Grafts et al. — 150— Water in Plants 



ency being to establish a uniform concentration. The metabohc gradient 

 across the tissue on the other hand as determined by the O2 : CO2 rela- 

 tions would enhance accumulation of solutes to a concentration that could 

 not be retained within the stele ; leakage of solutes must take place and 

 osmotic movement of water would tend to flush these along the cell walls 

 and into the xylem vessels for these constitute the only avenue of escape at 

 a pressure near atmospheric. This mechanism seems quite compatible with 

 known root structure and it fits in fairly well with the gross picture of 

 solute absorption by roots, xylem exudation by excised roots, and guttation 

 from intact plants. 



McDermott (1945j has shown that exudation from the roots of excised 

 sunflower plants growing in soil is greatest when the soil moisture is near 

 the moisture equivalent. At higher moisture contents exudation is lower, 

 probably because of poor aeration. At lower moisture contents exudation is 

 reduced until it stops. The exudation stream reverses as the wilting percent- 

 age is approached. The moisture equivalent mentioned by McDermott 

 is a water content near field capacity, the amount of water a soil can hold 

 against gravity. For an extensive report of experimental work on root 

 pressures and exudation see Sabinin (1925). 



A detailed study of the dynamics of xylem exudation indicates a more 

 complex mechanism than the above discussion would indicate. Experi- 

 ments by Grossenbacher (1938, 1939) prove that xylem exudation fol- 

 lows a fluctuating diurnal pattern that seems to be impressed upon it by the 

 previous history of the plants. And auxin has been shown to greatly stimu- 

 late the flow of xylem exudate (Skoog, Broyer, and Grossenbacher, 

 1938). If it could be shown that the diurnal fluctuation in root pressure is 

 caused by an auxin induced fluctuation in metabolic rate of the root cells, 

 the above observations might still fit into the mechanism of Crafts and 

 Broyer. 



An even more serious departure from a strictly osmotic mechanism 

 is indicated by results reported by van Overbeek (1942). Tomato plants 

 growing in culture solution were decapitated and the excised roots placed in 

 distilled water whereupon each plant was attached by rubber tubing to a 

 small bore U-tube and two or more milliliters of xylem exudate collected. 

 Then the U-tubes were replaced by potometer tubes and mannitol was mixed 

 in the distilled water around the roots until flow in the potometer stopped. 

 When flow had remained static for 15 minutes the mannitol solution was 

 sampled and its osmotic pressure determined cryoscopically as was that 

 of the exudate. Theoretically, if water movement into the roots were 

 osmotic, these two solutions should be isotonic. In the experiments the 

 concentration of mannitol required to stop flow was from twice to several 

 times that of the exudate. However, the rate of exudation and concen- 

 tration of the exudate were related to the concentration of mineral nutrients 

 in the solution bathing the roots, the rate being lower and the concentration 

 higher if culture solution were used in place of distilled water, van Over- 

 beek concluded that the pressure with which tomato plants absorb water is 

 composed of two fractions 1) an active pressure and 2) a pressure of 

 osmotic origin. The nature of active water movement has been considered 

 in a previous chapter ; the above case is a good example of the way in which 

 such active water movement may afifect the water economy of plants. 



In contrast to the above explanation of the differences in the concen- 

 trations of exudation sap and external solution, Lundegardh (1946) sug- 

 gests that the bleeding sap is frequently more dilute than the external solu- 



