Interrelation of Plant and Animal Pathology 59 



root system will develop. You may even in some cases destroy 

 both root and stem, and a new plant will grow out from the den- 

 tations of the leaf. All these facts must be given due consideration 

 when we compare the pathological behavior of plants and animals. 

 Especially must we remember the plant has no specialized nervous 

 system, the power of response being shared almost equally by all 

 its cells, though stimuli can be transmitted along definite directions 

 in some cases. 



When we examine the aetiology of plant diseases we find a pre- 

 dominance of those due to fungi. As a typical example of plant 

 diseases due to obligate parasites of this class we may take the 

 common rust of wheat. This disease while not usually proving 

 fatal to the plant attacked, does often greatly impair the vitality of 

 the host and result in greatly diminished harvests. The world's 

 toll to this parasite has been estimated at a hundred million dollars 

 annually. 



Another interesting typical obligate parasite is the smut of corn. 

 Infection may occur in this disease at any point of the host plant 

 where active growth is in progress. The vegetative filaments of the 

 fungus ramify sparsely through normal tissue of the host, and often 

 cause no abnormal appearance until fruit begins to form. The 

 enormous tumors produced by the rapidly dividing and growing 

 cells of the host plant, filled with the powdery black mass of myri- 

 ads of spores, are only too familiar objects in almost every corn 

 field. 



The production of tumorous growths is also well exemplified by 

 the Exoascus species producing the disease commonly known as 

 leaf curl of the plum and peach. 



Other fungi not to be considered typical obligate parasites often 

 produce most serious and destructive plant diseases. They seem to 

 take advantage often of any mechanical injury caused by insects, 

 for example, and attack the dead host cells as saprophytes. From 

 an entrance thus effected, they advance into adjacent healthy tissues, 

 destroying often an entire plant. They often become extremely 

 virulent and are then able apparently to infect the host plant di- 

 rectly. To this class belong the various anthracnoses which are 

 well exemplified by the clover anthracnose, which by a careful and 

 very conservative estimate made a few years ago, was costing the 

 State of Tennessee at least three million dollars annually. 



