248 J. F. BIERHUIZEN 



obtained from the Physical Laboratory at Wageningen, situated within 2 

 miles from the greenhouse. 



From a physical point of view, it would be expected that pan-evapora- 

 tion and radiation are related to each other. In Fig. la, daily values calcu- 

 lated from periods of 10 days are represented. A curvilinear relation was 

 obtained in this way. According to Makkink (1957) a linear relation occurs, 

 plotting pan-evaporation against Rm • (A/A + y) (where Rm is radiation in 

 mm per day, A is the slope of temperature-maximal vapour pressure curve, 

 and y the psychrometer constant, for which 049 was taken). For further 

 details, the reader is referred to Makkink (1957) and to Rijtema (1958). 

 It is obvious from Fig. ib that although the curve straightened, it stiU 

 remains curvihncar. This apparent contradiction might be ascribed to the 

 fact, that extremely high values of pan-evaporaton and those below 0-5 

 mm per day, do not occur in Makkink's figures probably because average 

 monthly values were taken. 



EFFECTS OF TOTAL RADIATION AND HEAT 

 SUM ON GROWTH 



For every growth period the total radiation and the heat sum were calcu- 

 lated. The latter was computed by substracting the minimum temperature 

 for growth (4°C) from the average daily temperature and integrating all 

 those values over the whole growth period. 



In Fig. 2 the total radiation is plotted against the heat sum. It is obvious, 

 that with longer growth periods both values increase. In the autumn, 

 however, the heat sum is much higher than in spring for the same total 

 radiation. This fact, as mentioned in the introduction, is due to a phase lag 

 of temperature behind radiation. 



If temperature were the most important factor for growth, one would 

 expect for both seasons a similar relation with heat sum, and two distinct 

 ones for total radiation, according to Fig. 2, or vice versa if radiation were 

 the most important factor. In Fig. 3 dry- and fresh-weight yield of various 

 vegetables has been plotted agahist heat sum and total radiation. Autumn 

 yields are underlined. It is evident that, plotting against the heat sum two 

 different lines for weight can be distinguished, the one for spring with a 

 steeper slope than that for autumn. The differences between seasons nearly 

 disappear when using total radiation. This result indicates that radiation is 

 the most important factor for growth in a greenhouse. 



Supposmg that the effects of radiation and temperature are linear and 



