78 
Adams .— The Effect on Certain Plants of 
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lamp Klebs was able to make a small beech-tree develop leafy shoots con¬ 
tinuously for eight months and ail oak-tree for seven months, three of which 
were in winter. He also states that humidity promotes growth and at the 
same time hinders the formation of flowers. He further adds that the 
essential condition for flower-production is the accumulation of carbo¬ 
hydrates in the plant, a result which is directly dependent on the amount 
of photosynthesis. Hence, by using artificial illumination from an Osram 
lamp, he was able to make Sempervivum flower at any time of the year. 
It is evident from the above summary that considerable difference of 
opinion exists among observers as to (i) the effect of light on growth, that 
is, extension in length, and (2) the effect of light on the production of 
flowers. In order to throw some further light on this problem two series 
of tests with different plants were carried out during the summers of 1920 
and 1921. The experiments made in 1920 were carried out in the green¬ 
house, while those of 1921 were conducted on plants growing in open ground 
for the most part. 
Greenhouse Experiments, 1920. 
Four species of plants were used for the experiments, namely, Flax, 
Wheat, Sunflower, and White Mustard. The seeds were sown in six-inch 
pots. Six pots were planted with each species of seed, three being exposed 
to the light, while the other three were kept dark for a short time each day 
while the experiments lasted. The experimental pots were darkened by 
being covered with large flower-pots inverted, in which the hole in the 
bottom had been tightly closed with a cork. At first pots of 9^ inches in 
diameter were employed, and towards the end of the experiments, when the 
Sunflowers were too tall to be covered by pots, they were placed in a wooden 
cupboard from which light was excluded. 
In experiments with environmental factors it is extremely difficult so 
to arrange the conditions that only one factor will vary at the same time. 
The pots were not watered during the time of darkening, so that both the 
pots exposed and those covered received the same amount of water each 
day. It is conceivable, however, that slightly more water would pass into 
the air from the soil of the pots left exposed than in the case of the pots 
covered. It is also possible that the temperature inside the cover might 
differ slightly from that of the rest of the greenhouse. It is not probable, 
however, that the difference in either case would be so appreciable as to 
have much effect on the final result. Some observations on the tempera¬ 
ture inside the cover were made in the case of the experiments carried out 
in 1921. 
To indicate the exact procedure followed, a more extended account of 
the operations in the case of Flax is given, while for other species only 
a summary of results is given in each case. 
