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WEAVER &• MOGENSEN— TRANSPIRATION 



395 



or 16.58 gm. per 100 gm. of fresh weight in foliage trees, and 

 8.18 gm. in conifers. He also noted that the time of minimum 

 transpiration agrees with that of the minimum temperature, which 

 occurred at the end of January. He states further that the 

 difference in the amount of transpiration of different species of 

 evergreen trees becomes smallest at the time of minimum tran- 

 spiration; and a change in the external conditions, especially in 

 temperature, does not necessarily produce a corresponding change 

 in transpiration in different species. In average cases the amount 

 of water transpired by foliage evergreen trees is one and a half or 

 two times greater than that transpired by conifers if we reduce 

 the amount either to the fresh weight or to the dry weight of the 

 transpiring part. 



Von Hohnel (19) estimated that a birch tree, with about 

 200,000 leaves and standing perfectly free, would evaporate 400 

 liters of water on a hot dry day. He also has calculated that 

 during the period of vegetation the beech requires 75 liters, and 

 the pine only 7 liters for every 100 gm. of leaf substance. The 

 same writer gives us the following table on the relative amount 

 of water transpired from June i to November 30 per 100 gm. dry 

 weight of leaf: 



Experiments on the transpiration of seedlings of Acer sacchari- 

 num in prairie and shrub thicket have been carried out by Weaver 

 and Thiel (21). Trees placed in the latter habitat lost only 30 

 per cent as much water per sq. dm. as those in the prairie. Similar 

 experiments with Quercus macrocarpa gave comparable results. 

 The high transpiration losses in the prairie help to explain the 

 absence of trees from such localities. 



Bergen (2) compared the transpiration rates of a number of 

 broad-leaved evergreens, including Olea europaea, Quercus Ilex, 

 and Pistacia Lentiscus, with those of equal leaf surfaces of Ulmus 

 campestris and Pisum sativum. He found that the losses in the 



