Coleoptile
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.
Coleoptile consists of very similar cells that are all specialised to the fast stretch growth. They do not divide and just increase in size, accumulating more water. Coleoptile also have several (frequently two) water vessels along its axis to provide a water supply.
When coleoptile reaches the surface, it stops growing and the flag leaves penetrates its top, continuing to grow along. The wheat coleoptile is most developed in the third day of the germination (if in the darkness).
Tropisms
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 stimulates growth on that side. The natural plant hormone responsible for phototropism is now known to be indoleacetic acid (IAA).
Coleoptiles also exhibit strong gravitropic reaction, always growing upward and correcting direction after reorientation. Gravitropic reaction is regulated by light (more exactly by phytochrome conversions). The direction of this regulation (stimulation or suppression) varies between species and even between varieties.
