Why and how do fallen trees, such as this white gum from Western Australia (pictured above), grow stems vertically from a horizontal trunk?
Pat French
Telford, Shropshire, UK
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Two major factors that contribute to the specific shape of a tree are its genetics and the flow of auxins, their growth hormones.
A tree inherits the density, position and angle of its buds from the DNA of its species. In early years, branches may be staggered or paired depending on bud position and orientated by their angle to give a typical tree shape.
However, there is also the auxin effect. These hormones are controlled by external factors.
The auxins in tree roots tend to be 鈥減ositively geotropic鈥�: they accumulate in such a way that a root grows towards a gravitational pull, in other words, downwards.
In the branches, the hormones are negatively geotropic and positively phototropic, causing growth away from gravity and towards light.
An unshaded stem such as the central one tends to grow straight up even when the sunlight in its location always falls at an angle (that is, beyond the tropics of Capricorn and Cancer). This is because growth in opposition to gravity predominates. For shaded lower branches, growth towards light predominates and they tend to grow outwards towards it. These factors combine to give the tree its recognisable shape.
When the gum in question fell, a number of dormant buds along the trunk were exposed to clear sunlight and therefore started to grow vertically upwards, as seen in the picture. As competition develops and subsequent sub-branches become shaded, each treelet will form the typical white gum shape. Eventually, the whole group of new trees will combine. Some stems will be so shaded that they die out and the whole group will take on a shape still recognisable as a white gum.
Recently, an iconic sycamore tree, growing by Hadrian鈥檚 Wall on the border between England and Scotland, was felled. One option is to let the stump regenerate by growing many new stems, forming a coppice that will eventually adopt the shape of a wide sycamore tree.
Chris Daniel
Glan Conwy, UK
Plants respond to external stimuli such as light, moisture and gravity. The new vertical growth from the tree in the picture shows the tendency of plant stems to grow in the opposite direction to the acceleration due to gravity, a property that is known as negative geotropism, or negative gravitropism.
Gravity-sensing cells called statocytes in the plant shoots contain starch-producing organelles called statoliths, which are denser than the surrounding cytoplasm in the cell and sink to the bottom under the influence of gravity. This chemically triggers an asymmetric distribution of auxin, a hormone that regulates plant growth. This can, for example, cause increased growth and lengthening of the cells on the lower side of a shoot, so that it curves upwards against gravity and towards the light. Strangely, in roots, auxin has the opposite effect, called positive geotropism or positive gravitropism, causing the roots to grow downwards.
Horticulturalists and farmers make use of negative gravitropism in plants. The can be trained horizontally in espaliers to improve fruit production. The low auxin content on the upper side of the branch allows shoots called watersprouts to grow, orienting themselves vertically against gravity. In traditional , the main stems of shrubs are trimmed and partially cut through near the ground, then bent over at 35 degrees or more and tied into position. This stimulates new growth from the stems, generally in the vertical direction, to create a dense, living fence that provides a refuge for wildlife and containment for livestock.
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