240 THE IVIOVEIVIExNT OF WATER THROUGH THE PLANT 



physical system which is analogous to that which is believed to operate in the 

 plant. 



Experiments similar to that just described have also been performed using 

 small branches or twigs of plants as an evaporating surface instead of a porous 

 clay surface. These are attached at the upper end of a glass tube in place 

 of the clay cup ; otherwise the set-up for such an experiment is similar to that 

 just described. While it has not been possible to obtain as great a rise of 

 mercury in these experiments as when a porous clay evaporating surface is 

 used, upward traction of mercury for distances well in excess of atmospheric 

 pressure can be demonstrated if a suitable technique is followed (Thut, 1932). 



Development of Tension in the Water Columns. — Convincing evidence 

 that the water in the xylem vessels is often in a state of tension has been 

 obtained by direct observations of vessels under a microscope. The stems of 



Fig. 66. Daily variations in the diameter of the trunk of a Monterey pine {Pinus 

 radiata) as measured with a dendrograph. Data of MacDougal (1936). 



some species of herbaceous plants, especially the Cucurbits, are especially 

 suitable for such observations. Such a stem of an intact, rapidly transpiring 

 plant can be fastened in position across the stage of a microscope and, by 

 careful dissection, vessels examined individually. If one of the vessels under 

 observation be jabbed with the point of a fine needle, an immediate jerking 

 apart of the water column at the point of the rupture will be seen to occur, 

 indicating that the water in the intact vessel was actually in a state of tension. 



Interesting evidence that the water in the xylem ducts of woody stems 

 is, at times at least, under tension has been obtained by means of an instru- 

 ment known as the dendrograph. This is a self-recording instrument which 

 measures variation in the diameters of tree trunks. It is so constructed that 

 its sensitivity is very great and that its recordings are not influenced by tem- 

 perature effects upon the instrument. Dendographs are used principally to 

 measure periodic variations in the diameter growth of trees. However, even 

 in trees in which diameter growth has ceased, slight diurnal periodic variations 

 in the diameter of trees have been found to be of regular occurrence. 



A record of the periodic variations in the diameter of a tree trunk for a 

 period of several days at a season when little diameter growth was occurring 

 is pictured in Fig. 66. The trunk attained its minimum diameter during the 



