350 
PHYSIOLOGY. 
BOOK ir. 
have such an origin ; as, for instance, the roses obtained 
between R. indica and moschata, the different mule Poten- 
tillae and Cacti, the splendid Azaleas raised between A. 
pontica and A. nudiflora coccinea, and the magnificent 
American-Indian Rhododendrons. By crossing varieties of 
the same species, the races of fruits and of culinary vege- 
tables have been brought to a state as nearly approaching 
perfection as we can suppose possible. And if similar 
improvements have not taken place in a more important 
department, namely, the trees that afford us timber, 
experience fully warrants the belief that, if proper means 
were adopted, improved varieties of as much consequence 
might be introduced into our forests, as have already been 
created for our gardens. 
It is, however, to be regretted that those who occupy 
themselves with experiments of this kind do not confine 
them to woody or perennial plants which can be perpetuated 
by cuttings. Mule annuals have the great fault of perishing 
almost as soon as they are obtained, and they serve no other 
purpose than that of encumbering the records of science with 
accounts of so called species which, from their transitory ex- 
istence, can never be re-examined. 
These, however, are considerations which belong to Hor- 
ticulture rather than to Botany. The reader who would 
make himself acquainted with the practical bearing of the 
subject should study Mr. Herbert’s work on Amaryllidaceae, 
p. 335. to 380. 
The cause of the frequent sterility of mule plants is at 
present 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 mule 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. 
