TRANSPORT PROCESSES IN THE SOIL-PLANT SYSTEM 711 



gradual decrease. When the roots were transferred from a cold to a 

 warm environment, there was an initial decrease in transpiration, fol- 

 lowed by a gradual increase. Such initial responses could be interpreted 

 as evidence that the permeability of the cytoplasm had temporarily 

 changed, causing a change in the rate of transport of the portion of the 

 water that was flowing through the cytoplasm. Experiments of this 

 type must be interpreted with care, however, because the plant itself 

 can act as a source or sink for water. Only about 5 per cent of the water 

 contained in the corn plant of our calculation, for instance, would be 

 required to increase the rather high transpiration rate by a factor of 10 

 per cent for an hour. A temporary change in transpiration rate might 

 actually indicate a permanent change in permeability or merely in 

 hydration of the tissue, with the plant finally reaching a new stress 

 equilibrium at practically the same transpiration rate as was originally 

 observed. 



The functional xylem consists essentially of cells interconnected so 

 as to form long tubes with no protoplasmic barriers to flow. Therefore 

 a rough approximation of the flow resistance can be obtained by the 

 use of the Poiseuille equation, if the dimensions of the tubes and the 

 rate of flow are known. The xylem path extends from the stele of the 

 roots through the stem and into all parts of the leaves. The nodes of 

 the stem of the corn plant have sometimes been regarded as barriers to 

 movement, because certain substances apparently collect at these 

 points in some diseased plants. In actuality, however, the nodes are lo- 

 cations in which a great amount of interconnection of conducting tissue 

 takes place, so that there may be less resistance to flow at the nodes 

 than in the internodes. Probably the reason for the apparent collection 

 of substances at the nodes of corn plants is that at these places one can 

 obtain a less diluted sample of the contents of the xylem than can be 

 obtained at the internode. 



The smallest veins of a corn leaf are about 0.016 cm. apart, and 

 rows of stomata alternate \\dth veins across the corn leaf, stomata being 

 about the same distance apart in the rows. Thus the average molecule 

 of water has to pass through about 0.08 mm. of mesophyll tissue on its 

 journey from the vein to the surface of the mesophyll cell where it will 

 evaporate. The resistance of the mesophyll tissue is probably about 

 the same as that of the root cortex, but in the mesophyll intercellular 

 spaces offer alternate pathways in which water can move as vapor. 

 The resistance of an air pathway to the transport of water ( as vapor ) is 

 about 40 times that of a cellulose pathway of the same dimensions. Be- 

 cause the cross-sectional area of the air path in the mesophyll may 

 often be many times as great as the area of the cellulose path, a signifi- 

 cant fraction of the water movement in the leaf mesophyll may take 

 place as vapor movement in intercellular spaces. 



