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.

Hairy root of paphiopedilum

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.

If the root is pulled apart carefully, the outer layers can be pulled away from the thin wiry central core - as can be seen in photos F and G. This shows the central pith clear in the centre. The balance of the true root tissue - the pericycle - and vascular tissue (xylem and phloem) can be clearly seen in photo A, surrounding the pith, in an unstained cross section of this area.
Most orchid roots grow downwards into the media, or on the surface on which the plant is established. There are, however, in some genera (some oncidiums, catasetums) different roots which grow upwards, which involve an interesting problem of root growth control. The upwards growing roots are usually distinct in their structure from normal roots, and appear to function as debris traps, catching plant detritus which releases nutrients to the orchid as the material decays.

All photographs by P C Tomlinson

Site established 9th May 1998