In 1792 Dr. Diel, of Germany, published a classification of apples based on natural affinities and resemblances, and the merits of this system were soon generally recognized and the system adopted. It was subsequently modified and extended by various writers, but may still be considered the basis of all the natural systems now in use. Diel's classification is as follows:
Ribbed Apples (Kantaepfel).
1. Around the eye, as well as the fruit as a whole, the apples are very conspicuously ribbed; the ribs are regular and do not make the form irregular.
2. Compared with size of fruit the core is large, open, and often very irregular.
Order 1. True Calvilles.
1. The taper toward the eye does not begin until at least past the middle of the apple.
2. They are covered with bloom when on the tree.
3. The skin is unctuous, or becomes unctuous in storage.
4. The entire coloring never consists of pure, distinct stripes only.
5. The flesh is light, loose, delicate.
Order 2. Schlotter Apples.
1. The skin is never unctuous when handled.
2. They are never covered with bloom.
3. The form is either flat, conical, cylindrical, or tapering.
4. The flavor is not balsamic, but mostly sweetish or sourish.
5. The flesh is granular, loose, and mostly coarse-grained.
Order 3. Gulderlinge.
1. Flavor is not balsamic, like Order 1, but spicy.
2. Flesh is delicate, almost like that of the Reinettes.
3. Form is conical or flat.
4. The ribs are most prominent only around the eye.
Rose Apples (Rosenaepfel).
1. They are covered with blue bloom when on the tree.
2. The core is not unproportionately large, but often only regular.
3. They emit a pleasant odor, at least when briskly rubbed.
4. The skin is not unctuous when handled.
5. They are handsomely and regularly ribbed around the eye and often also out over the fruit.
6. Flesh is tender, loose, spongy, fine-grained.
7. They have a delicate rose, fennel, or anise flavor.
8. They are mostly not late keepers, their good flavor terminating with the same year they ripen; often only summer or fall apples. Exceptions from this are the Wintercousinotten and Winterrose apple families.
9. They are mostly striped like a tulip.
Order 1. Fruit pointed or longish. Order 2. Fruit globular or flat.
1. They are all large, and include the largest sorts.
2. They have mostly, or almost always, two unequal halves, i.e.", one side is lower than the other.
3. They are always ribbed around the eye; the ribs are broad and prominent, and rise, irregularly and obliquely or broadly depressed, the one above the other, out over the fruit.
4. They are always broader than high, and often only apparently high (elongated) in form.
5. The flesh is loose, coarse-grained, and often very pleasant.
Order 1. Core with wide cells. Order 2. Core with narrow cells.