July 7, 1923 
Temperature Effects in Plant Metabolism 
15 
l 
RED CLOVER (TRIFOLIUM PRAETENSE) IN SOIL CULTURES WITHIN GREEN¬ 
HOUSES 
The soil employed was Miami silt loam as described in a previous pub¬ 
lication (jo, p. 237). It was compacted moderately in glazed stoneware 
jars 21 cm. in diameter and 13 cm. deep (1-gallon crocks). These held 
conveniently 5 kgm. each of the air-dried soil. Two gm. of CaC 0 3 
were mixed with each portion of soil. While filling the jars two of the 
cylindrical form of auto-irrigator (unground atmometer cups) intro¬ 
duced by Livingston (21, 11 ) were placed in the soil. These were con¬ 
nected with water reservoirs which could be adjusted vertically to regu¬ 
late the plane of water in the soil of each jar separately. 
On January 2, 1918, when the soil masses had attained a moisture 
content of 14 per cent (by weight), seeds from a vigorous commercial 
stock were sown in four jars. The jars remained in a greenhouse with a 
temperature range of 15.5 0 to 21 0 C. until the seedlings appeared, a 
period of six days. * 4 Two of the jars were now transferred to another 
greenhouse with a temperature range of io° to 15.5 0 C. The two pairs 
of cultures were placed in the same relative position in the southwest 
corner of the two greenhouses. A thermometer was plunged near the 
center of the soil mass in one pot of each pair and another was suspended 
with its bulb about 15 cm. above and midway between the two jars. 
The readings of these instruments were recorded daily, usually at about 
4.30 p.m. . 
By occasional adjustment of the height of the water column connected 
with the irrigators the moisture contents of the soil in the several jars 
were increased and equalized. On January 14 the plane of soil moisture 
was 17.8 and 19.8 per cent in the cultures of the colder house and 19 
and 20.3 per cent in those of the warmer one. These values approximate 
40 per cent of saturation. The optimum content of this soil for red 
clover under similar greenhouse conditions, but in large containers, has 
been found to be 50 per cent of saturation. 
The plants were reduced in number by removing the poorer individuals 
from time to time. This process was discontinued on February 10, 
when 8 plants per jar remained. After the plants reached considerable 
size and drew moisture rapidly from the soil the latter contracted, thus 
breaking contact with the irrigating cups. This caused variations in the 
plane of soil moisture among the several cultures. 5 
During the growth period the humidity of the air and approximate 
degree of illumination in the two houses were compared by means of the 
spherical form of the white and black atmometers devised by Livingston 
(22, 23, 24). In this case the water loss from the standard white porous 
clay instrument is employed as an index of the moisture deficit of the 
atmosphere, while the added evaporation from the blackened sphere 
serves as a comparative measure of light intensity. 
The tops of the plants were harvested when the seventh and eighth 
leaves were emerging from the stools. This occurred on March 29 and 
April 13 at the higher and lower temperature ranges, respectively. On 
the former date the soil moisture was 10.8 and 15.4 per cent in the cooler 
house and 7.8 and 9.6 per cent in the warmer one. The value in the 
former case decreased to 7 per cent at the time of harvesting. After 
< In view of the results of Kidd and West (25) relative to physiological predetermination, it would have 
been preferable to rear the seedlings at the temperatures in which the plants were to be reared. 
5 An improved form of irrigator which corrects this difficulty is described by Livingston ( 26 ). 
