ABSORPTION OF CARBON 



41 



plants occur and compared it with the intensity of light in an 

 open place. The fraction obtained shows with what part of full 

 daylight a given plant may be satisfied and is, consequently, an 

 indicator of their shade tolerance. The following values of the 

 relative light minima (in fractions of full daylight) were obtained 



Fig. 15. — Leaves of the hazelnut: a and b, leaves exposed to the sun; c, d, c, 

 shaded leaves (after Hesselman from Lundegardh) . 



for some common trees: larch, }; birch, T to £; pine, £ to TT ; 

 oak, 2V; spruce, ■& to -gV; maple, 3V; beech, -fa to ¥ V; Buxus 

 sempervirens, t ^q. These values show that the most light tol- 

 erant of our trees are larch, birch, and pine, which like but 

 very little shading. The woods and groves formed by these 



Fig. 16. — Comparative size of plastids in sun and in shade plants: a, Taxus 

 (shade); b, larch (sun); c, locust (sun); d, beech (shade) (after Lubimenko) . 



trees, therefore, are thin and light and the soil is covered by a 

 thick, herbaceous growth. The spruce and the beech, on the con- 

 trary, are shade-enduring trees, and the forests formed by them 

 are thick and gloomy. Their heavy crowns almost permit no 

 light to pass and the soil in these woods is devoid of herbaceous 

 cover. 



