METHOD FOR LEARNING

Those practicing will find that their greatest difficulty in drawing will be rendering trees. However, here it is only about giving beginners a general idea and helping them develop a habit of depicting them well.
To properly depict a landscape, one must somehow follow the movement of branches through clusters of leaves, much like one follows the nude in figures through drapery, so that one can feel the way they emerge from each other. Even though one can give a lot of freedom in the depiction of branches, it should not be thought that this can be done without choice, nor that there is no semblance to observe; generally, the more one sees the trunk and the birth of branches in a tree, the lighter and more pleasant the landscape looks. One must also ensure that the leaves are positioned at the end of branches precisely according to their position. Such an inclination of branches necessarily leads to such an arrangement of leaves. It is unnecessary to point out that branches always diminish as they move away from the trunk, even though one often sees paintings, otherwise good, where this principle, simple as it is, is not observed.
The best technique for leaves and the most grand is that of Titian and the Carracci: it is broad, with large clusters, and divided with an elongated and undulating form. The method of Berchem, Ruysdael, and some other Flemish artists, as spiritual as it is in their hands, is nevertheless small, and only suitable for depicting holm oaks or other shrubs, whose leaves appear bristly, and are small: it also fairly represents the oak seen...

Translation Notes:
Titian and the Carracci refer to renowned painters known for their grand technique in depicting foliage. Berchem and Ruysdael were Dutch artists known for their particular styles in painting landscapes and foliage. "Bristly leaves" refers to leaves that have a stiff, hairy appearance, typical in certain types of shrubs.