Coleoptile

From Free net encyclopedia

Coleoptile is the pointed protective sheath covering the emerging shoot in monocotyledons such as oats and grasses. Coleoptiles have two vascular bundles, one on either side. Unlike the flag leaves rolled up within, the pre-emergent coleoptile does not accumulate significant protochlorophyll or carotenoids, and so it is generally very pale. Some preemergent coleoptiles do, however, accumulate purple anthocyanin pigments.

Phototropism of the Coleoptile

Early experiments on phototropism using coleoptiles suggested that plants grow towards light because plant cells on the darker side elongate more than those on the lighter side. In 1880 Charles Darwin and his son, Francis found that coleoptiles only bend towards the light when their tips are exposed. Therefore the tips must contain the photoreceptor although the bending takes place lower down on the shoot. A chemical messenger or hormone called auxin moves down the dark side of the shoot and stimiulates growth on that side. The natural plant hormone responsible for phototropism is now known to be indoleacetic acid (IAA).

Reference

Charles and Francis Darwin, "The Power of Movement in Plants" (1880)