354 B. ORCHARD 



Strikes the erect beet leaves obliquely and its effective intensity is brought 

 to a level at which it is used efficiently and, at the same time, more leaf area 

 can be illuminated by a given area of hght beam. This mechanism, how- 

 ever, does not operate when the incident level does not greatly exceed 

 saturation or when the Hght is difflise, and the optimum for cultivated beet 

 should then be the same as for kale. The Dutch-hght structure intercepted 

 at least 30% of the incident radiation and introduced some diffiision, and 

 the optimum L for sugar beet should be considerably lower than in the 

 open field. 



In the first two experiments the L of the A plots was probably near the 

 optimum, hence the decrease in L caused by drought did not influence dry 

 matter production significantly. 



In the third experiment L was below the optimum but the effect of the 

 drought was obscured by the large experimental errors. 



REFERENCES 



BiERHuiZEN, J.F. (1959) Some observations on the relation between transpiration and 



soil moisture. Nether., J. agric. Sci. 6, 94-98. 

 Gaastra, p. (1958) Light energy conversion in field crops in comparison with the 



photosynthetic efficiency under laboratory conditions. Meded. LandbHoogesch. 



Wageningen, 58, 1-12. 

 Gates, C.T. (1955) The response of the young tomato plant to a brief period of 



water shortage. II: The individual leaves. Aust.J. biol. Sci. 8, 215-230. 

 Gates, C.T. (1957) The response of the young tomato plant to a brief period of water 



shortage. Ill: Drifts in nitrogen and phosphorus. Aust.J. biol. Sci. 10, 125-146. 

 KuiPER, P.J. & BiERHUiZEN, J.F. (1959) The effect of some environmental factors on 



the transpiration of plants under controlled conditions. Meded. LandbHoogesch. 



Wageningen, 58, 1-16. 

 MoNTEiTH.J. L. & Owen, P. C. (1958) A thermocouple method for measuring relative 



humidity in the range 95-100%. J. Sci. Instmm. 35, 443-446. 

 Orchard, B. (1961) An automatic device for measuring leaf area. J. cxp. Bot. 12, 458- 



464. 

 Owen, P.C. (1957) Rapid estimation of the areas of the leaves of crop plants. Nature, 



Lond. 180, 6x1 only. 

 Owen, P.C. (1958) The growth of sugar beet under different water regimes. J. agric. 



Sci. 51, 133-136. 

 Owen, P.C. & Watson, D.J. (1956) Effect on crop growth of rain after prolonged 



drought. Nature, Lond. 177, 847 only. 

 Philip, J. R. (1957) The physical principles of soil water movement during the irrigation 



cycle. C.S.LR.O. Inter. Coinm. Irrigation Drainage, 125-154. 

 Slatyer, R.O. (i960) Absorption of water by plants. C.S.LR.O. Div. Land Res. Reg. 



Survey, Australia, }ii--ii9. 



