Light

Light is essential for the core physiological processes of a plant — photosynthesis, transpiration, seed germination and flowering all depend on it. The portion of sunlight the human eye can detect is called visible light, spanning roughly 400 nanometres (violet) to 700 nanometres (red). Not all of this range is equally useful to a leaf: photosynthesis runs fastest in the blue band (400–500 nm) and the red band (600–700 nm), while the green band (500–600 nm) in between is only weakly absorbed by chlorophyll — which is also, incidentally, why healthy leaves look green: that band of light is reflected rather than used. Beyond driving photosynthesis directly, light also regulates a whole set of other plant behaviours — the opening and closing of stomata, various plant movements, seed germination, flowering, and even the production of runners and tubers. Plants differ in how much light they need or can tolerate: heliophytes are light-loving plants (most angiosperms fall into this group), while sciophytes are shade-loving plants (bryophytes and pteridophytes are typical examples). Some plants respond to the daily light cycle with a diurnal rhythm — flowers of poppy, chicory and dog rose open before dawn, while evening p …