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 

 also possible to distinguish the whites and the 

 blues, and experience shows that the correlation 

 of the flowers is quite constant. The color can 

 always be relied upon; if lacking in the seed- 

 lings, 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 beauty 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 



