The Role of the Velamen of the Aerial Roots of Orchids

1. The number of cell layers and the volume of cells in the velamen of the aerial roots of the several species of orchids studied are not constant but may be affected by the relative humidity in which the roots are growing. 2. In the plants studied, the pneumathodes do not have modified tissue, as reported by Haberlandt. The pneumathode cells are not wetted by water but are rendered wettable by addition of a detergent. Cell walls of the pneumathode and of the exodermis are more heavily lignified than are those of other velamen cells. The lignin of the pneumathode cells may differ in composition from that of the other cells. 3. The intact mature velamen and exodermis of the aerial roots of certain orchids seem to be nearly impermeable to water and certain solutes. Though the velamen may absorb both, little of either seems to penetrate into the living cortex within. Radioactive phosphorus could not be detected in leaves of orchids even after aerial roots had been immersed in a solution containing it for as long as 96 hours. That the velamen is not a barrier to carbon dioxide and oxygen was shown by studies on photosynthesis and respiration of detached roots. The velamen was not found capable of "condensing" gaseous vapors in a measurable quantity. If the velamen is modified by mechanical injury or after the root becomes attached to a solid substratum, then the root may serve as an absorbing organ. The principal roles of the velamen in free aerial roots are mechanical protection and the prevention of excess loss of water from the cortex.