The Effect of Gravity on Gum Tree Roots

The Gum Trees

• “Gum tree” is one common name for certain trees in the related genera Eucalyptus, Angophora and Corymbia, of which the vast majority of species reside in Australia, with a few types indigenous to New Guinea, Indonesia and Southeast Asia. These trees are collectively called “eucalypts." While “gum” reflects the gummy substance produced by only some of the species, the term "gum tree" is generally taken to be synonymous with eucalpyt. Hundreds of species inhabit a huge diversity of Australian ecosystems, from the high country of the Great Dividing Range to the shoulders of ephemeral watercourses in arid plains. While much physiological variety exists within the eucalypts, most have distinctive, oily leaves that in juvenile form are opposite in arrangement but are alternate as mature. The hard seedpods of gum trees come in a range of shapes and sizes. The trees' bark comes in several patterns and textures, such as the patchy arrangement of box eucalypts or the fibrous coat of “stringybark” gums.

Gravitropism

• The roots of gum trees grow downward in response to the Earth’s gravitational forces, a phenomenon of higher plants called gravitropism. “Tropism” stems from the Greek “tropos,” which means “a turn” and in biology refers to the directional submission of a part of an organism’s growth to some stimulus; in gravitropism, this stimulus is gravity. As the direction of root growth aligns with the direction of gravity’s pull, the situation is one of “positive gravitropism.” By contrast, the upward growth of gum-tree stems is, as in other plants, an example of “negative gravitropism”: That is, the stems “defy” gravity by growing in reverse to its tug.

Mechanics

• In eucalypts and other plants, the downward, gravity-compliant growth of roots is regulated by special hormones such as cytokinin and auxin, created in the root-cap and young shoots, respectively. Through complex chemical signals, these and other hormones trigger or stall cellular development both in the primary root and the lateral systems.

Other Examples

• In trees of the genus Eucalyptus, gravity doesn’t just dictate the directional growth of roots and stems. Its influence also helps in the development of a specialized organ in the seedling. A peglike structure under gravitational control develops at the bottom of the germinating seedling’s stem or hypocotyls. This organ helps open the seedling’s coat as it germinates.