TRANSLOCATION OF WATER 



185 



The root parenchyma cells are able to pump water into the 

 vessels with a force of 2 to 3 atmospheres. Hence, with feeble 

 transpiration the surplus of water may be forced out of a plant in 

 the form of drops. The importance of this lower motive force, 

 however, is not nearly so significant as is that of the upper ter- 

 minal motor, the transpiring cells of the leaf parenchyma. 



That transpiration is a sufficient force to convey the water sup- 

 ply of the plant may be seen from the well-known fact that por- 

 tions of a plant, when cut off and placed in 

 water, preserve, under favorable conditions, 

 their freshness for a long time, using up a 

 considerable amount of water. The mechan- 

 ism of the transpiration pull is not so very 

 complicated after all. It is based on the fact 

 already discussed that every cell, not fully 

 saturated with water, shows a suction ten- 

 sion of several atmospheres; hence, when 

 brought into contact with water, it absorbs 



\ 



water with a proportional force. The pa- Fig. 82.— Diagram show- 

 renchyma cells of a leaf, for example, will ing the a " an s emen * of 



the terminal mechanisms 



absorb water from a vessel with the force of the water current: leaf 

 of several atmospheres. This avidity for and root parenchyma and 



, -n i ,i , ,i ,i ii vessels connecting them: 



water will be the greater, the more the cell ( a ) leaf parenchyma; (b) 

 loses it, or the stronger the process of trans- vessels ; (c) root paren- 



,. mi j -i j ,i chyma; (d) root hair 



piration. 1 he upper terminal system thus (adapted from Walter). 



represents a self-regulating mechanism which 



absorbs water in 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 pur- 

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

 water, the lower end of which dips in mercury (Fig. 83). After a 

 short time 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 consider- 

 able 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 connec- 

 tion between the vessels of the plant and the water. 



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

 determination of suction tension of the leaves as measured by the 



