Travis Lick Plant movement Plants, with their roots firmly fixed in the earth, seem immobile and vulnerable compared to animals, but this does not prevent them from reacting to stimuli in their environment. Plants react to much of the same environmental changes that humans and other animals respond to, such as light, pressure, temperature, water, contact and even the change of day into night. The term used to describe this movement of a plant in response to an external stimulus is called tropism. Plants accomplish this my manipulating chemicals inside their stems, leaves and roots. Some plants even use these non-muscular movements in order to gain nutrients. Carnivorous plants have developed the ability to gain valuable nitrogen from living invertebrates in areas where poor soil conditions occur. There are several different types of entrapment methods that include but are not limited to: the snaptrap, flypaper-trap and bladder trap. Approximately 640 species of carnivorous plants use these methods to help them gain the nutrients they need. Both carnivorous and non-carnivorous plants use tropisms to help grow properly, these tropisms and other more aggressive plant movements will be discussed in the following report. Plants are extremely sensitive to light and respond to it in a positive and a negative way. The stems and leaves of a plant have positive tropism which means they grow in the direction of light, while roots exhibit negative tropism in that they grow in the opposite direction of light. The change in the directional growth of a plant is caused by a chemical called auxin that resides in the coleoptile of a maturing plant shoot. See the picture below for an example and explanation of how this phenomenon occurs.
Figure 1: Phototropism Auxin is a chemical that promotes the rapid elongation of growth cells in the shoots of a plant. Auxin also helps the plant to remember where it has branched off in the past and also in which direction it needs to grow. In the figure, you can see the auxin gathering on the left side of the coloptile, which is the covering of a plant shoot that enables it to grow. As auxin moves down the shoot, the redistribution stops when it has evenly distributed itself along the shaded portion. Once this happens, the auxin stimulates cell growth and cell division only in the region opposite the light source. The plant cells react to the high auxin levels by transporting Hydrogen ions into their cell walls and raising the ph. Due to this, the plant elongates more rapidly on the auxin rich side causing the overall result, the extension of the plant in the direction of the light source.
In order for a plant to successfully grow from a seed, the roots and the shoots need to differentiate between up and down. The reaction of the pre-emergent stems and roots is the result of gravitropism, or the directional growth caused by the gravitational pull on the seed. Just as the shoot and the root have opposite reactions to light, they also have opposite reactions to gravity s pull. Roots exhibit positive gravitropism and therefore grow in the downward direction with gravity while the shoot has negative gravitropism and grows against gravity. This reaction to the force of gravity is undertaken in the nodule at the tip of a root called the root cup. Inside the root cap, there are sensors called statocytes that contain starch sediments. These sediments settle on the bottom most point in the root cap and indicate to the root cells the direction they need to grow. Figure 2: Gravitropism In the figure above, there are two normally growing corn kernels that were turned ninety degrees horizontally. The plants were then given six hours to grow and the results are pictured here. The top kernel labeled C, has an intact root cap, while the bottom kernel labeled NC, had the root cap removed. Both kernels grew at the same rate, however, the NC kernel was unable to correctly adjust it s growth to the change in gravitational pull, while the C kernel, containing the statocytes and starch sediments was able to correctly change it s directional growth. In order for a plant to survive, it must be able to extend its roots towards a source of water and nutrients. This movement, called hydrotropism, is carried out, once again, in the root cap at the tip of the root. This root cap, in addition to statocytes, contains other sensors that are able to identify water pressure and in turn, stimulate growth using auxin concentration in the root. At times, the gravitational pressure and the water availability may
be in opposing directions, when this is the case, the hydrotropic response overrides the gravitropic response. Figure 3: Hydrotropism In the figure above, the root had been growing downward by following the gravitropic response of the root cap until the water gradient was changed. The hydrotropic sensors inside the root cap respond to this by diffusing auxin to the bottom of the root cap causing the elongation of the lower level root cells in the direction of the water concentration. In addition to the plants ability to react to differences in light, pressure and water location, they also have the ability to change growth direction based on physical contact with a stimulus called thigmotropism. Thigmotropism in the root system occurs when the root cap contacts an object that is cannot penetrate such as a rock layer. Figure 4: Thigmotropism (root system) These carrot roots were part of an experiment in which they purposely obstructed the root growth to see them effect. The outer epidermal cells of the root cap have tactile papillae that act as touch sensors, when these papillae contact an object the cells become deformed and therefore cannot grow. This continued process of touch, deformation, touch, deformation, causes the root to grow
around the object obstructing it. When the papillae sensors no longer contact the object, then continue to grow normally. This reaction by the root is called negative thigmotropism. Conversely, the stem system of plants exhibits positive thigmotropism and grows towards objects that its papillae contact. This can be seen in plants that are viney like the sweet pea, cucumber, hops and kudzu. Figure 5: Thigmotropism (stem system) The tiny hair-like papillae on this plant have contacted the metal wire growing near it, this causes the positive response of auxin, causing elongation of cells on the side opposite when the papillae touched. This positive response causes cell growth in a circular pattern, encompassing the wire and giving the plant extra support. While most plants exhibit these tropisms for general growth and water intake, carnivorous plants have adapted additional methods of moving in order to procure necessary nutrients lacking in the soil. Carnivorous plants use trap systems, powered by water pressure and enzymes to snare and even digest live invertebrates. In areas of high rainfall and poor soil nutrients, sundews and pitcher plants have evolved the ability to capture small prey in a bladder trap in order to supplement their diets.
