252 DR J. M. MACFARLANE ON THE 



The epidermal papillae are absent as such in R. ciliatum, but the free surface of each 

 epidermal cell is slightly convex (Plate VII. fig. 6) ; and as incident light rays are not 

 greatly interfered with, the surface has a dull green aspect. In R. glaucum each epidermal 

 cell grows out as a little wart or papilla, measuring 7 fx (fig. 8), and light rays falling on the 

 sides of these and on the epidermal surface from which they spring interfere with each 

 other, so that a leaden-white colour results. In the hybrid R. Grievei* each epidermal 

 papilla is 4 to 5 /a in height (fig. 7), since the convex cell surfaces of the first-named parent 

 aid in making each slightly longer than half that of the other parent. 



A cross of R. ciliatum with R. Edgeworthii was effected by Mr Lindsay. The leaves 

 of the hybrid have a good deal of the wrinkled character of the latter parent, but are 

 entirely destitute of the dense woolly covering to the under epidermis, which is so con- 

 spicuous a feature of that parent. Transverse leaf sections of R. Edgeworthii expose 

 the scale hairs cut across ; also each epidermal cell grows out into a straight papilla 

 slightly constricted in its middle, and measuring 14 to 16 jx in height (fig. 12). Depres- 

 sions in the epidermis are occupied by knob-like outgrowths of it, from each of which a long 

 twisted hair arises. The hybrid shows, in addition to scale hairs, papillae 9 to 10 [x in 

 height, but though there are depressions in the epidermis that appear to correspond to 

 those of R. Edgeworthii, the long hairs are never produced. 



The well-known hybrid between R. formosum and R. Dalhousice may now be taken. 

 Leaf sections of the former (fig. 9) show short epidermal papillae, which may be straight, 

 but mostly are slightly inclined, so as collectively to form a broken circle round each stoma, 

 and, therefore, the rudiment of a wind chamber. They measure 12 to 14 /x, in height. 

 Leaf sections of R. Dalhousice (fig. 11) present papillae that are 16 to 18 fx in height, and 

 are curved inwards in groups round the stomata, so as to form very efficient wind 

 chambers. The hybrid exactly blends the extremes of the parents in the size and angle 

 of bending of the papillae (fig. 10). 



In the above set of Rhododendron parents and hybrids, therefore, a complete grada- 

 tion in size and position of epidermal papillae is established. If an equally gradual 

 transition could be traced among existing species, not only in the size and position of 

 the papillae, but in other structural minutiae, a key to specific relationship would 

 undoubtedly be obtained. 



Colour of Hybrids and of their Parents. 



Reference may now be made to certain features which are better treated of as a 

 whole, though isolated references have been made to most of them in the foregoing 

 descriptions. First we may deal with hybrid colour distribution, and in doing so we 

 must keep in view the surface area over which any pigment is to be distributed, specially 

 in the case of dissolved pigments like red and blue, or their combinations. Since these 

 depend in most cases on relative acidity (for the red) or alkalinity (for the blue) of the 



* I am greatly indebted to Mr Grieve of Messrs Dickson's nurseries for a liberal supply of this hybrid that 

 was raised by him. 



