MadSci Network: Botany |
Plants have evolved ways to get what they need without being able to move about. Generally this is done by growing in the direction of a desired nutrient or condition. This growth toward (or away from) something is called a "tropism". You may have heard of phototropism (shoots growing toward light) or gravitropism (roots growing toward gravity, shoots growing away from gravity). The process by which vines wrap around something for support is called "thigmotropism", growth stimulated by touch or contact. Like the other tropisms, it is the result of a stimulation of growth in a specific region of the plant. As in phototropism, where the growth is faster on the side away from the light, in thigmotropism, the growth is faster on the side away from the contact, thus causing the stem to bend in that direction (toward the contact). If you think about it you will see how this will cause the plant to bend toward the source of contact. Since it can't (usually) go through the contact, it will grow around it, continually curving as long as the contact is maintained. As to exactly how this increased growth rate is achieved, in most tropisms the plant growth hormone "auxin" is implicated and thought to accumulate on the side where increased growing is to take place. How does the auxin know to go there? Not much work has been done on thigmotropism, but scientists are looking at these questions, and seem to be finding involvement of calcium ions and/or calmodulin (a protein found in many animal and plant systems which controls developmental processes) or derivatives of fatty acids inducing the response. Is that what causes the auxin to accumulate? Don't know yet. This is an area which bears watching. You can find out more about auxins and tropisms in a Plant Physiology textbook (I use one by Taiz and Zeiger) or in a Botany textbook and in some general biology texts. For the truly serious about the physiology of all this (and you need a serious library also): Blechert, S; Bockelmann, C; Fusslein, M; Von Schrader, T; Stelmach, B; Structure-activity analyses reveal the existence of two separate groups of active octadecanoids in elicitation of the tendril-coiling response of Bryonia dioica Jacq. PLANTA, 207: (3) 470-479 JAN 1999 Authors: Liss, H; Bockelmann, C; Werner, N; Fromm, H; Weiler, EW Title: Identification and purification of the calcium-regulated Ca2+-ATPase from the endoplasmic reticulum of a higher plant mechanoreceptor organ Source: PHYSIOLOGIA PLANTARUM, 102: (4) 561-572 APR 1998
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