Following is an overly simplified representation of how the lights and shades form around clumps of leaves. The same types of value clumpings can be seen in fluffy clouds and in cauliflower. Especially in watercolor, which is known for its poetic simplicity, drawing a tree leaf-by-leaf is not advised. It is better to suggest the clumps of leaves by depicting their values. The original of this drawing exercise can be found: http://arcmel.com/archives/799
A typical oak tree is topped by an arching, umbrella, like system of many smaller umbrella clumps. Each umbrella is shaded separately; and is lighter on the top and darkest on the bottom. The clumps often touch and even overlap each other; but think of the clumps as individual and make alterations as necessary.
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