76 Diseases of Greenhouse Crops 



which is really a fonn of sunburn. This is un- 

 doubtedly caused by the dryness of the atmosphere 

 in which the moisture in the leaves is given off faster 

 than the roots can supply it. 



Humidity as Affected by Walks in the House 



There seems to be a tendency among modern 

 greenhouse men to build benches and sidewalks of 

 cement. While this may be very desirable from a 

 hygienic viewpoint, it is objectionable for the main- 

 tenance of the proper degree of humidity. It is 

 extremely difficult to raise the humidity of a house 

 with cement walks. This is true even when water 

 is applied to the walks and sprinkled on the plants. 

 In houses devoted to roses and similar plants, where 

 it is desirable to maintain a humidity of at least 75 

 per cent., cement walks become quite objectionable; 

 cinder walks are to be recommended instead. Ce- 

 ment walks may be readily transformed by cover- 

 ing them with a layer of six inches of cinders. This 

 covering will make it possible to maintain a higher 

 and more uniform humidity. 



Humidity is also an important factor in the heat- 

 ing of glass houses. The greater the humidity the 

 greater is the evaporation of moisture, and the 

 greater is the amount of heat required to maintain 

 a uniform temperature. Without proper attention 

 to these conditions, the hothouse crop may be 

 doomed to failure. 



