Plant Tropisms And Other Movements

[Music] do observation of certain plants such as the common sunflower will reveal one of the types of movements that plants are able to make in the morning the flowers are turned toward the sun somewhat toward the east a few hours later responding to the direction of the sunlight the flowers have turned toward the southeast as the hours pass they continue to turn about midday they are facing south and in late afternoon they face southwest this response to a directional stimulus is a form of tropism one kind of plant movement we'll observe the tropism again under controlled conditions using time lapse photography again the stimulus is light this time from a light bulb watch the movement of the rhubarb plants as the bulb is moved over a period of several hours leaves and stems turn toward the stimulus this particular tropism is phototropism response to light tropisms are turnings or growth curvatures in relation to the direction of the stimulus another response to a directional stimulus is geotropism the effect of gravity on the direction of plant growth the trunk of a tree grows upward away from the center of the earth the root system grows laterally and downward under the influence of gravity geotropism is easily demonstrated with seedlings three grains of corn are planted the first one is upright the second on its side the third upside down plant number one is growing normally in number two the root is curving downward notice that in number three root and stem make reverse turns no matter how the seed is planted the root goes downward and the stem upward it is believed that such growth responses are controlled by hormones in the growing cells observe what happens when a root strikes an obstacle such as this stone even though deflected the root continues its downward path another tropism is the response of tendrils in certain climbing plants the tendril a slender whip-like organ of the stem is sensitive to contact when the tendril of the sweet pea is stroked on the lower side we can see in time lapse how it curves toward the stroke side tendrils help support climbing plants here a support is placed near the sweet pea when the tendril touches the support the tendril is stimulated to twine about it this is thigmotropism a response to touch or contact in addition to trophisms plants exhibit other movements that can easily be observed on a bright summer day dandelions are in full flower the blossoms are fully open let's look at these same dandelions when there is little or no sunlight they are closed later as the light increases we see them beginning to open again this is a gnastic movement a response to changes in light intensity and to temperature let's observe this gnastic movement in the tulip in time lapse as the light intensity is varied we see the closing and opening of the flower illustrating nick tenacity one of several gnastic movements sleep movements is another name for nick dynasty this response to diminishing light and temperature protects the pollen from dew and rain nicktonasty occurs in leaves as well as flowers the leaves of the common wood sorrel fold when the light intensity is less a typical gnostic movement they open again when the light stimulus is stronger the leaves of the mimosa plant are so sensitive to stimuli that we can observe responses without time lapse we'll demonstrate a reaction to a marked change in temperature watch the response along the leaf stems this is an example of thermonasty another kind of gnostic movement all nastic movements are non-directional external responses to stimuli we'll try another non-directional external stimulus the stimulus in this case is shock and it travels along the leaf stem or petiole the gnostic movement of the leaves is called seismonasty let's consider the nature of this movement at the base of the leaf stem or petiole is a hinge-like enlargement the pulvinus when the cells of the pulvinas are turgid that is filled with fluid the leaves stand erect the reaction to a shock is a loss of fluid from the cells in the lower portion of the pulvinus and the leaf reflexes downward one interesting aspect of seisminasty concerns the way reaction to shock can be prevented by use of an anesthetic the mimosa plant is now in a glass tank into which some ether is poured the tank is covered and within a few minutes the plant is anesthetized this temporarily renders it insensitive to shock thus far we've been seeing two major kinds of plant movements gnastic movements responses to non-directional stimuli and tropisms growth curvatures in response to directional stimuli but plants exhibit a third kind of movement related to their growing called mutational movement the waving motion of these bean plants is a mutational movement unlike tropisms or gnostic movements mutational movements are responses to internal stimuli in the plants if we observe these plants from above we see that the mutational movement is circular this is termed circumnutation circumnutation is easily seen in long slender climbing plants or twiners such climbing plants usually grow on some sort of a support let's see how we'll place a support near the plant then in time lapse will follow the growth pattern the plant sweeps round and round in a circumnutational movement when the vine touches the support the growth pattern is changed and the plant twines tightly about the stick as it grows here we have two variations in mutational movements the plant at the left exhibits mutational movements in the leaves and stems the circumutation of the other plant results in the stem twining about the support let's review the three major types of plant movements we've seen atropism is a growth curvature response to a directional stimulus this particular tropism is thigmatrapism an external touch stimulus illustrated by the action of a tendril this tropism geotropism is a growth curvature in response to gravity a gnastic movement represents another type of movement a common gnastic movement is illustrated by nick tenacity the sleep movements of plants and by thermonasty the response of plants to an external variation in temperature the third kind of movement mutational movement is any growth curvature in response to internal stimuli mutations represent a basic growth pattern of most plants all plant movements are responses to stimuli some external some internal responses to these stimuli produce the various kinds of plant movements