LETTUCE 285 



after the heads begin to form. When surface watering of this kind 

 is practiced, the soil is thoroughly saturated, and as soon as the sur- 

 face becomes sufficiently dry to admit of cultivation, it is stirred 

 to prevent baking and to lessen direct evaporation. This makes a 

 surface mulch of dry soil which helps to conserve the moisture of 

 the soil and at the same time maintains a surface condition more 

 satisfactory for the healthy growth of the plant than that produced 

 by subirrigation. 



Excessive moisture in the atmosphere, a wet surface soil, and 

 sudden temperature changes are conditions most conducive to the 

 development of rot and mildew. It will therefore be seen that 

 watering is one of the most essential and most exacting operations 

 connected with the cultivation of lettuce under glass. Some grow- 

 ers who make a specialty of head lettuce overcome the dangers of 

 surface watering by providing a very deep soil containing a large 

 percentage of organic matter. Such soils have a large water-holding 

 capacity, and if they are thoroughly saturated before the crop is 

 transplanted, it is found possible to bring the crop to maturity with- 

 out overhead watering after the plants are once well established. 



Ventilation of forcing structures. In constructing the forcing 

 houses adequate provision should be made for ventilation, 1 for this 

 is one of the most important factors in lettuce culture, whether in 

 frames or in forcing houses. Intelligent ventilation aids in the 

 control of the moisture conditions of the air and also of the tempera- 

 ture. With well-arranged ventilators it will be possible to provide 

 for a constant change of air without permitting cold drafts to blow 

 over the plants. Sudden changes in temperature are as detrimental 

 to the crop as currents of cold air, and both should be avoided. 



Insects. In both field and house culture lettuce is subject to dis- 

 eases which often cause the grower much trouble, and sometimes 

 result in a marked loss in the crop. 



The green aphis, or green fly, is one of the most persistent and 

 annoying insect enemies of lettuce, both in the open and under 

 glass. This insect is a plant louse and, like all insects of its 

 class, secures its nourishment by sucking the juices of the plant 

 upon which it lives. Insects of this class are not provided with 

 mouth parts for biting or eating the tissues of the host plant, but 



1 See Chapter IV, Forcing and Forcing Structures. 



