LETTUCE 227 
die, new ones form, but the plants do not thrive. 
Doubled or twisted roots (Fig. 74), due to careless trans- 
planting, may result in dwarfed plants. Excessive 
applications of fertilizers, or unfavorable soil conditions, 
may cause the formation of rosettes. Gray mold or other 
diseases which attack and cause the loss of the outer 
leaves may have the same effect in causing the develop- 
ment of tufts of short leaves instead of fully developed 
heads. When a fungus is the direct cause of this ab- 
normal growth, sterilization with steam or formaldehyde 
is effective as a preventive measure. 
The excessive drying out of the soil frequently pro- 
duces a “rosette” appearance of the plants. 
Sometimes the margins of the leaves wilt and die, thus 
injuring the selling quality of the plants. This is a dis- 
ease, the result of a physiological disturbance called “tip 
burn,” that may occur on bright, clear days when the 
temperature of the houses is 70 degrees or above, follow- 
ing a season of cloudy weather. With good management 
in the regulation of soil and atmospheric conditions in 
the house, tip burn is not likely to occur. 
Electro-culture—Experiments made at several agri- 
cultural experiment stations show that electric light is 
beneficial to the growth of lettuce. The most extensive 
studies were made at the stations of Cornell University, 
West Virginia and Massachusetts. W. W. Rawson, a 
large commercial grower near Boston, was pleased with 
the results for a number of years, but he finally abandoned 
the use of electric lights for hastening the growth of let- 
tuce. While reports of the stations are rather favorable, 
commercial growers have not regarded electro-culture as 
a practical business proposition. 
Harvesting.—No general rule can be given concerning 
the proper time to harvest lettuce, because so many 
factors enter into the question. If the heads are to be 
sold by the dozen or hundred, they should be cut just as 
soon as they are large enough to satisfy the market re- 
