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Normal plants respire and use sunlight for photosynthesis during the day, and rest at
night. Some orchids in arid habitats can split this food production process into two stages, breathing during the
night when there is less risk of water loss, and storing carbon dioxide for use during daylight hours when there
is plenty of sun for photosynthesis. During these daylight hours, when there could be significant water loss, the
stomata remain closed. Dendrobium speciosum is a typical plant showing this adaptation. The plants response is
shown in the chart. It can be seen that at dawn the air temperature falls to 15oC. rising to 32oC. during the day.
Solar radiation levels (sunshine) increase, and the water vapour differential increases indicating the plant stops
respiration and controls potential water loss. As photosynthesis is completed, the leaf acid content falls as food
is manufactured. In the evenings, temperatures fall and humidity increases, allowing respiration to start again
and more carbon dioxide stored for use the next day.
Not all plants show this metabolic adaptations, but if heavy leaves and other noted physical
adaptations are present, there is a good chance that the plant will have this adaptation. Plants known to have
this metabolic process include many laelias and cattleyas, Epidendrum radicans, Schomburgkia crispa, Sophronitis cernua, Dendrobium taruinum, speciosum,
Aerides odoratum, Angraecom sesquipedale, Arachnites flox-aeris and hookeriana, Ascocentrum ampullaceum, Phalaenopsis amabilis.
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