290 
PHYSIOLOGY. 
BOOK II. 
nate in the flowers and parts of fructification ; while the foliage 
and general 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. 
