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Cahilog, Ma. Reza

Garcia, Bea Clarise B.

Tueres, Alyssa Nicole

Plant Movements

Thigmonasty refers to the non-directional

response of a plant to :

touch flexure vibration blowing slight heat chemical stimulus electrical stimulus lack of water

which results to the rapid loss of water from its turgid

motor cells. This loss of turgidity then causes the leaf to

temporarily droop downwards. Makahiya is a good

example of a plant which exhibits thigmonasty.

Figure 1. Makahiya leaves before (left) and after (right)

stimulation of leaves.

Makahiya (Mimosa pudica Linn.) is a commonly

known weed having compound leaves and pink

inflorescences. Here are the parts of the makahiya

exhibiting thigmonastic response.

Figure 2. Parts of makahiya.

Pulvini are responsible for the thigmonastic

response of makahiya. They are enlargements located at

the end of each pinnule, pinna and petiole, named as

tertiary, secondary, and main pulvinus respectively.

Figure 3. Tertiary, secondary and main pulvinus of

makahiya.

Each pulvinus has extensor cells located at the

upper portion and flexor cells at the lower portion.

Extensor cells contain sugar molecules while flexor cells

contain potassium ions and water. The turgidity of flexor

cells and flaccidity of extensor cells maintain the erect

position of pinnule, pinna or petiole before a stimulus.

Figure 4. Pulvinus before a stimulus.

Upon application of a stimulus, the flexor cells release the

potassium ions causing the water to flow out through

osmosis. The water then also flow towards the extensorcells through osmosis, due to the high sucrose content.

Figure 5. Pulvinus upon stimulation.

The turgidity of the extensor cells and the flaccidity of the

flexor cells now cause the pinnule, pinna or petiole to

droop.

Figure 6. Pulvinus after a stimulus.

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Meanwhile, the release of potassium ions

triggers the neighboring pulvini to release their potassium

ions as well. This is comparable to the process of

propagation of action potential in an axon of a nerve cell.

Figure 7. Propagation of action potential in an axon of a

nerve cell.

Thus the stimulus causes the spread of the thigmonastic

response by the sequential drooping of the pinnules, pinna

and petioles.

The effect of heat stimulus on the apical and

lower parts of the plants was compared. A flame was

placed underneath the end of a pinna, causing a domino

effect on the drooping of pinnules, pinna and petioles.Several trials were done for the apical and lower stimulus.

In a selected recording of a trial in the lower

stimulus, seven pinna drooped for 34 seconds, some of

which are located at the other leaves. In another selected

recording of a trial in the apical stimulus, six pinna

drooped for 36 seconds, some of which are also located at

the other leaves. Thus for both parts, there was a

generalized response.

However, comparing the rest of the trials, there

were not as much significant difference in the number of

pinna which had drooped. This could be attributed to the

unequal application of heat stimulus and the wind whichcaused the heat to be dispersed. But the expected result is

that the lower stimulus should be more responsive than

the apical stimulus. Since the lower parts of makahiya are

more mature than the shoot parts, they should also have

bigger pulvini, thus creating more drastic spread of the

response.

The effect of touch stimulus on the uppermost

and lowermost pinnule was also compared. A pencil was

placed underneath the pinna and triggered an equal

amount of pressure for both locations in different

makahiya plants.

Figure 8. A pinnae after a touch stimulation of the

uppermost pinnule.

Figure 9. A pinnae after a touch stimulation of the

lowermost pinnule

The results showed a more drastic response

upon stimulation of the lowermost pinnule than the

uppermost pinnule, due to the difference of distances of

the origin of response from the secondary pulvinus. In the

lowermost pinnule, the stimulus was passed on to the

secondary pulvinus faster before it diminished, thus

triggering a more drastic response by drooping of the

entire leaf. In the uppermost pinnule, the stimulus

diminished before it had reached the secondary pulvinus,

thus triggering a faint response by drooping of some of the

pinnules.

In conclusion, the lower leaves and the

lowermost pinnule of makahiya showed a more drasticthigmonastic response. The possible adaptations of

thigmonasty are as follows:

protection less appetizing to herbivores less transpiration in hot conditions removal of harmful insects