William Sutherland


used during the middle ages) in less than 0.5 milliseconds (ms)” while the White Mulberry (Morus alba) engages in flower movement with its petals moving at “velocities in excess of half-the speed of sound” or 25 microseconds (µs) (or millionths of a second).1


A tropism is often a slow movement, growth or turning in response to sustained environmental stimuli. Tropisms may be either positive (turning towards the stimulus) or negative (turning away from the stimulus).2 The response in tropisms is dependent on the direction of the stimulus and thus directional.


Both tropisms and nastic movements are named for the stimulus they are associated with as illustrated by the table below:


Nastic Stimulus Movement


Chemical Chemonasty Gravity


None


Light/Dark Photonasty Night Nyctinasty Sunlight None


Temperature Thermonasty Touch Haptonasty Vibration Seismonasty Water Hydronasty


Tropism Chemotropism


Gravitropism or Geotropism Phototropism Nyctitropism Heliotropism


Thermotropism Thigmotropism Seismotropism Hydrotropism


Out of tropisms, photo- and gravi-/geo-tropisms are the most important and prevalent among plants. tropisms.


Consquently, each stem branch is capable of both Phototropism


Phototropic movement or growth in response to light (predominantly of the blue spectrum – scientific studies have shown that blue light elicits the greatest phototropic response; red light detected by phytochromes causes only slight stem bending or curvature (positive phototropic movement) and green light elicits no response) is critical for plant survival since it positions shoots (of the majority of plants) where the most light is available for photosynthesis. Phototropic


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