PLATE 18 
A. —Silverskin onions, grown with different conditions of light intensity and with 
differences in the length of the daily light period. The plants at left were exposed 
to full intensity of the sunlight for the entire day. Plants' at right were exposed to 
sunlight for the entire day, but the intensity of the light was reduced to a third of 
the normal by means of a cheesecloth shade. The plants in the center were exposed 
to the full intensity of the sunlight, but the duration of the daily period of illumina¬ 
tion was reduced to io hours (5.30 a. m. to 3.30 p. m.). It is easily seen that the 
strong reduction in intensity of the light has had but little effect on bulb formation 
and entrance into the summer rest period. Shortening the length of da y by less than 
a third (and thus reducing the total radiant energy of the sunlight received by a far 
less proportion), however, has effectively prevented the formation of bulbs and the 
appearance of the summer rest period. 
B. —Violet. Plant at left in flower shows the characteristic behavior under the 
relatively short days of spring and fall. The plants in the center and at the right 
were exposed to electric light from sunset till midnight as a supplement to winter 
daylight. Beginning January 30, however, the daily light period of the center plant 
was reduced to 7 % hours. In a few days there was partial loss of turgidity in the leaf 
stems, resulting in the prostrate condition to be noted in the photograph. Subse¬ 
quently the characteristic blue blossoms of spring appeared in this plant. This ex¬ 
periment seems to suggest that permeability of the plasmic membrane or the degree 
of dydration of the protoplasm is affected by the length of day. This prostrate type 
of development is a characteristic feature in the development of many plants under 
short-day conditions. Photographed April 17. 
