92 
Adams .— The Effect on Certain Plants of 
1 and 3. The behaviour of Soy Bean and Tomato as compared with 
the other plants tested would tend to indicate that there is a certain 
optimum relation between the daily amounts of light and darkness within 
which a plant will attain its best development. But probably further 
experiments with these two species are necessary before any broad conclu¬ 
sions can be drawn. 
In the case of the other species experimented with, which grow 
naturally farther north, they were able to utilize the full period of daylight 
during the summer months with good effect. Kjellman’s experiments 
showed that continuous illumination for twenty-four hours in the arctic 
regions produced better results than twelve hours of light and twelve of 
darkness. Until, however, some experimenter tests the result of exposure 
to eighteen hours of light and six of darkness or twenty-one of light and 
three of darkness daily, it would probably be rash to assume that in the 
case of plants growing in temperate climates there is no upper limit to the 
daily optimum amount of light. 
Summary. 
The effect on certain plants was noted of excluding light for a number 
of hours during the months of June and July, when the normal period of 
daylight in latitude 45i° is greatly in excess of the period of darkness. 
Some of the plants were covered for a period of three and a half hours, thus 
equalizing the duration of light and darkness ; in other cases the plants 
were darkened for one, two, or five hours, on the average, for each day. 
Some of the experimental plants were grown in pots in a greenhouse, 
others were planted inside a frame, while still others were planted in open 
ground. To exclude the light, large inverted flower-pots were used, also 
sheets of brown paper fastened to frames, and in other cases large wooden 
boxes were employed for the purpose. Care was taken to keep all the 
other conditions as uniform as possible. 
The plants experimented with were Wheat, Indian Corn, Liver-leaf, 
White Mustard, Soy Bean, Wax Bean, Flax, Coolwort, Tomato, Sunflower, 
and Dandelion. 
In almost all cases the plants exposed longest to the action of light 
gave the following results: 
(a) Greatest average weight, (b) greatest average height, (6*) earliest 
flowers. 
The conclusion is drawn that growth or extension in length can take 
place both in light and in darkness, and that in both cases the amount of 
growth within a definite period of time is largely determined by the supply 
of available reserve material and the readiness with which this can be 
drawn upon by the growing parts. 
