18U7.] PUBLIC DOCUMENT — No. 38. 68 



and contrast prominently with the surrounding green tissues. 

 Sometimes all the leaves on the plant are aflected ; again, 

 only a few show any spotting. Sometimes almost the entire 

 leaf is dead ; in other cases, onl}^ a small spot. Such plants, 

 if at all seriously affected, are of course almost valueless 

 for decorative purposes, and even in less serious cases their 

 beauty is greatly impaired ; consequently it is well worth 

 an effort to get rid of sucli disfigurations, and prevent their 

 reappearance. In order to do this, we must first know the 

 cause or causes of the difficulty. They are extremely vari- 

 ous. Any injury, or weakening of the vitality of the plant 

 in any way may produce the effect indicated by the well- 

 known expression "leaf spot." It may be nothing more 

 than a simple burn, produced by the sun's rays concentrated 

 in passing through the glass roof and drops of water on the 

 leaves, or, as frequently happens, by contact of the leaves 

 with the heating pipes. The attacks of insects also some- 

 times have quite a similar effect. But the trouble is not al- 

 ways so obvious. Various other agencies conspire to 

 produce the effect which we are considering. 



It may be stated, as a general principle, that the healthy 

 and rapidly growing plant is the least likely to fall a prey to 

 disease. Exceptions to this may be found in the case of un- 

 usually vigorous outbreaks of the most destructive diseases, 

 but in the long run the rule holds good. Let the plant be- 

 come weak and sickl}' from improper and insufficient nour- 

 ishment, too much or too little heat, light, water, etc., poor 

 ventilation or drainage, or any other disturbance of its nor- 

 mal functions, and its liability to disease becomes largely 

 increased. At such a time the weakening of the plant's vital- 

 ity may proceed so far as to cause a gradual dying away of 

 the leaves and thus produce spotting, or it may, and always 

 does, favor the attacks of parasitic vegetable organisms, most 

 of which belong to the class called the fungi. Such attacks, to- 

 gether with those of bacteria and other vegetable organisms 

 of low rank, are alone properly considered as plant diseases. 

 The fungi are true plants, but of low order and microscopic 

 in size. Some of them are strictly parasites, i.e., they can 

 live only upon the tissues of other organisms. Others, like 

 the toad-stools, are strictly saprophytes, i.e., they live only 



