A few explanations for why trees grow the way they do… and how this can be good to know when developing your bonsai tree’s…
Gravitropism (also known as Geotropism) is growth in response to gravity so the roots grow down and shoot groves upwards and auxin is the key hormone in this process. With bonsai trees you can use this to your advantage by changing the planting angle of the tree in the pot, or alternatively tilt the pot in a desired direction. The tree will still want to grow shoots in an upwards direction.
Phototropism is growth in response to directional light and again is controlled by the hormone auxin. Therefore it is important to regularly turn your bonsai trees to ensure that the foliage and branches receive an even distribution of light over the growing season for even foliage and crown development.
Hydrotropism is growth in response to a moisture gradient so roots will respond to differences in soil moisture content by growing towards regions of greater water potential. Therefore it is important that the bonsai tree soil is even in structure and allows moisture to be evenly accessible for the tree roots throughout the pot to avoid congested root development in one area.
Thigmotropism is growth in response to touch and it is the response of a root or shoot once it comes into contact with a solid object. Tendrils are a common example for climbers.
Heliotropism is the plants ability to carry out solar tracking where the leaves and flowers have the ability to move diurnally orientating themselves either perpendicular or parallel to the sun’s direct rays.
Thigomomorphogenesis is a growth response unrelated to the direction of external stimulus. It is that of mechanical stimulus such as rubbing or bending of stems which inhibits their elongation and stimulates their radial expansion. A great example is the bark and trunk on Sweet Chestnut which can often have a helical twist.
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