ABSORPTION OF WATER BY AEROPHYTES 



97 



FIG. 46. Showing at / the felty body 

 cDvering 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 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 not 

 develop, although their fundaments are present in the young 

 seedlings. This plant hangs from trees of various kinds, but 

 has no organic connection with them and derives no materials 

 from them. The branches are wiry and the leaves slender, 

 7 



