PRECIPITATION IN A SPRUCE STAND 121 



transpiration increased after cutting the stem, whereas in other cases it 

 decreased. The number of decreases was, however, greater than the number 

 of increases, and transpiration was about 10% lower after cutting the 

 sample than before it. 



Allerup (1959), using 7- to lo-day-old wheat plants grown in water 

 culture, found an increase in transpiration after cutting stem or root in air, 

 immediately or shortly after this process. He assumed that the plants were 

 well supphed with water, since they were taken from a water culture. It 

 therefore appears, he says, 'reasonable to assume that in order to find an 

 increase in transpiration upon cutting, it is necessary to ensure that trans- 

 piring parts of plants have ample available water reservoirs . . .' (p. 912). 

 For this reason, Allerup criticised my experiments with spruce (1944), and 

 stated that my failure to find an increase in transpiration in all my experi- 

 ments must probably be attributed to the fact that the plants used had 

 different water reserves. He also suggested that the plants I used had injured 

 roots, since they had been freed from soil by suspension directly before the 

 experiment. 



With respect to the question of whether or not the experimental plants 

 should be water-saturated, it must be stressed that my control experiments 

 were not only intended to test how the plants reacted when water-saturated, 

 but also how plants with a varying water deficit reacted to cutting. My 

 transpiration measurements on spruce were, in fact, made at different times 

 of the day, and on days with varying weather- during both rainy and dry 

 periods. Consequently, they were made on objects which almost invariably 

 had a water deficit of varying degree. Normally, there is an initial water 

 deficit in the morning- when transpiration starts -and it increases in the 

 course of the day. Furthermore, the deficit is lowest immediately after a 

 rainy period, and rises with increasing length of the subsequent dry period. 

 It was in order to make the control measurements comparable with 

 measurements of the transpiration of forest trees under normal conditions- 

 that is, with varying degrees of water deficit-that I used spruce plants that 

 also had a varying water deficit. 



I do, on the contrary, attach greater importance to Allerup's other 

 criticism, namely, that possible injury to the roots may have influenced the 

 reaction after cutting the stem. I therefore repeated these experiments, but 

 using spruce plants that had been allowed to grow in a nutrient solution for 

 some weeks beforehand. The plants were 2 years old, and were taken from a 

 plant nursery. 



After the roots had been freed from soil by suspension in water, the plants 

 were placed in a nutrient solution for 6-8 weeks before starting the experi- 



