DISEASES OF PLANTS. 219 



manifestation depends for its character, to a great extent, 

 upon the alteration and changes in the plant-cells themselves. 

 Now, whenever a foreign body of an irritating character, not 

 sufficiently strong to destroy the cells at once, makes its way 

 into a plant, the first effect on the plant-cells is to stimulate 

 them. The cells which normally produce resin, under the 

 irritating influence of an insect or a fungus, produce at first a 

 greater quantity of resin ; those which produce a sugary or 

 milky juice produce it in larger quantities ; those cells which 

 are undergoing cell-division pass through this change all the 

 more rapidly : consequently, when a plant is attacked by 

 insects or fungi, we usually see a swelling, owing to a more 

 rapid increase of the plant-cells, caused by the irritation pro- 

 duced by the foreign body, and also an exudation arising 

 from the undue stimulation of cells whose normal office is to 

 secrete juices appropriated by the plant itself. 



The diseases produced by insects are innumerable, but can- 

 not be discussed by one who is not an entomologist. The 

 diseases produced by fungi may conveniently be divided into 

 two classes: first, tumoes, or swellings, and blights, or 

 moulds. This classification is not, of course, scientific, but will 

 serve as a basis for the present lecture. Tumors are, as you 

 all know, of comparatively slow growth, often lasting several 

 years : on the other hand, what are popularly called blights 

 appear suddenly, and do their destructive work quickly. 

 Let us start with the subject of tumors. You see before you 

 three specimens. One is what is familiar to you all under the 

 name of "black knot," and is growing on the branch of a 

 cultivated plum-tree. The second specimen is on the stem 

 of a cultivated blackberry. The third is a knot which many 

 of you must have seen distorting the branches of hickory- 

 trees. To the naked eye, the swellings on these different 

 plants resemble one another very closely : the limbs afi"ected 

 are cracked ; the surface is black and rough ; and there is on 

 the plum and blackberry more or less gummy matter exuded. 

 If you were told that the swelling in one instance was caused 

 by a fungus, you would naturally assign the same cause to 

 the other tumors ; but such is not the case. The swelling on 

 the plum is caused by a fungus ; that on the blackberry, it is 

 said, by an insect ; while that on the hickory has not as yet 

 been traced either to an insect or a fungus. If the tumors, 



