BROMELIAD TRICOMES

by Penrith Goff, S.E.Michigan Bromeliad Society

 


    The epidermis of many plants grows attachments consisting of one or more cells and taking many different forms. These attachments are called trichomes (TRIH-combs), a word derived from Greek "hairy" and indeed, the trichomes we are most familiar with are the ones which give plants a downy or furry appearance. The Great Mullein, for example, or artemesia and the many other plants which grow under hot, arid conditions are covered with "hair" which protects against the glare of the sun, shelters against drying out in the wind, and helps perhaps to keep predators at bay. A more aggressive defense is mounted by the stinging nettle. Under the slightest pressure the tips of its stiff trichomes break off, forming virtual hypodermic needles which inject the hapless intruder with its "venom." Trichomes can be utilized to help a plant climb. One of the most interesting and successful adaptations to a nutrition-starved environment is the development of trichomes in the sundews, which exude nectar to attract insects, which are then trapped and digested.


   Bromeliad trichomes are complex cellular structures somewhat similar to an umbrella with a short shaft, the "shaft" being stalk cells, the "screen" being a disc-shaped shield. In Fig. d. above, the tillandsia trichome shields lie fairly flat against the epidermis so that the leaf is smooth, perhaps slightly velvety like Tillandsia xerographica. Not only does each bromeliad have its own unique trichomes, the trichomes on the upper (adaxial) side of the leaf are different from those on the lower   (abaxial) side of the leaf. If the shield edges turn up, the leaf surface will be rough as in Tillandsia ionantha. The disc may be more fully developed on one side, producing a fuzzy surface (T. crocata). The extreme is the hair-like extensions on the trichomes of T. tectorum.

     The trichomes have two important functions: to protect the plant from too much sun and to acquire and conserve moisture. Tillandsias (and other bromeliads) which grow in a shady, humid environment have fewer trichomes than those exposed to full sun, and are green. Depending on the amount of sun exposure to which they have adapted, the density and extensions of the trichomes cause the leaves to appear gray, silver, or white The cells of the extensions are hollow, so that they reflect light (up to 45%) and form good insulating barrier. When the leaf is wet, the cells fill with water and reflect very little light; the leaf appears green. The trichomes channel water very quickly through the stalk cells into the leaf interior but prevent water (water vapor) from escaping. With good air circulation the trichomes quickly dry out again and the plant regains its normal gray to silver luster.  


 
This sketch oversimplifies a fascinating but complex aspect of bromeliad anatomy an physiology. For greater detail, the reader is urged to read David Benzing, The Biolo o the Bromeliads, 1980, and Paul Isley, Tillandsia, 1987. 


 


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