108 THE STRUCTURE OF FLOWERS. 



The next point to notice is that when the anterior petal 

 is enlarged, the posterior one or more often enlarges also, 

 while a corresponding tendency to atrophy affects the lateral 

 ones. This is seen in many species of Leguminoso}, Scrophu- 

 larinece, and Labiatce, and in zygomorphic flowers generally. 

 It occurs thus in the wing petals of many papilionaceous 

 flowers, as is particularly well seen in Onobrychis. The 

 immediate causes, I repeat, I would recognize in the weight 

 of the insect in front, the local irritations behind, due to the 

 thrust of the insect's head and probing for nectar, coupled 

 with the absence of all strains upon the sides. In some 

 papilionaceous flowers the wing petals form a landing-place,, 

 as in Indigofera and Phaseolus. Whenever this is the case, 

 they too are enlarged, as the lateral ones are in Fig. 31, and 

 undertake the duty impressed upon them. 



When, therefoi"e, one finds as an invariable rule how the 

 fi'ont petals enlarge when flowers are compacted and visited 

 only from the front, and thus become irregular ; and as 

 such often occur in orders where flowers are normally regular, 

 as Iheris, Centaurea, Heracleum, etc. ; and, moreover, when 

 the same phenomena appear in orders having no affinity 

 between them, as in Labiatce and Orchidece; and are, indeed, 

 to be found throughout the length and breadth of the floral 

 world, one is justified in attributing such irregularities to a 

 common cause, that being, according to my theory, the 

 responsive power of protoplasm to the irritations from with- 

 out, set up by insect and other agencies. 



Many other special cases might be described from the 

 different orders of plants, but the above will suffice to illus- 

 trate this principle of responsive action with resulting correla- 

 tions to insect agency. I would here, howevei', call the reader's 

 attention to the mechanical arrangement of forces as shown 

 in Lamium and Echium, whei'e it will be seen that the 



