An Orchidist's Miscellany
Orchids never produce a primary 'carrot-like' tap root but rather many finer secondary roots are formed from the stern. Orchids are unusual in that the root structure proper (photo A) is surrounded by an outer layer of spongy whitish cells which have lost their living contents - the velamen. The actual function of this white velamen has been subject to some controversy. It has been suggested that one of its greatest values lies in its ability to catch and hold the first, relatively mineral rich, run-off from the host tree when it rains. Aerial roots have been found capable of absorbing nutrients through the valernen layers. It is also possible that the velamen on the root is not primarily a structure for water absorption, but rather of water conservation. Plants growing in dry exposed habitats where periods of moisture stress occur generally have thicker velamen layers, while those from moister habitats generally have a thinner layer. Epiphytes, therefore, generally produce heavy velamen, with terrestrials having a thinner coating to their roots.
The roots produced vary in size between genera. When actively growing a light green growing tip is apparent, with the white velamen formed some distance back - photo B. When active growing ceases, the velamen will virtually cover the growing point, protecting the generative tissue. Few orchid roots are covered by hair, paphiopedilums (photo C) being one of the few orchids of this type.
Photomicrographs of orchid roots are illustrated (Epidendrum ibaguense unless other- wise noted). both in cross
(photo D) and longitudinal (photo E) section. There exists an outer band of white (dead) epidermal tissue in a
mature root - the velamen, which can vary from 1 to 18 layers in thickness. Inside this lies the green 'living'
cortical tissue, the cortex proper of usually some 15 layers sandwiched between an outer exodermis and inner endodermis.
All photographs by P C Tomlinson
Site established 9th May 1998