Retrograde Varieties 143 



jection can be made to our using them as a case 

 of correlative variability. 



The essential character of the purple thorn- 

 apple lies in the color of the flowers, which are 

 of a very beautiful pale blue. But this color 

 is not limited to the corolla. It is also to be 

 seen in the stems and in the stalks and veins of 

 the leaves, which are stained with a deep purple, 

 the blue color being added to the original green. 

 Even on the surface of the leaves it may spread 

 into a purplish hue. On the stems it is to be 

 met with everywhere, and even the young seed- 

 lings show it. This is of some importance, as 

 the young plants when unfolding their cotyle- 

 dons and primary leaves, may be distinguished 

 by this means from the seedlings of the white- 

 flowered species. In crossing experiments it is 

 therefore possible to distinguish the whites and 

 the blues, even in young seedlings, and ex- 

 perience shows that the correlation is quite 

 constant. The color can always be relied upon ; 

 if lacking in the seedlings, it will be lacking in 

 the stems and flowers also; but if the axis of 

 the young plant is ever so slightly tinged, the 

 color will show itself in its beautv in the later 

 stages of the life of the plant. 



This is what we term correlation. The colors 

 of the different organs are always in agreement. 

 It is true that they require the concurrence of 



