PLANT MOVEMENT Higher plants, being fixed to soil, cannot move from place to place. Really, have you ever seen a tree with legs?! Yikes.
However….. within the plant body various mechanisms are in constant motion! For example, there is movement of: water minerals food hormones and, more! Just when you thought plants just stood there looking green… now you know there’s a lot of action going on inside those plants!
Certain parts of the plant body (in response to external stimuli) show physical displacement called movement. Even lower unicellular (one-celled) plants also show movement from place to place. Such movements may be autonomic (automatic) or induced (created from another place).
Plants move in response to: * light * touch * chemicals * temperature * gravity * H2O
Factors that cause movement are called stimulus. The place that receives the stimulus on the plant is called the perception site. The plasma membrane is necessary for the message to be able to pass from one place to another so that the plant can respond properly.
There is a time delay between the time the stimulus is applied and the time the response begins. This time is called reaction time. It may vary and depends upon the intensity of stimulus and the kind of response. If the stimulus is weak, there may not be any response at all, but if the stimulus is adequate or in right quantity the response is positive. The time required to cause the proper stimulus is called presentation time.
Once the plant responds to the stimulus the plant structures always come back to their original position. This process is called recovery and the time required is referred to as relaxation time, which varies from species to species. Ahhhh…..I’m chillin’ now!
Wow! Did you know that there are many types of plant movements?… So cool!
We will discuss the following plant movements and how they occur…. Drum roll, please…..
Growth movements We will talk about several categories of plant movements: • These that happen because of: • Internal stimuli • External stimuli Turgor movements • Turgor movements happen because of changes in the internal water pressure of the plant. Turgor movements are often (but not always) started by contact with objects outside of the plant. They are: • “Sleep” movements • Solar tracking • Water conservation movements
Internal Stimuli Nutations – Charles Darwin observed spiraling movement of the growing tip. This type of movement is common to many plants. Nodding Movements – This is a slow, oscillating movement that appears like the emerging stem (hypocotyl) “nods” from side to side like an upside down pendulum. Twining Movements – Tendril twining, which starts once the tendril comes into contact with an object. This action happens when the cells elongate on one side and shrink on the other side. Contraction Movements – contractile roots of bulbs Nastic Movements -occur in response to environmental stimuli but unlike tropic movements, the direction of the response is not dependent on the direction of the stimulus. Some of the most spectacular plant movements are nastic movements. These include the closing of the carnivorous Venus Flytrap leaf when it captures prey or the folding of the mimosa leaf when it is disturbed.
External Stimuli • Tropisms are movements in one direction that occur as a result of an external stimuli. Tropic movements are in three phases: • Perception phase when plant receives a stimulus on one side. • Transductionhappens when one or more hormones are unevenly distributed which results in • Asymmetrical growth (where one side of the plant has greater cell elongation than the other.) • There are two tropisms we will investigate:
Phototropism Phototropism is growth in the direction of the light source (which is the external stimulus). The cells on the plant that are farthest from the light source activate the hormone auxin that then causes the plant to have elongated cells on the farthest side from the light then causing the plant to bend twoards the light. Growth towards a light source is a positive phototropism, while growth away from light is called negative phototropism. Most plant shoots exhibit positive phototropism, while roots usually exhibit negative phototropism.
Gravitropism Gravitropism is a growth movement by a plant in response to gravity. Charles Darwin was one of the first to scientifically document that roots show positive gravitropism and stems show negative gravitropism. So, roots grow in the direction of gravitational pull (i.e., downward) and stems growing the opposite direction (i.e., upwards). Example of gravitropism in the remains of a cellar of a roman villa in the Archeologic Park in Baia, Italy.
Turgor Movements Turgor movements are reversible changes in the position of plant parts due to a changes in internal water pressure and are often, but not always, started by contact with objects outside of the plant. The movement of Mimosa pudica (Touch-me-not) leaves when touched is an example of turgor movements. Another example of this is the reaction seen in the Venus fly trap when an insect lands inside its leaves. http://www.youtube.com/watch?v=PRo4rg07_gg&feature=fvwp&NR=1 http://www.youtube.com/watch?v=o3hk9Sz--h4