LEAF-BURN OF LETTUCE. 107 
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.”’ 
