120 THE STRUCTURE OF FLOWERS. 



they be flowers visited; or by fluctaating conditions of nutri- 

 tion, if not ; and then, arrested in that state. 



A further remark on a significant point may be added on 

 Petunia. In this flower, as in Verbascum, the limb of the 

 corolla 'stands in a vertical plane, the anterior lobe is a 

 trifle larger than the others, the five stamens have a slight 

 tendency to be atrophied on the posterior side, while the 

 stigma has become just so much displaced as to hinder self- 

 fertilisation. This property is, however, by no means yet 

 lost. Florists are aware of it, and find it necessary to self- 

 fertilise, but not to cross, these flowers artificially to secure 

 plenty of seed ; Mr. Darwin corroborates this {Cross and Self, 

 etc., p. 193). 



We have, then, here a case, but by no means an isolated 

 one, in which the forms of the floral organs are undergoing 

 a change, but the physiological characters of the essential 

 organs have not yet been influenced by the external stimulus, 

 so as to become more or less inert upon one another, as is 

 sometimes the case in highly differentiated flowers. 



Indeed, it would seem to be a universal rule that morpho- 

 logical changes are more readily acquired than physiological 

 barrenness ; as by far the great majority of plants have 

 retained their self-fertilising powers ; and, when they have 

 lost it, it is easily and rapidly reacquired when the necessary 

 conditions are supplied. 



Echium is another instance of almost a single genus 

 amongst others of the same order characterized by great and 

 persistent regularity. Bhododendron and Azalea may be 

 compared with, other genera of Ericaceae, and the reader will 

 readily suggest others. 



Sometimes the irregularity is confined to the stamens or 

 style, or both, which may have a tendency to become decli- 

 nate, as in Calluna, in some Liliaceous and Amaryllidaceons 



