TRANSPIRATION 



99 



series of observations. Less accurate observations can be 

 made by substituting for the capillary tube a tube of wider 

 bore bent at right angles a little below the orifice of 

 the potometer, and affixing to it a scale by means of which 

 the rate of passage of the 

 column of water in the tube 

 can be observed (fig. 63). 



According to the varia- 

 tions in the external condi- 

 tions of the plant, including 

 all the features already 

 alluded to, the amount of 

 watery vapour transpired 

 is continually changing. 

 The most favourable con- 

 ditions being afforded in 

 summer, it is not to be 

 wondered at that tran- 

 spiration attains an annual 

 maximum during that sea- 

 son. It does not, however, 

 entirely cease during the 

 winter, though it is reduced 

 to a minimum, especially 

 in the case of such trees 

 as shed their leaves in the 

 autumn. 



Apart from such changes 

 in the external conditions, 

 transpiration appears to 

 show no independent pe- 

 riodicity, differing in this 

 respect conspicuously from 

 root-pressure. It is, how- 

 ever, very sensitive to only slight changes in the environ-, 

 ment. 



It was mentioned in an earlier part of this chapter that 



FIG. 69. APPARATUS TO SHOW THE 

 SUCTION CAUSED BY TRANSPIRA- 

 TION. (After Detmer.) 



