. Physiological botany; I. Outlines of the histology of phænogamous plants. II. Vegetable physiology. Plant physiology; Plant anatomy. 84 MOKPHOLOGY OF THE CELL. The cambium-cells and tbe youngest tracheids have uniform and smooth walls, but in those next older there appear thin spots, which are well defined above and below, but not on the sides, for here the}- grade off into the thicker part of the wall. In the cells which are still older the thin places take the shape of discoid markings, and are clearly seen in anj- radial view. Com- parison of radial with transverse sec- tions shows that a
. Physiological botany; I. Outlines of the histology of phænogamous plants. II. Vegetable physiology. Plant physiology; Plant anatomy. 84 MOKPHOLOGY OF THE CELL. The cambium-cells and tbe youngest tracheids have uniform and smooth walls, but in those next older there appear thin spots, which are well defined above and below, but not on the sides, for here the}- grade off into the thicker part of the wall. In the cells which are still older the thin places take the shape of discoid markings, and are clearly seen in anj- radial view. Com- parison of radial with transverse sec- tions shows that at the margins of the thin places a portion of the wall ex- tends as a slight projection upwards, and partly over the spot. In the more mature form the thin jilace is still re- tained as a delicate plate separating the two cells, but easilj- broken down peihaps in further growth. 270. Scalarifurm markings (see 134) are especially abundant in ferns. The bordered pits are much elongated, and appear as clefts with onlj- narrow portions of the wall between them (Fig. 64 l). They often follow each other with as much regularity' as the "rounds" of a ladder, whence the name (from scalaria, — a flight of steps). They are more commonly found in DUCTS. 271. Ducts, or Trachece, are variously marked by pits, and by the thickenings described in Chapter I. Some of the more common forms of dots are shown in Fig. 64. Spiral, annular, and reticulated markings are all formed by the thickening of parts of the wall by which narrow lines or bands are produced on the inner surface. In these cases the poitions of the wall which are not thickened are often of extreme tenuity, and break upon slight pressure or strain, permitting the spiral to uncoil or the rings to separate (Fig. 64, s s'). 272. Spiral marking's. The number of threads or narrow bands varies from one to fifteen or even twenty, the latter in the petioles of Musa.^ They wind, as a rule, from right to left;. ' De Rary : Verglei
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