1908.] PUBLIC DOCUMENT — No. 33. 145 



certain unfavorable symptoms may escape his notice. In the 

 same way that a physician can diagnose a patient's condition 

 by an examination of certain organs, or gain an idea 'of tlie 

 state of his general health by considering various symptoms, 

 can one familiar with the normal functions of a plant ascertain 

 its condition by observing certain features which it may display, 

 and then discover the cause of the trouble. 



The highest conception of health and vigor in plants is 

 brought to a realization through the remarkable skill of expert 

 gardeners, and it is no exaggeration to say that this class of 

 men possess the most profound knowledge of a plant's require- 

 ments and limitations. Those trained men who have made a 

 specialty of greenhouse crops for years are unexcelled in their 

 skill and knowledge of the plant's needs, and this is also true 

 of many intensive agriculturists. Some of these specialists 

 have gained remarkable insight into the nature of plant re- 

 actions, the slightest change which takes place in the plant 

 organism being noticeable to them ; but such a large percentage 

 of this knowledge is intuitive or instinctive, as it were, that it 

 cannot be conveyed to others. The best gardeners are in sym- 

 pathy with all that pertains to the well-being of their plants, 

 and they are continually observing each minute change which 

 the plant may undergo, thus gaining a knowledge of the influ- 

 ence of the external factors which in any way affect the organ- 

 ism. A slight modification in the light intensity or in the 

 temperature for even a brief period is suflicient to cause varia- 

 tions in the plant development which are discernible to the 

 expert gardener. The conditions which both directly and in- 

 directly afiect a plant in respect to susceptibility to disease are 

 various. A plant, both in its chemical and physical character- 

 istics, is affected by light, heat, electricity, gravity and soil, 

 moisture, air, biological relationships, etc., and in greenhouses 

 by such factors as ventilation, air space, quality of glass, and 

 in fact the simplest features connected with greenhouse con- 

 struction. It is in a greenhouse that we gain the most insight 

 into the relationship existing between the condition surround- 

 ing plants and their susceptibility to disease, for here the gar- 

 dener has the environment largely under his control, and can 

 therefore regulate the conditions to meet the requirements of 



