=Nastic Responses= '''Nastic Responses are '''non-directional''' responses to the intensity of a stimulus''' Because nastic responses are non-directional they ''cannot'' be described as positive or negative. [image:] '''Thigmonasty''' Nastic responses are usually faster, reversible movements rather than growth responses. ''Mimosa pudica'' also known as the ‘the sensitive plant’ displays a very rapid response to touch. This is known as a '''thigmonasty'''. This response is '''non-directional''', that is the plant will collapse it’s leaves regardless of where it has been touched, rather it responds to how hard it has been touched (the intensity of the stimulus). This response is shown in real time. The footage has not been sped up. [image:] '''Thermonasty''' ''Mimosa pudica'' will also respond in the same way to fire. Regardless of where the flame is held the leaves will collapse, a small singe will cause several leaflets to close; exposure to a larger flame for a longer period of time will cause every leaf on the entire plant to close. Because this is not a directional response it cannot be described as either positive or negative. [image:] '''Thigmonasty''' The ''venus flytrap'' exhibits one of the fastest known nastic responses. The trap of the venus flytrap is actually a highly modified leaf containing 3 hair cells on each half which act as motion detectors, also a non-directional nastic response. [image:] '''Etiolation''' Etiolation occurs when a plant is grown in darkness. Two plants are shown; the right was grown under normal conditions the left in complete darkness. The plant grown in the dark grows / elongates much faster without producing any additional leaves. The plant also produces less chlorophyll (appears yellow) and only has small leaves until it finds a light source. Although this is a growth response, it is a response to the intensity of the stimulus (the lack of light) rather than the direction of the stimulus. Because this is a non-directional response, it can also be considered a nastic response. [image:] '''Photonasty -Sleep Movements''' Some plants lower or fold up their leaves at night to prevent water loss and possibly reduce unnecessary exposure to harsh weather or predation. During the day the leaves are held in a relatively horizontal position to maximise their exposure to sunlight. This too, is a non-directional response (and therefore considered a nastic response). [image:] '''Photonasty / Thermonasty -Flowering''' Most flowers will open and close in response to light intensity and / or temperature. Most flowers open during the day to be pollinated by insects such as bees or by small birds. Some flowers open during the night to be pollinated by nocturnal insects such as moths, etc.
Credit: Ben Himme