54 Diseases of Greenhouse Crops 



process is known as photosynthesis. It consists of 

 an intake and assimilation of carbonic acid by the 

 plant, and a simultaneous liberation of oxygen. 

 The carbonic acid breaks down and combines with 

 the water to form the sugars and starch. The spec- 

 trum rays which are most concerned with the manu- 

 facture of starch are the orange and the red. The 

 blue rays aflFect growth. Success with greenhouse 

 plants depends largely on the intensity and the na- 

 ture of the light rays which are permitted to pene- 

 trate through the glass. At best, these rays differ 

 materially from the normal sunlight. 



Contrary to general belief, plants make most 

 growth at night or in the dark. On the other hand, 

 photosynthesis takes place during the daytime and 

 under the direct influence of light. "While light does 

 not favor growth, it assists in the development of 

 supportive tissue which enables the plant to resist 

 attacks of various diseases. The lack of a proper 

 amount of light in the greenhouse causes the plants 

 to possess little or no resistance to disease. This is 

 especially true in the winter months. However, 

 while insufBcient light is conducive to disease, an 

 excess of it, such as occurs in the summer months, 

 is also detrimental to plant health in the hothouse. 

 In that case, shading the glass becomes necessary. 

 Moreover, there are numerous hothouse plants, such 

 as palms for instance, which naturally require less 

 light. On the other hand, lettuce, tomatoes, cucum- 

 bers, roses or carnations require more light in the 



