VARIEGATION. 63 



purposeless products of special modification ; they 

 clearly subserve a function in the economy of the 

 plant, and that function is the direction of the 

 insect towards the proper place for effecting the 

 fertilization of the ovary. In the common rhodo- 

 dendron, the connection can be most readily observed 

 with the naked eye, and the honey tested by the 

 tongue. In this case, one lobe of the corolla secretes 

 a very large drop of nectar in a fold near its base, 

 and the lines of dark spots appear on this lobe alone, 

 pointing directly towards the nectariferous surface. 



The Geraniacece afford an excellent illustration of 

 the general principle. They are on the whole a com- 

 paratively high family of polypetals, for their ovary 

 tends to become compound and very complicated, 

 and they have many advanced devices for the disper- 

 sion of their seeds. Oxalis corniculata, our simplest 

 English form, is pale yellow : O. acetosella is white, 

 with a yellow base, and its veins are delicately tinged 

 with lilac. The flowers of Er odium and Geranium, 

 which are much more advanced, are generally pink 

 or purplish, often marked with paler or darker lines. 

 For the most part, however, these regular forms are 

 fairly uniform in hue ; but many of the South African 

 Pelargoniums, cultivated in gardens and hot-houses, 

 are slightly bilateral, the two upper petals standing 

 off from the three lower ones ; and these two become 

 at once marked with dark lines, which are in some 

 cases scarcely visible, and in others fairly pronounced. 

 From this simple beginning one can trace a gradual 

 progress in heterogeneity of colouring, till at last the 

 most developed bilateral forms have the two upper 

 petals of quite a different hue from the three lower 



