1122. The leaves can be regarded as gigantic and unrolled spiral vessels, and these again as microscopic and involuted leaves.
1123. As through the root the water-process enters the plant, and through the stalk the earth-process, so does the air-process through the leaves.
1124. It is probably the stomata through which the air is conducted into the tracheæ; the connexion, however, has not yet been proved.
Gemmæ or Buds.
1125. With the formation of branches there is at the same time a diminution of the cellular tissue, and an increase of the tracheæ. Entirely new spiral vessels commence in the branches, and are not continued or prolonged into the stalk. The further indeed the extent of the ramification, by so much less is the quantity of cellular tissue, and by so much greater the number of the tracheæ. It comes at last to this, that the tracheal fascicles, which were from all sides surrounded by dense cellular tissue, are only then loosely connected by a thin layer of such substance. This ramuscule is still therefore only a hollow stalk, consisting of fascicles of tracheæ disposed in circles, and so united by a thin cellular integument or membrane, that the whole forms a vesicle.
1126. This vesicle is a Bud. A bud is at bottom none other than the end of a twig that has become hollow.
1127. Several buds are usually involved in each other, i. e. many vesicles of tracheal rings have been encased in each other. They issue gradually forth and become shoots or buds. Buds are bulbs at the end of the branches.
Leaves.
1128. If the bud or the external vesicle ruptures, while the cellular substance becomes consumed at the apex or between two or more tracheal fascicles, it is then manifested as a leaf or Leaves.
1129. Then the second vesicle grows forth, becomes petiolated, ruptures and becomes leaf or leaves. In this manner a twig is formed, surrounded in a spiral manner by leaves.