556 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



directed to go ; the spots and streaks called " guides " are localized on 

 one or more petals and so direct the insect to the right spot ; they are 

 often on the " lip " only, as in Orchids, or on the hindermost petals, as 

 in Pelargoniums and Rhododendrons. In the last cases the stamens 

 and style are " declinate " ; that is, they curve downwards, and then 

 raise the anthers and stigmas to strike the under-side of the insect ; 

 whereas in the Dead Nettle and Foxglove they stand erect at the back 

 of the flower, and so strike the visitor on the head or back. 



So innumerable and universal are the adaptations to insect visits 

 that the probabilities of there having been a " direct cause and effect " 

 accumulate until the conclusion becomes a " moral conviction " or 

 "inductive proof" that such has been the case. In other words, 

 they would appear to be responsive growths, and the honey a respon- 

 sive secretion to the irritations set up by the visitors themselves. As 

 a corroborative illustration to this theoretical origin of honey-glands, 

 we may recall the adhesive pads of the Virginia creeper, which are 

 only formed after contact with a wall. Other examples might be 

 given, such, e.g., as the developments to meet strains and weights ; for 

 if a weight be hung on to the growing petiole of a leaf, the mechanical 

 or supportive tissues will be increased to meet the unusual strain. 

 Similarly the number of supportive vascular bundles in the tubular 

 calyx of a Labiate may be increased from five (the midribs of the 

 calycine leaves) to ten, thirteen, or fourteen (in Salvia). This increase, 

 with the enlargement of the front petal into a lip, is thus enabled 

 to bear the weight of a comparatively heavy insect. 



The accompanying diagram will explain how the supportive 

 " ribs " are added in proportion as the weight of the insect increases. 

 d stands for the " dorsal " or midribs of the five sepals. These five 

 are the only ones the sepals possess when free. When they cohere 

 by their margins, one marginal cord (m) is added between the hinder- 

 most sepals, but, the weight being on the anterior sides, two are inserted 

 there (mm). In Salvia, in which the calyx is greatly " stretched," a 

 supernumerary cord (s) is inserted between the two anterior marginal 

 cords. The number of ribs or cords varies in different genera. 



d 



m m 

 d d 

 m m 

 m m 

 d d 

 m m 

 s 



We cannot corroborate this " inductive proof" by experimental 

 evidence, as we do not know how long it took to convert a regular 

 flower into an irregular one; but "reversions" in a way 

 corroborate it. 



It is a universal fact both in the vegetable as well as in the animal 



