Flowering plants also need a communication system in order to respond adaptively to changes in their internal and external environments. A seed must germinate in suitable conditions, and the radicle and plumule must grow downwards and upwards respectively. The plant must then grow adaptively with respect to light intensity and variations in water content of the soil. A plant growing in a region with seasons must respond to those seasons and flower at the right time. The plant may need to respons to predators or abiotic stress.
sensor(receptor) >>> plant hormone >>> effector
Many plant responses are by means of growth. A growth response is called a tropism when the growth is in a direction determined by a stimulus. A response towards this is said to be positive and one away from the stimulus is negative.
Light = phototropism
Gravity = geotropism
Touch = thigmotropism
Much work has been done on the phototropic response of cereal coleoptiles, in which the tip of the coleoptile is the receptor region of the plant. A cereal coleoptile responds most to wavelengths in the blue region of the spectrum around 450nm. It is not yet certain what the photoreceptor is, but two molecules in the coleoptile tip have absorption spectra with maxima in that region: carotene and riboflavin. It is likely riboflavin is the receptor.
The plant's effector is the region of cell elongation, just behind a shoot, coleoptile or root tip. In…