714 PLANT GROWTH AND PLANT COMMUNITIES 



By making certain approximations, the resistance of the various 

 tissues to the passage of water may be calculated for various transpira- 

 tion rates. For such a calculation the mature corn plant is assumed to 

 have the following (not unreasonable) characteristics: 



Leaf area (both sides ) : 2 m-. 



Area of leaf mesophyll exposed to intercellular space: 2 m^. 



Root area ( excluding root hairs ) : 3 m^. 



Root area effective in water uptake ( excluding root hairs ) : 2 m^. 



Diameter of stern at internode: 3 cm. 



Root weight: 500 gm.; above-ground weight: 1,500 gm. 



Diameter of xylem lacunae: 0.012 cm. 



Total cross-sectional area of vessels in 3-cm.-diameter stem: 0.20 



cm^. 

 Rate of transpiration: 200 gm. per hour. 

 Using these values, the following quantities may be calculated: 

 Velocity of transpirational stream: 1,000 cm. per hour. 

 Average velocity on a total stem-area basis: 30 cm. per hour. 

 Hydraulic head loss in distance of four meters (root tip to leaf 



end ) : 3.2 atmospheres. 

 Velocity of transpiration stream into roots: 0.01 cm. per hour. 

 Pressure drop from epidermis to endodermis: 3.5 atmospheres. 

 Pressure drop from leaf xylem to surface of mesophyll: 1.7 



atmospheres. 



For the sake of simplicity, the root hairs have been ignored in this 

 discussion even though under certain circumstances the inclusion of 

 the root hairs may significantly change the calculations. Root hairs may, 

 for example, increase the surface area within the root-hair zone ten- 

 fold, and the length of the path traversed by water from the epidermal 

 surface to the endodermis may be increased several-fold if the water 

 enters the distal portion of a root hair. These two factors would, of 

 course, tend to cancel each other out in calculations of potential drop, 

 but they might be quite significant in other ways. Probably the prin- 

 cipal effect of root hairs is to increase the volume of soil from which 

 a given root can draw water, and therefore greatly increase the effec- 

 tive radius of the root in terms of water uptake. The advantage given 

 to the plant by the root hairs lies in the fact that, in fairly dry soil, 

 transfer of water to the root through the root hair is easier than transfer 

 of the water through the soil, and it seems to be under just such condi- 

 tions that root hairs reach their maximum development. 



Several investigators have pointed out the similarity between the 

 flow of water in porous media and other transport phenomena, such as 

 the flow of heat, the diffusion of gases, and the transport of ions. In 

 each case the flow obeys the General Transport Law, and the flux is 



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