Note 46 — Chapter III · DIFFERENTIATIONS AMONG THE OUTER TISSUES OF PLANTS.

A critical comment made on this sentence runs as follows:—“The aërial roots of most epiphytic orchids contain chlorophyll in their cortex throughout their length, but the cortex being covered by a ‘velamen’ of air-containing cells which break up and reflect incident light, the green colour is not visible through this opaque coat. When moistened the cells of the velamen take up water and the green colour immediately shows through. Such roots do not however possess stomata. The roots of certain species of Angræcum, however, contain the whole of the assimilating tissue of the plant.”