Water 



333 



and transpiration rate on the development of vascular tissue. Does a 

 strong transpiration stream stimulate the formation of conducting cells 

 and thus serve as a "functional stimulus"? 



There is clearly a quantitative relation between a transpiring surface 

 and the vascular tissue supplying it. D. J. B. White (1954), in a study of 

 the relation of laminar area to petiolar xylem in the bean leaf, found 

 that there is an allometric developmental correlation between the two, 

 the cross-sectional area of xylem growing about two-thirds as fast as 

 laminar area (Fig. 14-7). There have also been a good many measure- 

 ments of the amount of vascular tissue at different levels in the stem, both 

 absolutely and in proportion to the area of leaf lamina above. Reliable 



L09X 

 2-5 



" Immature 



20 



V5 



0-5 10 Log L 15 20 



Fie. 14-7. Relative growth of area of lamina (L) and cross-sectional area of petiolar 

 xylem (X) in immature leaves of bean. (From D. ]. B. White.) 



data as to the amount of transpiration in relation to cross section of 

 vascular tissue are difficult to obtain. Riibel (1920) measured the xylem 

 area at different levels on a sunflower plant and the total leaf area above 

 each level. In plants grown in a normally sunny situation there was 

 about 0.21 sq. mm. of vascular tissue per square decimeter of leaf area, 

 as compared with 0.10 sq. mm. in shaded plants. Since there is more 

 transpiration in the sun, there is evidently a relation here between 

 transpiration and the amount of conducting tissue. In the lowest stem 

 levels there was from Vi to V3 sq. mm. of cross section of conducting 

 tissue to every gram of dry weight of leaves above it, but in young and 

 vigorous leafy plants this increased to V2 sq. mm., again showing a 

 presumptive relation to transpiration. The proportion of wood to phloem 



