. Physiological botany; I. Outlines of the histology of phænogamous plants. II. Vegetable physiology. Plant physiology; Plant anatomy. 30 THE VEGETABLE CELL IN 133. Bordered pits are a very common modification of the last. A comparatively large spot remains unthickened, but becomes covered b}' a low dome whicii has at its top a small aperture ; at a corresponding point of the wall of the neighbor- ing cell another thickening produces a similar dome, so that the two domes constitute a double convex body which appears as a disc with a central perforation. These bodies are known as disc
. Physiological botany; I. Outlines of the histology of phænogamous plants. II. Vegetable physiology. Plant physiology; Plant anatomy. 30 THE VEGETABLE CELL IN 133. Bordered pits are a very common modification of the last. A comparatively large spot remains unthickened, but becomes covered b}' a low dome whicii has at its top a small aperture ; at a corresponding point of the wall of the neighbor- ing cell another thickening produces a similar dome, so that the two domes constitute a double convex body which appears as a disc with a central perforation. These bodies are known as discoid markings. 134. Sometimes the spot covered by the arched projection or dome is elliptical instead of round. When this kind of marking becomes linear, or nearly so, it is termed scalariform. 135. When annular and spiral thickenings occur the cell-wall lying between them remains so thin tliat a slight strain suf- fices to break it, releasing the rings and coils. The number, the direc- tion, and the stee[)ness of the spi- rals furnish in some cases diagnostic features. 136. Besides spirals and rings, there are intermediate forms, which pass easily over into netted or reticu- lated thickenings. It happens some- times that the reticulated markings are so regular that their interspaces appear as regular polj-gons. 137. Tlie external sculpturing of the cell-wall can be seen in many pollen-grains, and in the hairs of many plants, though in the latter case the projections ma}' be partly due to irregularities in the form of the cell. Stratiflcation and striation. The cell-wall, even at an early stage, frequently exhibits a distinctly stratified structure. In some cases, at least, removal of all the water whicli forms a constituent of the wall obhterates every trace of sta'atification, and this fact supports the hypothesis that the appearance of lamination is caused by differences in tlie amount of water con- tained in alternating layers of the wall. The less strongly refractive
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