Figure 6: Sarracenia leucophylla Plants like the one listed above capture rainwater inside the cuplike folded leaf and attracts insects inside. Once its prey enters the cup, digestive enzymes, proteases and phosphotases, are secreted to help break down the insect. The released nutrients can then absorbed by the lining of the cup, enabling the plant to survive. Another adaptation that carnivorous plants use is called a flypaper trap. In the case of the Drosera binata plants, the leaves are covered by mucilage-excreting glands that secrete a sticky glue-like substance that traps insects when contact is made. Figure 7: Drosera binata
Once an insect becomes ensnared by the plant, thigmotrohic responses cause an elongation of the leaf, causing it to curl around its prey. This growth response stops the insect from being washed away by rainwater and also creates a small pool around the insect where it will be digested and absorbed. Possibly the most recognizable of the carnivorous plants in the venus fly trap. The Venus fly trap is the third type of carnivorous adaptation known as snap-traps. Snap traps use tiny trigger mechanisms and lobed leaves that act in the same manner as a mouse trap. Figure 8: fly trap 1 Figure 9: fly trap 2 In figure 8, you can see the unsprung fly trap with the trigger hairs lining the interior of the lobes. The closing mechanism occurs when two or more hairs are stimulated, causing an ion release and rapid osmosis to take place. This osmotic action forces the two lobes of the trap to close on its prey, figure 9. Once the prey is ensnared, thigmotropic responses cause an accelerated growing rate completely enclosing the insect, creating a sealed chamber where the prey will be digested. At first glance, it may seem that plants have very little control over the environment in which they are living. However, there are numerous processes working inside plants that allow them to respond and react to the changing world around them. Prior to ever growing a single leaf or root follicle the development of these complex processes have given plants the ability to not only inhabit, but thrive is almost every corner of the earth.
References: Figure 1: Phototropism: http://resources.ed.gov.hk/biology/english/images/environme nt/coleoptile.jpg Figure 2: Gravitropism: http://images.google.com/imgres?imgurl=http://www.bio.psu.e du/people/faculty/gilroy/ali/graviweb/images/lastphoto.jpg& imgrefurl=http://www.bio.psu.edu/people/faculty/gilroy/ali/ graviweb/toc.htm&h=329&w=325&sz=19&hl=en&start=16&tbnid=wfi tk2x_farvpm:&tbnh=119&tbnw=118&prev=/images%3fq%3dgravitrop ism%26svnum%3d10%26hl%3den%26lr%3d%26sa%3dn Figure 3: Hydrotropism: http://images.google.com/imgres?imgurl=http://www.learn.co. za/04/g12/bio/ang/pla/images/hydrotropism.gif&imgrefurl=htt p://www.learn.co.za/04/g12/bio/ang/pla/page4.html&h=138&w=5 90&sz=20&hl=en&start=4&tbnid=DcbI2Ru9fm2j7M:&tbnh=30&tbnw=1 32&prev=/images%3Fq%3Dhydrotropism%26svnum%3D10%26hl%3Den%2 6lr%3D%26sa%3DG Figure 4: Thigmotropism (root system): http://images.google.com/imgres?imgurl=http://www.kidsgarde ning.com/growingideas/projects/july04/tendril.jpg&imgrefurl =http://www.kidsgardening.com/growingideas/projects/july04/ pg2.html&h=188&w=250&sz=13&hl=en&start=1&tbnid=lrjgdcoxl7xk am:&tbnh=83&tbnw=111&prev=/images%3fq%3dthigmatropism%26svn um%3d10%26hl%3den%26lr%3d%26sa%3dg Figure 5: Thigmotropism (stem system): http://images.google.com/imgres?imgurl=http://botit.botany. wisc.edu/courses/img/botany_130/physiology/thigmotropism.jp g&imgrefurl=http://botit.botany.wisc.edu/courses/botany_130 /Physiology/thigmotropism.html&h=309&w=400&sz=22&hl=en&star t=1&tbnid=4bvhbwxjymokom:&tbnh=96&tbnw=124&prev=/images%3fq %3Dthigmotropism%26svnum%3D10%26hl%3Den%26lr%3D%26sa%3DG Figure 6: Seracenia leucophylla: http://www.rbgkew.org.uk/plants/carnivorous/images/serencen ia_leucophylla.jpg Figure 7: Droserra binata: http://www.rbgkew.org.uk/plants/carnivorous/images/droserra _binata.jpg
Figure 8: Venus fly trap 1: http://www.en.wikipedia.org/wiki/carnivorous_plant Figure 9: Venus fly trap 2: http://www.rbgkew.org.uk/plants/carnivorous/images/fly_trap.jpg Vartarian, Steffan (1997). Thigmotropism in Tendrils. Retrieved August 1, 2006, Web site: http://biology.kenyon.edu/edwards/project/steffan/b45sv.htm Takahashi, Nobuyuki (2003, April). Plant Physiology. Retrieved August 3, 2006, from American Society of Plant Biologists Web site: http://www.plantphysiol.org/cgi/content/abstract/132/2/805 The Gilroy Lab. Retrieved August 2, 2006, from Root Gravitropism Web site: www.bio.psu.edu/.../ gilroy/ali/graviweb/toc.htm Meškauskas A., Moore D., Novak Frazier L. (1999). Mathematical modelling of morphogenesis in fungi. 2. A key role for curvature compensation ('autotropism') in the local curvature distribution model. New Phytologist, 143, 387-399. National Gardening Association. Retrieved August 7, 2006, from Growing ideas, classroom connections Web site: http://www.kidsgardening.com/growingideas/projects/july04/p g2.html