. Plant anatomy from the standpoint of the development and functions of the tissues, and handbook of micro-technic. Plant anatomy. ABSORPTION OF WATER BY AEROPHYTES 97. Fig. 46.—Showing at / the felty body covering the passage way through the exodermis of the aerial root of Sobralia macrantha. (After Haberlandt.) the cell cavities become alternately filled with air and water as a dry interval is succeeded by a wet one. The necessary soil constituents are doubtless obtained from the dust which gathers on the roots or is washed down to them from the other parts or from overhanging branches of th
. Plant anatomy from the standpoint of the development and functions of the tissues, and handbook of micro-technic. Plant anatomy. ABSORPTION OF WATER BY AEROPHYTES 97. Fig. 46.—Showing at / the felty body covering the passage way through the exodermis of the aerial root of Sobralia macrantha. (After Haberlandt.) the cell cavities become alternately filled with air and water as a dry interval is succeeded by a wet one. The necessary soil constituents are doubtless obtained from the dust which gathers on the roots or is washed down to them from the other parts or from overhanging branches of the tree on which the aerophyte is encamped; and from particles washed out of the atmosphere by the falling rain drops. Separating the velamen from the rest of the root is a cell-layer known as the exodermis (Fig. 45) which is similar to the endo- dermis (page 43) of the ordinary roots in having the cell-walls more or less thickened and suberized, with the exception of cells at intervals whose thin cellulose walls permit the passage of water and solutes which the velamen has gathered in. In some instances the outer wall, of these passage cells is covered externally by a felty mass of interlaced fibrous outgrowths (Fig. 46). It has been conjectured that this is a device to condense moisture from the atmosphere when rain and dew are not keeping the velamen supplied; but conclusive evidence is lacking to show that the vela- men has the power to condense water from the vapor state by this or any other device. In any event the felty covering may help to retard evaporation through the walls of the passage cells when the velamen is dry. When the velamen is wet the root is as if embedded in a saturated sponge, and when dry the velamen acts' as a niulch 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 not develop, although their fundaments are present in the young seedlin
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