58 HATCH EXPERIMENT STATION. [Jan. 



normal condition of the plant, due to improper care and sur- 

 roundings. While the distinction between the two classes 

 of disorders can in many cases be readily discerned, in other 

 cases it is indeed difficult to discriminate between them, as 

 physiological disorders of the phmt so frequently produce 

 just the conditions which are most favorable for the devel- 

 opment of parasitic fungi and bacteria. Thus the original 

 cause of the trouble is liable to be entirely lost sight of. 

 Bearing in mind this fact, it must be clear that to recom- 

 mend fungicides for the treatment of physiological diseases 

 is about as absurd as it would be for a physician to treat a 

 person for consumption who was suffering from malaria or 

 indigestion, and simply required a change in his food or the 

 conditions which surrounded him. The only logical method 

 of treatment under such circumstances is to restore the nor- 

 mal and proper conditions. On the other hand, parasitic 

 fungi which cause serious disorders in our cultivated plants 

 are also found on plants which w ould pass for quite normal 

 and healthy ones. In fact, probal)ly no plant is entirely ex- 

 empt from parasites ; and here we are ])rought face to face 

 with the question, What constitutes a plant disease? It may 

 be defined as a disorder caused by any failing in or diversion 

 of the normal physiological actions of the plant. Practically, 

 we include as plant diseases the effects of all of those forms 

 of parasitic fungi which occur on plants, although it is doubt- 

 ful whether many of them really cause any perceptible harm 

 to their hosts. 



Of the two classes of diseases, the parasitic and physiologi- 

 cal, those of the latter are more likely to be prevalent in 

 greenhouse plants, inasmuch as the conditions to which the 

 latter are subjected are very artificial, and cannot coincide 

 very closely with those of their normal habitat. The physi- 

 ological disorders, moreover, are much less likely to be dis- 

 cerned, and, when found, are more difficult to contend with 

 than parasitic attacks, for they are more com})licated in their 

 nature, as well as less thoroughly understood. In all our 

 dealings with the plant we must bear in mind that it is 

 a plastic organism, capable of responding, within certain 

 limits, to a g-reat varietv of external factors which act as 

 stimuli. These external stimuli are principally to be found 



