Question

Difference bt internal and external cause of plants movement? In thigmotropism the stimulus is light but changes are internal so why is not in autonomic movement?

Answer 1

Movements are generally considered as a sign of life. Movements are the characteristics of both animals and plants.

However, the larger and more complex plants are fixed in position, but many of their parts or organs carry on several kinds of movements. Many of these movements are generally overlooked because of the slowness with which they take place.

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Important among these movements are curvatures, twining, leaf movements, and the opening and closing of flowers. All such movements are the result of protoplasmic sensitivity which quickly responds to external and internal stimuli.



Plant Movement Type # 1. Movements of Locomotion:

Streaming of protoplasm within the cells, free movements of naked masses of protoplasm (e.g., the slime fungi) and those of uni-and multicellular structures or entire body of the organism (e.g., several uni-and multicellular algae) are called movements of locomotion.

Such movements maybe of two types:

(A) Spontaneous:

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These movements are the result of internal stimuli and are relatively independent of the environment. Common examples of such movements are ciliary movement of ciliated protoplasmic bodies like zoogametes, amoeboid movement of protoplasmic masses like plasmodium of slime molds, streaming of protoplasm within the cell, oscillating movements as shown by oscillatoria—a blue green alga (Figs. 7.1 to 7.4).









(B) Induced:

The result of response to stimuli coming from outside the plant. Among the external stimuli causing plant movements are chemical substances, contact, shock and fluctuations in light and temperature.

Depending upon the nature of the stimuli, induced movements may be of three types:

(i) Chemo-taxis,

(ii) Photo-taxis and

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(iii) Thermo-taxis.

(i) Chemo-taxis movements:

Such movements are brought about by certain chemical substances. For instance, in most of the bryophytes, pteridophytes and gymnosperms, certain chemical substances are secreted by their female reproductive organs in order to attract male cells (i.e., antherozoids) for fertilization. In ferns malic acid is secreted by archegonia which attracts antherozoids.

(ii) Photo-taxis movements:

In this case the external stimulus is light. Fluctuations in light intensity induce photo-taxis movements. Such movements are of common occurrence in algae which are repelled by strong light. Photo-taxis movements are also shown by chloroplasts within the cells. According to the intensity of light chloroplasts arrange themselves in such a manner so as to avoid any decomposition of chlorophyll.

(iii) Thermo-taxis movements:

These movements are caused by fluctuations in environmental temperature. A number of freely moving organisms quickly respond to any change in the temperature. Streaming of protoplasm in the cells is greatly accelerated by an increase in temperature.

Plant Movement Type # 2. Movements of Curvature:

The larger and more complex plants being fixed to the ground are not capable to move freely from one place to another, but their various organs carry on several kinds of movements. By means of curvatures these organs may change their positions according to their requirement in order to carry on their functions more effectively. Movements of curvature may be either mechanical or vital. Vital movements are again of two kinds—spontaneous and induced.

(A) Mechanical Movements:

Such movements are exhibited by certain non-living organs of plants. For instance, many fruits, when they are fully ripe, burst open to disperse their seeds. These movements are caused by hygroscopic changes in the tissues of the fruit wall and as such certain fruits burst open when their walls are completely dry (e.g., Lupinus perennis, Iris versicolor) (Figs. 7.5 & 7.6) while others rupture after absorbing moisture from outside (e.g. Ruellia). Such movements are common in lower plants also, like bryophytes and pteridophytes; hygroscopic movements greatly help in the dispersal of spores from sporophytes in these plants.





(B) Spontaneous Vital Movements:

These movements are exhibited by various plant organs of their own accord. No external stimulus is involved in such movements.

These movements may be of two kinds:

(1) Movements of variation and

(2) Movements of growth.

(1) Movements of variation:

These movements are also known as turgor movements because these are brought about by changes in the turgor pressure of certain sensitive cells of the plant organs. These movements are mostly initiated in compact groups of relatively large and thin walled cells which occur in ‘motor organs’ of plants, but they may also occur in any tissue which is largely composed of living, thin-walled cells. Such movements are exhibited by lateral leaflets in Desmodium gyrans (Fig. 7.7), ‘sleep’ movements of the leaves and leaflets of many legumes, guard cells of stomata.



(2) Movements of Growth:


(i) Nutation:
.

(ii) Hyponasty:
.

(iii) Epinasty:


(iv) Circumnutation:



(C) Induced Vital Movements:
.