. Plant anatomy from the standpoint of the development and functions of the tissues, and handbook of micro-technic. ration through the walls of the passage cellswhen the velamen is dry. When the velamen is wet the root isas if embedded in a saturated sponge, and when dry the velamenacts as a mulch to keep the rest of the root from drying. Tillandsia usneoides, the hanging moss of the southern states,represents the second class of aerophytes where the roots do notdevelop, although their fundaments are present in the youngseedlings. This plant hangs from trees of various kinds, buthas no organic
. Plant anatomy from the standpoint of the development and functions of the tissues, and handbook of micro-technic. ration through the walls of the passage cellswhen the velamen is dry. When the velamen is wet the root isas if embedded in a saturated sponge, and when dry the velamenacts as a mulch to keep the rest of the root from drying. Tillandsia usneoides, the hanging moss of the southern states,represents the second class of aerophytes where the roots do notdevelop, although their fundaments are present in the youngseedlings. This plant hangs from trees of various kinds, buthas no organic connection with them and derives no materialsfrom them. The branches are wiry and the leaves slender,7 98 ABSORPTION OF WATER AND MINERALS and both are thickly beset with overlapping scales under whichrain and dew gather and find entrance by osmosis into the cellcavities. Here the scales, like the velamen, serve both for theabsorption of water and protection against its loss. The scaleswhen dry are shrunken and lie close against the stem or leaf;but when wet their thicker outer wall swells and bulges outward,. FIG. 47.—Cross section through a water-absorbing scale of Tillandsia usneoides; a, a, water-absorbing cells partially filled with water. (After Schimper.) water is drawn into the cell cavities, and by their turgidity thescales rise and make room for more water beneath them (Fig. 47).Practically the whole plant body is thus enabled to imbibe water,and solutes that have come in the form of dust. Other Methods of Absorbing Water.—Some desert plantshave devices for absorbing water into the leaves. DiplotaxisHarra, for example, a cruciferous plant of the Egyptian andArabian deserts, has its foliage beset with stiff hairs which, actingas points for the radiation of heat, after sunset gather dew. Thehair is practically waterproof excepting at its base, where thedew, running down from above, forms a film over the wall andis quickly absorbed. There are some interesting anatom
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