152 THE BIOLOGY OF FLOWERING PLANTS 



Lundegardh found that in the shade plants he examined 

 it lay between 1/120 and 1/140 sunlight, and in the sun 

 plants between 1/40 and 1/60. Boysen- Jensen found this 

 light intensity for shade plants about 1/5 that for sun plants. 

 The shade plant is therefore able to accumulate a carbo- 

 hydrate surplus at a lower light intensity than the sun plant. 

 One interesting fact is brought out — that the carbon 

 dioxide content of the atmosphere near the ground in a 

 forest is about 25 per cent, higher than that over open ground, 

 and that this is due to a much greater evolution of carbon 

 dioxide from the soil. Whether this is of great importance 

 to the plants of the forest floor where conditions of tempera- 

 ture or light are usually limiting is much less certain than 

 Lundegardh seems to think ; it would be interesting matter 

 for exact investigation. Lundegardh also attempts an 

 evaluation of the actual amount of assimilation by Oxalis 

 throughout the day, and finds that it barely covers the 

 loss by respiration for the twenty-four hours, and indeed may 

 fall below it. It rises above the respiration loss consider- 

 ably, only when the leaf is illuminated by sun spots pene- 

 trating the forest canopy, and this calls attention to the 

 importance for shade plants of this variable and uncertain 

 type of illumination. Lundegardh concludes that plants 

 like Oxalis can build up a surplus of carbohydrate only 

 when light travels freely through the leafless trees in spring 

 and autumn, and that in summer they just cover wastage 

 without increasing dry weight. This investigation empha- 

 sises the interest of the conditions of existence in the forest. 

 The question is one which might well be investigated in 

 the tropics, where the shade vegetation is much more 

 luxuriant than in our latitudes, a fact that points to an 

 important influence of temiperature, as well as of light, in the 

 assimilation of shade plants. Differences similar to those 

 between the leaves of sun and shade species also occur 

 between leaves of a single species. 



Sun and Shade Leaves. — The structure of the assimi- 

 lating tissue of the leaf— the number of layers of paHsade, 



