This color is a result of the thing called velamen which usually is composed of single or even of a number of levels of epidermal cells.    From Wordnik.com.  [EzineArticles] Reference

The velamen takes up moisture from the ambient atmosphere.    From Wordnik.com.  [EzineArticles] Reference

The roots are thick, spongy and are covered with white velamen.    From Wordnik.com.  [CreationWiki - Recent changes [en]] Reference

• Orchid roots are covered by a thin membrane referred to as velamen.    From Wordnik.com.  [EzineArticles] Reference

The velamen functions as a covering of the root system except for the short terminal tip.    From Wordnik.com.  [EzineArticles] Reference

When the velamen is too dry, its color is silvery or white while a watered velamen will appear green or slightly mottled.    From Wordnik.com.  [EzineArticles] Reference

Purple was the favourite colour for this velamen, or veil; because, when the sun shone through it, it cast such beautiful rosy tints on the snowy arena and the white purple-edged togas of the Roman citizens.    From Wordnik.com.  [A Book of Golden Deeds] Reference

Y tiemble su velamen de sombra cuando de mí se burlen los dioses invisibles cotidianos orfebres inmensa aventura de haber sido un acaso.    From Wordnik.com.  [veruscio Diary Entry] Reference

The corporal was described as palla corporalis, or velamen dominic mens, or opertorium dominici corporis, etc.; and it seems generally to have been of linen, though we hear of altar-cloths of silk (Greg. of Tours, "Hist.    From Wordnik.com.  [The Catholic Encyclopedia, Volume 4: Clandestinity-Diocesan Chancery] Reference

A profession of continence, and was called velum, velamen, maforte, flammeus (flammeum), flammeus virginalis, flammeus Christi (Wilpert.    From Wordnik.com.  [The Catholic Encyclopedia, Volume 15: Tournely-Zwirner] Reference