202 THE STRUCTURE OF FLOWERS. 



absolute in nature, so in this case, plants respond to the 

 influences brought to bear upon them, and each individual 

 may vary accordingly, but if the influence be permanent, then 

 the variation becomes hereditary, and one or other character is 

 fixed, and may be regarded as specific or generic as the case 

 may be. Should the environment change again, what may 

 have been constant for generations vyill be once more broken 

 up, and instability ensues. 



Miiller records several cases of such oscillations, as in 

 Pidsatilla vernalis, Dryas odopetala, Bibes petramm, Gentiana 

 cani/pesfris, Veronica serpyllifolia, V. spicata, Walnut, Hazel, 

 etc. These vary from protandry through homogamy to 

 protogyny. He also mentions species which have not yet 

 arrived at complete protogyny, such as Sihbaldia prociimbens 

 and Ranunculus alpestris, mountain species which are homo- 

 gamous ; while R. glacialis is sometimes even slightly pro- 

 tandrous. Papaver alpinum, Arabis alpina, and Biscuiella 

 Icevigata are also described as horaogamous. 



As the transitions from a conspicuous, protandrous, and 

 entomophilous or insect-fertilised flower to a homogamous 

 and autogamous or self-fertilised one, as well as to anerao- 

 phily, are the effects of degeneracy, they will be considered 

 more fully when that peculiar condition of floral structure 

 comes to be discussed.* 



* See Chaps. XXVI. aud XXVII. 



