MECHANISM OF THE ABSORPTION OF WATER 277 



on the average, each day during the life of this plant the aggregate increase 

 in the length of its root system was more than three miles. Furthermore, new 

 root hairs developed on this plant at an average rate of more than 1 00,000,000 

 per day. 



This picture of the role of root elongation in the absorption of water 

 (and mineral salts) from the soil is probably an accurate one for many and 

 perhaps most species of plants. The root systems of some species such as 

 orchids, however, are of a stubby, sparingly branched tj'pe which would seem 

 to indicate that the quantities of water which they can absorb in the manner 

 just postulated would be relatively small. 



The root tip population of any plant is usually so large that often not 

 all of the root tips are subjected to the same soil water conditions. Some may 

 be located in lower soil horizons which contain more water than the upper 

 layers of the soil. At other times, or under other climatic conditions, the 

 reverse situation may prevail. After light rainfalls on a comparatively dry 

 soil, for example, the root tips closer to the surface may be in contact with 

 soil at a higher water content than those at greater depths. Hence some of 

 the root tips are usually absorbing more water as they progress through the 

 soil than others. 



As a general rule, however, the water content of any soil must exceed 

 a certain value if roots are to continue to grow through it. Sufficient data are 

 not at hand to define exactly the soil water content at which root growth 

 ceases, but apparently it corresponds approximately with the wilting percent- 

 age as would be theoretically expected. The results of some experiments of 

 Walter (1924) substantiate this statement. He found that the rootlets of 

 germinating seeds of water-cress and peas were not able to grow at relative 

 humidities of appreciably less than 98 per cent. Such a value is equivalent 

 to a diffusion pressure deficit of the soil of about 27 atmos. (Table 24) which, 

 in general, would correspond to a soil water content not greatly below that of 

 the wilting percentage. Hendrickson and Veihmeyer (1931) have shown 

 experimentally that roots do not grow into zones of soil which have a water 

 content less than the wilting percentage. 



Mechanism of the Absorption of Water. — In Chap. XV it was shown 

 that the development of a diffusion pressure deficit in the mesophyll cells of 

 leaves causes the water in the xylem vessels or tracheids to pass into a state 

 of tension, which is equivalent to an increase in the diffusion pressure deficit of 

 the water in the xylem ducts. As a result of the development of a tension in 

 the water columns a gradient of diffusion pressure deficits is established in 

 the absorbing region of the root, increasing consistently from cell to cell across 

 the root from its peripheral layer to the xylem conduits. 



