LEAF-BURN OF LETTUCE. IO7 



ment of the water and heat. Burn is the direct result of 

 the collapse and death of the cells composing the edges 

 of the leaves. It is most likely to occur just as the plant 

 begins to head, and may be induced by a number of 

 causes. The trouble is most likely to result on a bright 

 day following several days of cloudy, wet weather. Dur- 

 ing cloudy weather in winter the air in a greenhouse is 

 practically saturated, and in consequence there is com- 

 paratively little transpiration on the part of the leaves. 

 The cells, therefore, become excessively turgid, and are 

 probably weakened by the presence of organic acids. 

 When the sun suddenly appears, as it often does after 

 a cloudy spell in winter, there is an immediate, rapid rise 

 in temperature and a diminution of the amount of moist- 

 ure in the air in the greenhouse. Under these conditions 

 the plant rapidly gives off water, and if the loss is greater 

 than the roots can supply the tissues first wilt, then col- 

 lapse and die. The ability of the roots to supply the 

 moisture is affected by the temperature of the soil, the 

 movement of water in the latter, and the presence or 

 absence of salts in solution. In this soil the temperature 

 rises rapidly as soon as the air in the greenhouse becomes 

 warm, and the roots in consequence immediately begin 

 the work of supplying the leaves with water. The move- 

 ment of the water in the soil is also rapid, so that the 

 plant is able to utilize it rapidly." 



