ture conditions of the soil and air in greenhouses are affected by 

 the sun's beat in summer, making imperative more ventilation to 

 counteract their effect. 



THE RELATION OF LIGHT TO PATHOLOGICAL CONDI- 

 TIONS. 



As previously stated, light exerts an inhibitory effect on growth, 

 and develops a firm, resistant tissue. For instance, the sprouts 

 of a potato stored in a dark cellar are long, slender and whitish, 

 with little rigidity, and they will immediately wilt and collapse 

 if exposed to the light and heat found in an ordinary greenhouse. 

 Darkness or even long continued cloudy periods will produce a 

 slender, light colored, spindling plant. 



While light plays an important part in the development of 

 normal tissue, a lack of it is responsible for many abnormal condi- 

 tions, and there are a number of diseases common to plants under 

 glass which are traceable to insufficient light. Cucumbers furnish 

 a good illustration of the truth of this statement. In some sec- 

 tions it is still the custom to grow cucumbers under two layers of 

 roof glass to keep in the heat, but in the short winter days the 

 glass, which is usually dirty from collected dust and moisture, 

 often shuts out such a large percentage of light that the plants 

 wilt badly. The amount of heat saved does not compensate for 

 the loss of light; besides, this method of construction is based on 

 wrong scientific principles and is poor economy. Plants grown 

 under the poor light common to November and December have 

 leaves of poor color, slender and elongated petioles, and little 

 mechanical or resistant tissue, and when subjected to the bright 

 sun in the early spring every plant in the house will wilt. In 

 houses running north and south all the plants in the east side, 

 growTi in double roofed houses, will wilt in the morning, and when 

 the sun reaches the west side, those plants will wilt badly in the 

 afternoon. In the night the plants recover, only to repeat the 

 wilting the following day if conditions are favorable. Plants 

 growing under these conditions may be said to be in a state of 

 "partial etiolation." Besides receiving insufficient light, the 

 plants gro-wing in houses of this primitive construction are often 

 subjected to higher temperatures, and this has a tendency to ag- 

 gravate the trouble or produce a spindling growth. Cucumber 

 crops grown under practically normal light and temperature con- 

 ditions seldom suffer from wilting, although in any house cucum- 

 bers will wilt slightly at times. 



Poor light also renders cucumber plants more susceptible to 

 powder}^ mildew, and possibly to timber rot, and often causes 

 the tender edges of the leaves to wilt, turn brown and die. The 

 large number of leaves produced in lettuce plants prevent light 

 from reaching the stem, and stem rot {Sclerotinia) or "drop" 



