90 LIGHT, VEGETATION AND CHLOROPHYLL 



produces with it 30 grammes of starch or substances of 

 similar composition. 



Respiration 



Photosynthesis has the reverse effect to that of respiration. 

 In respiration, the oxygen of the air combines with organic 

 substances and this combustion produces carbon dioxide and 

 water vapour, which are given off. 



Like all other organisms, plants breathe, day and night. 

 Even the green parts, while they are receiving light and are 

 assimilating through photosynthesis the carbon dioxide of the 

 air, are, at the same time, performing the opposite function 

 of respiratory combustion. 



At night, only respiration takes place, but during the day 

 photosynthesis predominates. There is a level of illumination 

 at which respiration and photosynthesis compensate each 

 other exactly and at which the overall gaseous exchanges 

 balance. This point, called the point of compensation, gives 

 an idea of the relative intensity of the two processes. It is 

 generally reached at a much lower illumination than that of a 

 dull day; the highest is at 4,200 lux for a hchen, Pelligera 

 canina, but this is exceptional. For shade plants, compensation 

 occurs at less than 100 lux, for plants which grow in full 

 sunlight, at a few hundred. These illuminations are 100 or 

 1,000 times lower than those of full dayhght. We may therefore 

 conclude that at a medium latitude photosynthesis generally 

 predominates from dawn to dusk. In July, at 69° N., it 

 continues without interruption for twenty-four hours a day. 



The Influence of Illumination 



Certain species of plants, for example deep-water algae, 

 can maintain life, and therefore effect photosynthesis, in 

 extraordinarily low illumination — no higher than moonUght. 

 One green moss, Schistostega ormundacea, Uving in dark 

 caves where the illumination is of this order, is adapted to 

 such conditions and possesses a layer of superficial cells in 



