. A textbook of botany for colleges and universities ... Botany. 492 ECOLOGY tage derived from root hairs seems to be the increase of permeable sur- face, which sometimes is as much as five or ten times that of a hairless root of equal size. The youngest hairs emerge a short distance behind the tip; farther back they are of mature size, and still farther back they are withered and dead. Most root hairs are ephemeral structures, lasting only a few days or weeks. Indeed, the entire epidermis is soon sloughed off, and the hypodermis (here called the exodermis) becomes the outer layer of the root,
. A textbook of botany for colleges and universities ... Botany. 492 ECOLOGY tage derived from root hairs seems to be the increase of permeable sur- face, which sometimes is as much as five or ten times that of a hairless root of equal size. The youngest hairs emerge a short distance behind the tip; farther back they are of mature size, and still farther back they are withered and dead. Most root hairs are ephemeral structures, lasting only a few days or weeks. Indeed, the entire epidermis is soon sloughed off, and the hypodermis (here called the exodermis) becomes the outer layer of the root, which through cut- inization is thenceforth relatively im- permeable to water. The continual dying of the older hairs, as new hairs develop toward the tip, gives rise to the migration of root hair zon^s, mak- ing absorption possible from new soil regions (figs. 703, 704). Furthermore, the ever increasing development of the root system is accompanied by a con- tinual increase in hair development, thereby enlarging the aggregate area of absorption and the total absorption capacity. When unimpeded, root hairs grow at right angles to the root, showing no reaction to gravity stimuli. The hairs become variously gnarled and con- torted (fig. 705) through contact with soil particles, with which an intimate cementation is effected by the trans- formation of the outer layer into muci- lage. So close is this attachment that when a plant is pulled from the ground, considerable earth adheres to the root hairs (fig. 700), which are more apt to break than to separate from the soil particles; this indicates the important part played by root hairs in anchorage, especially in seedlings. The adhesion of hairs to soil particles is of still greater advantage in absorption, since most of the available water surrounds the particles as a film, in which are also most of the salts utilized by plants as food materials. The carbon dioxid excreted by the root hairs assists in dissolving the soil F
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectbotany, bookyear1910
