1065. The plant is not merely earth-and water-organism, but also air-organism; and there must therefore be developed in it an anatomical system, which coincides with the process of air.
1066. Besides the cells and tubes naught else is found in vegetable tissue but spiral vessels; what are called scalariform tubes, annular vessels, dotted ducts, vermiform or strangulated vessels, are no peculiar formations in themselves, but only different conditions of the spiral vessels.
1067. The spiral vessels are the air-system of the plant, and therefore rightly deserve the name of Tracheæ. They exhibit the structure of the air-tubes in insects, and contain, according to the most authentic observations, air, and not sap, except in the period of adolescence, as occurs in the animal kingdom.
1068. The spiral vessels consist of one or several filaments spirally contorted, and held together by a delicate tubular-shaped membrane.
1069. They must be regarded as elongated cells, upon whose parietes the starch-granules have been placed in serial juxtaposition with each other, so as to form spirally-twisted filaments, as is to be plainly seen in many cells and also in Confervæ. This spiral condition originates without doubt from the spiral-shaped motion of the granules in cell-sap.
1070. The ultimate cause of this spiral motion, as well as the position of the parts, appears to reside in the rotation of the sun.
1071. Upon this also depends probably the winding of the stem of plants, with the spiral-shaped position of the leaves and branches, as probably even the contortions of the snail's shell and of the hairs upon the crown of the head.
1072. The production of the spiral form originates from the antagonism of the light with the matter. The number of spiral vessels is therefore less in those parts that are beneath, than in those above, the earth, or less in the root than the stem. The more indeed an organ has been exposed to the air, by so much the more do the spiral vessels preponderate, as e. g. in the leaves.
1073. An organ must necessarily be nobler in character, the more spiral vessels it contains. The plant also that, with more spiral vessels, exhibits them particularly arranged, must take a higher rank. The lowest plants, as the mushrooms, lichens and mosses, consist therefore entirely of cellular tissue; in the ferns therefore only a single bundle of spiral vessels makes its appearance. When plants become nobler, several fasciculi of spiral vessels originate; and in tracing this feature we ascend from the ferns to the grasses and lilies, up to the lower Dicotyledons. In the higher Dicotyledons the packets of the tracheæ increase for the first time to such a degree, that they form a closed circle, the fibrous ring or zone of wood.
1074. The tracheæ extend from one end of the plant to the other; many are wont to terminate only in nodes, while these are to be regarded as arrested branches. The air can therefore penetrate through the spiral vessels from the leaves even to the apices of the roots.