290 



PHYSIOLOGY. 



Book II. 



nate in the flowers and parts of fructification ; while the fohage 

 and o-eneral constitution are chiefly those of the female parent. 

 Thus, in the celebrated mule Rhododendron, gained by Lord 

 Carnarvon by fertilising R. Catawbiense with R. arboreum, 

 the mule variety had the flowers and colour of R. arboreum, 

 but more the leaves and hardiness of constitution of R. Ca- 

 tawbiense. The same circumstance has been observed in 

 hybrid Amaryllises, Centaureas, &c. 



The cause of the sterility of mule plants is at present en- 

 tirely unknown. Sometimes, indeed, a deficiency of pollen 

 may be assigned ; but in many cases there is no perceptible 

 difference in the healthiness of structure of the fertilising 

 organs of a male plant and of its parents. I know of no 

 person who has attempted to prove this by comparative 

 anatomical observations, except Professor Henslow, of Cam- 

 bridge ; who, in an excellent paper upon a hybrid Digitalis, 

 investigated anatomically the condition of the stamens and 

 pistil, both of his hybrid and its two parents, with great care 

 and skill. The result of his enquiry was, that no appreciable 

 difference could be detected. 



