1897.] PUBLIC DOCUMENT — Xo. 33. 83 



market value, but the real damage to the lettuce plant is 

 never sufficient to destroy it. Microsco[)ic examination of 

 the blackened areas frequently shows bacteria in the cells, 

 but more often the "damping fungus" (Botrytis) is pres- 

 ent, and can ])c readily observed with the naked eye. In 

 this instance, however, neither of these forms of organisms 

 has anything to do with the cause of the disease. They are 

 simply accompanying factors, which are always ready to 

 seize upon any abnormal condition in the plant which is 

 especially favorable to them. The disease is a ph3'siological 

 one, and has its origin in the unfavorable surroundings of the 

 plant, especially those connected with transpiration and 

 sunlight. Mr. B. T. Galloway of the United States Depai-t- 

 ment of Vegetable Physiology and Pathology has made this 

 disease a study, and I can do no better than to quote his 

 views : — 



Top-burn, one of the worst troubles of the lettuce grower, does 

 comparatively little injury on this soil [Boston soil], providing 

 the proper attention is given to ventilation and the management 

 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 lilvely to 

 result on a bright daj^ following several days of cloudy, wet 

 weather. During cloudy weather in winter the air in a greenhouse 

 is practically saturated, and in consequence there is a compara- 

 tively 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 imme- 

 diate rapid rise in temperature, and a diminution of the amount 

 of moisture 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 collapse 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 tem- 

 perature 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 movement of the water 

 in the soil is also rapid, so that the plant is able to utilize it 

 rapidly. 



