ASSIMILATION OF CARBON BY PLANTS 213 



throughout the year, while in colder climates it may not exceed 

 6 or 7 months. The most vigorous development of plants is 

 found in the humid tropics, where the abundance of moisture 

 allows the plants to assimilate successfully during the whole 

 day. In dry tropical regions, on the other hand, growth of 

 plants is very slow. 



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

 through respiration in light-tolerant mustard plants is about 

 27 per cent of the total yield. For trees, he found approximately 

 the same magnitude, but here about the same percentage must 

 be added for losses on account of the leaves and branches that 

 are dropped. Thus the actual yearly increase in mass makes 

 up about 40 to 45 per cent of the total organic substance produced 

 in photosynthesis. Under conditions less favorable for assimi- 

 lation, for instance, with trees strongly shaded by their more 

 vigorous neighbors, the increase may diminish to 10 per cent of 

 all the assimilates, while the losses augment to 90 per cent. The 

 relative significance of respiration increases especially; for shading 

 leads to a decrease in the number of leaves on the shoots, and the 

 proportion between the producing and consuming parts of the 

 plant changes to its disadvantage. 



According to Boysen Jensen's data, the average production 

 of organic substance approximates 40 to 50 mg. per 50 sq. cm. 

 of leaf surface per day. Other authors give confirming data. 

 These data are related to conditions of sufficient water supply, 

 when the stomata are open during the whole day and photo- 

 synthesis continues uninterruptedly, and likewise to conditions 

 of moderate temperatures, when respiration does not exceed 

 5 to 10 per cent of photosynthesis. In hotter and dryer climates, 

 where the stomata are closed for many hours and photosynthesis 

 is inhibited by deficiency in water, the productivity of plants is 

 considerably decreased. 



In the favorable maritime climate of Denmark and under 

 conditions of perfect methods of tillage and abundant fertiliza- 

 tion, the maximum production calculated per unit surface, not 

 of leaves but of the soil occupied by the plants, may be expressed 

 according to Bondorff in the following figures: 



For wheat, 45 cwt. of grain and 75 cwt. of straw per hectare. 

 With 15 per cent of water content in the crop this makes about 



