. A textbook of botany for colleges and universities ... Botany. ROOTS AND RHIZOIDS 499. falls " a common sight. The effectiveness of roots as anchorage organs is well displayed along streams and shores, where erosive forces are active. In such situations the earth often is held firmly in place by matted roots and it is only by undermining the grasses and trees, which may even overhang in such places, that the erosive forces are able finally to dislodge the plants and to encroach farther upon the land. Even more striking is the behavior of roots in regions of shifting sands along seacoast
. A textbook of botany for colleges and universities ... Botany. ROOTS AND RHIZOIDS 499. falls " a common sight. The effectiveness of roots as anchorage organs is well displayed along streams and shores, where erosive forces are active. In such situations the earth often is held firmly in place by matted roots and it is only by undermining the grasses and trees, which may even overhang in such places, that the erosive forces are able finally to dislodge the plants and to encroach farther upon the land. Even more striking is the behavior of roots in regions of shifting sands along seacoasts, where many grasses and other plants are able not only to maintain themselves, but even to check the prog- ress of the sand (fig. 707). Such plants are known as sand hinders, and are of great impor- tance in preventing the encroachment of sand upon villages and farms, even being planted for that purpose in many places. Gravity and the direction of root growth.—The tendency of roots to grow downward { to ex- hibit progeotropism) makes possible a favorable relation to absorption and anchorage (figs. 691- 693). The geotropic reactions of roots are well shown when seeds germinate on the soil surface; if the root issues from the upper side, it may curve through an arc of 180°, and grow directly down into the soil. But while the tap root usually grows straight down in this fashion, the numerous side roots branch out in almost all directions. The advantage of such a habit is clear enough, but the following the moist- cause is obscure. If the growing part of a tap "^fi ="rf'''^«- i"f"=f,<i . , of growing vertically root is removed, some of the stronger side roots downward. commonly begin to grow straight down (figs. 708, 709). Apparently there is something in the tap root which inhibits the ejq^ression of progeotropism by the side roots. The removal of the tap root removes this inhibition and the side roots change their growth direction. Water and the d
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectbotany, bookyear1910
