72 Diseases of Greenhouse Crops 



However, if an excess of steam pressure is used, 

 the engine under an excessive strain will break or 

 explode. The same is true with plants. A low 

 temperature may not sufEce to awaken the active 

 life processes. With increased temperature, the 

 plant becomes capable of maximum activity. Tem- 

 perature beyond the normal requirements causes it 

 to suffer or even to die from weakness and disease. 

 The ideal development with forced plants is pos- 

 sible only if we consider the relationship of the soil 

 temperature to that of the air. For instance, the 

 bum of lettuce is brought about by rapid evapora- 

 tion of moisture from the leaves at a time when 

 the roots are unable to supply this excessive demand 

 of water. If the soil is cold, or its temperature dif- 

 ferent from that of the house air, the roots will be 

 unable to supply fast enough the water needed by the 

 foliage. This will result in their collapse or burn- 

 ing, and the ruin of the lettuce crop. The proper 

 temperature requirement for each crop will be taken 

 up when we consider further the cultural require- 

 ments of each. 



