Color Variations in the Canes of the Blackberry
Not only is the leaf a guide to the kind of fruit which a plant will produce, but in many cases the stem or cane gives a reliable indication. With most berries it is the rule that the light stalk will produce the lightest color berry-and the darker the stalk the darker the berry. From the variations above it will be seen that Mr. Burbank was able to select for lightness of color even before he used the leaves as an indication to size and form.
This image is from: Luther Burbank: his methods and discoveries and their practical application. Volume 2 Chapter 2