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- 



