffice to illustrate these facts. 

 Ranunculus acris, which is perfectly regular and with no 

 specialized structure, is visited, according to Miiller, by more 

 than sixty different species of insects ; whereas species of 

 Aconitum and Delphinium, the two most highly differentiated 



