TRANSLOCATION OF SUBSTANCES IN THE PLANT 327 



based on the fact already discussed that every cell not luUy 

 saturated with water shows a suction tension, which may reach 

 several atmospheres. Hence, when brought into contact with 

 water, it absorbs water with a proportional force. The paren- 

 chyma cells of a leaf, for example, will absorb water from 

 vessels with a considerable force. This avidity for water will 

 be the greater, the more the cell loses it, or the stronger the 

 process of transpiration is. In leaves of trees, it reaches usually 

 from 10 to 15 atmospheres. The upper terminal system thus 

 represents a self-regulating mechanism that absorbs water m 

 proportion to the rapidity with which it is spent. 



The suction of water by the transpiring leaves can be readily 

 measured. A leafy shoot may be conveniently used for this 

 purpose. It is inserted into the upper end of a long tube filled 

 with water, the lower end of which dips into mercury (Fig. 105). 

 After a short tune, it will be seen that as the water filling the 

 tube is used up, it will be replaced by mercury, which may rise 

 to a considerable height. The demonstration is brought to an 

 end, not by the ceasing of transpiration, but by an accessory 

 phenomenon. Air from the intercellular spaces of the shoot will 

 begin to enter the tube, finally filling its whole upper end and 

 breaking the connection between the vessels of the plant and the 

 water. 



This entry of the air into the tube interferes wdth the exact 

 determination of suction tension of the leaves as measured by 

 the rise of the mercury. Indirect measurements, based on the 

 determination of suction tension in separate cells of a leaf, have 

 shown, however, that a force of several atmospheres is attained. 

 Thus, the upper terminal motive force shows a number of advan- 

 tages over the lower one. It is able to develop a much greater 

 force; it is self-regulating; and it works at the expense of radiant 

 energy, which heats the leaf and increases transpiration. The 

 lower mechanism works at the expense of nutritive substances 

 supplied by the leaves. It is little wonder, therefore, that the 

 upper equipment is more efficient than the lower and that most 

 of the water used by the plant is supplied by its action. Only 

 in. early spring, when the leaves have not yet expanded, or in 

 case of a very moist atmosphere, when transpiration falls to a 

 minimum, is the principal role in raising water assumed by the 

 root cells. 



