PLATE io 
A. --Yam (Dioscorea alata) from the tropics. Tubers at right are from plants grown 
under a 12-hour day, beginning May 14, while the tubers at left are from control plants 
exposed to the full seasonal length of day. The equatorial length of day favors intense 
tuberization, while the long summer days of high latitudes causes marked develop¬ 
ment of fibrous roots, with very limited tuberization. The basal flattening of one of 
the large tubers is the result of “spreading” over the floor of the container in which 
the plants were grown, suggesting a sort of gravitational settling somewhat like that, 
for example, of fresh putty. This seems to represent a rather extreme case of loss of 
capacity for apogeotropic functioning. Photographed October 7. 
B. —Wild lettuce (Lactuca spicata). Plants at left, exposed to a 10-hour day begin¬ 
ning March 29, produced much larger, coarser leaves than the control plants (at right) 
exposed to the full summer length of day. Under the short-day exposure both leaf 
and stem were far more hairy than under the long-day conditions. The stature was 
greatly reduced, but the time of flowering was not affected. 
