Depths at Which Plants Absorb Water and Nutrients. 103 



had made a very poor growth during the 129-day period (November 4 to 

 March 12) when compared with the usual development under field conditions 

 (cf. p. 98). Moreover, when the containers were emptied on March 12, so 

 that they might be available for use in the field, it was found that the roots 

 had made even a poorer development relatively than the tops, since the latter 

 were somewhat attenuated because of unfavorable growth conditions. 



While the duration, and especially the intensity, of the hght were un- 

 doubtedly the chief hmiting factors to growth (cf. Garner and Allard, 1920), 

 brief mention may be made of the other environmental conditions. Thermo- 

 graph records show that the air-temperature averaged about 75° F. throughout 

 the period. The temperature was maintained within 5° F. of the average 

 more than two-thirds of the time. During November the extremes varied 

 from 50° to 93° F.; throughout December the range of variation seldom ex- 

 ceeded 15° F., a maximum of 90° F. being reached only during two days; 

 during January, owing to an unusual amount of clear weather, temperatures 

 of 90° to 95° F. were reached on several days, while during February and 

 March the extreme range was from 65° to 100° F. Hygrograph records gave 

 an average relative humidity for the several weeks varying from 45 to 65 

 per cent. During most of the period the average was 50 per cent. Corre- 

 lated with the rather uniform temperature, the humidity did not fluctuate 

 widely. The minimum fell to 28 per cent on a few occasions, but usually 

 varied from 33 to 38 per cent, while the maximum humidity of 85 per 

 cent was seldom reached, the usual maxima varying from 60 to 68 per 

 cent. The average daily evaporating power of the air recorded by atmom- 

 eters placed among the plants was 15.1 c. c. A maximum of 19.5 c. c. was 

 determined during the week of December 20 to 27 and a minimum of 12.7 c. c. 

 on December 6 to 13. A good water-content was present in the containers 

 at all times. 



Thermograph records secured at a depth of 8 inches in the sand, just outside 

 the containers, which was kept well watered, gave an average temperature of 

 about 70° F. This fell to 65° during one period of two weeks and reached 

 75° F. in March. The fluctuation from the mean during any week did not 

 exceed 5° F. Records of soil-temperature were also secured at a depth of 

 3 feet. Temperature fluctuations here took place very slowly; the total 

 variation during the whole period did not exceed 6° F., a temperature of 73° F. 

 being rather uniformly maintained. Thus the growth conditions as regards 

 soil and air temperature were not greatly unUke those of field conditions in 

 June, but quite unMke those under which the field crop makes its early growth. 

 Soil and air moisture were favorable, but the short days of relatively poor 

 illumination were made still less effective by the glass roof cutting off much of 

 the radiant energy. Normally an ordinary clean, double-strength window- 

 pane cuts off 40 to 50 per cent of the hght (as measured by its effect upon 

 soUo paper), while dust, moisture, etc., collecting upon the glass reduce this 

 amount still further. Aside from this, during midwinter the direct sunhght 

 was cut off for about an hour each day, owing to the proximity of other 

 buildings. On the other hand, weather records show the period to be one with 

 an unusual amount of clear weather. 



On March 12 the containers were opened, the root systems examined, and 

 soil samples for water-content and nitrate determinations were taken to com- 



