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I know of one or two exceptions to the claim that 24 hours of light is good for aquatic plants. Hygrophila polysperma and some other Hygrophilas on 24 hours light produce distorted leaves with no air filled spongy parenchyma between upper and lower epidermis. The leaf looks narrower, distorted and dark green because of the lack of any air in the leaf. Back in the days when H. polysperma was not on the Federal banned list I could always tell if a pet store kept their tank lights on for 24 hours by looking at the leaves on the Hygrophila. Also, the alga, Hydrodictyon, water net, can't form its nets with their six sided openings in 24 hours light apparently because mitosis is limited to the dark period. According to a paper I ran across, there are a number of algae, including Cladophora, that only do mitosis in the dark. I wonder if the absence of a dark period could kill Cladophora, if you kept it up long enough.
 

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I suppose you could give plants 'days' that are longer or shorter than 24 hours. One thing I know is that the photoperiod---the part of the day that plants measure and which determines whether they flower or grow vegetatively or go dormant---is the dark period. Long day plants are really short night plants that require a night shorter than some length of time in order to bloom. Short day plants are really long night plants that require a night longer than some amount of time in order to bloom.
 

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Can you please give some examples as to wich plants you are referring to?
Christmas cactus is a long night plant. It won't bloom unless it has a long, uninterrupted night. Some Echinodorus species bloom only with a short night and some bloom only with a long night. In general, tropical species near the equator are not sensitive to night length, but temperate species are. If temperate species bloom in late spring or summer, they are short night plants, and, if they bloom in the fall, they are long night plants.

During the day they soak up nutrients, and store them as food once broken down into something they can actually use. So I assume that it's safe to say, the growth rate is dependent upon the amount of nutrients stored readily as food, wich is dependent upon the light intensity, dependent upon the length of lighting time. Once lights are out, the plants grow in relation to amount of food stored? What is the ratio, term, length, timing of lights out time, when the plant says "Hey, look buddy, I just can't grow no more, show me the liiiiiiiiiiiiiiiiiiiight!"
I am pretty sure that most plants grow both when the light is on and when it is off. There are exceptions, as in some of the algae mentioned previously where they limit cell divisions to the dark period. It is also safe to assume that plants will grow more given longer days than shorter days at the same light intensity.
 
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