ASSIMILATION OF CARBON BY PLANTS 197 



of full daylight a given plant will be satisfied and is, consequently, 

 an indicator of the shade tolerance of such a plant. The follow- 

 ing values of the relative light minima (in fractions of full day- 

 light) were obtained for some common trees: larch, }i; birch, 

 H to }^; pine, }^ to Ki; oak, ^q; spruce, J-^s'to H3; maple, 

 Hb', beech, }yQO to }4o', Buxus sempervirens Hoo- These values 

 show that the most light tolerant of our trees are larch, birch, 

 and pine, which like very little shading. The woods and groves 

 formed by these trees, therefore, are thin and light, and the soil 

 is covered by a thick, herbaceous growth. The spruce and the 

 beech, on the contrary, are shade-tolerant trees, and the forests 

 formed by them are thick and gloomy. Their heavy crowns 

 permit almost no light to pass, and the soil in these woods is 

 devoid of herbaceous cover. 



The light and shade tolerance of plants is to a certain degree 

 determined by the intensity of their respiration. With a high 

 light intensity, the production of organic substance in the process 

 of photosynthesis exceeds about ten times its expenditure in the 

 process of respiration, and the ratio between photosynthesis and 

 respiration is of no consequence. But with a decrease of light 

 intensity, the overbalance of photosynthesis gradually diminishes. 

 As has been noted already, respiration does not change appre- 

 ciably with the intensity of light. Finally, a point is reached 

 where the processes of photosynthesis and respiration are in 

 equilibrium, the plants show neither increase nor decrease of 

 organic substance, and the surrounding atmosphere is enriched 

 by neither oxygen nor carbon dioxide. This state of equilibrium 

 has been termed the '^ compensation point," and its position is 

 of major importance for the successful growth of plants at low 

 light intensities. It has proved that differences between various 

 groups of plants are more frequently determined by differences 

 in intensity of respiration, and not of photosynthesis. Light- 

 tolerant plants are characterized by a higher respiratory intensity, 

 and their compensation point is therefore situated comparatively 

 high, while shade-tolerant plants are distinguished by a low 

 intensity of respiration and by a lower position of the compensa- 

 tion point. 



According to Boysen Jensen's determinations (1932), the 

 compensation point of the light-tolerant mustard plant coincides 

 with a light intensity of 900 candle-meters ; of the less light tolerant 



