314 



LEAVES AND ROOTS— THEIR FUNCTIONS AND STRUCTURE. 



The array of facts now presented, regard- 

 ing this malady, seem to leave scarcely a 

 doubt that malignant pear blight is an in- 

 fectious disease. 



We observe that European physiologists 

 have decided that a certain class of vegeta- 

 ble diseases are infectious. " The diseased 

 cells oi a vegetable are capable of commu- 

 nicating their diseased action to healthy 

 cells, just as the cells from an animal, af- 

 fected with the small pox, are capable of 

 ffivinar that disease to another." 



It has also been ascertained that "in 

 order that disease may be thus produced, it 

 is not necessary that the contagious matter 

 be conveyed from one plant to another in a 

 tangible form ; but the diseased cells may be 

 conveyed through the medium of the air.^^ 



Certain conditions of the atmosphere, as 

 well as of the plants themselves, favor the 

 development and spread of these epidemic 

 diseases. Only a portion of the individual 

 plants exposed will suffer, — some escaping, 

 as among animals. And, like epidemic 

 diseases among animals, it is rarely the 

 case that diseases of this kind occur largely 

 in the same districts in successive years. 



Taking this view of the matter, the pre- 

 sence of a single tree, in Mr. Bissell's 

 oTounds, thoroughly diseased with infectious 

 blight, would be sufficient to spread the 

 disease among his whole plantation, pro- 

 vided, as appears to have been the case, the 



condition of the atmosphere and of the trees 

 were such as to facilitate its extension 

 through the air. 



The sudden appearance of this blight in 

 healthy trees, and its rapid extension 

 among trees which themselves had not, as 

 our correspondent thinks, been exposed to 

 any causes which would originate the blight, 

 force us to look for an explanation in the 

 infectious character of the disease. 



The check given to its extension in the 

 row of trees, where the diseased branches 

 were cut out as fast as the symptoms ap- 

 peared, compared with the other row where 

 it was left to itself, seems to be a strong 

 argument in favor of the infectious character 

 of the disease. And, in the absence of any 

 known specific, it is plain that close watch- 

 ing, with the constant removal and destruc- 

 tion of diseased branches, is as yet the 

 most successful mode of checking its rava- 

 ges. 



We hope Mr. Bissell will pursue his 

 experiments with the disinfecting wash, 

 and give us the results another season. 



We will only remark, in conclusion, thqt 

 the pear blight, (as we learn from a culti- 

 vator now abroad, who is familiar with the 

 disease at home,) is common in the south of 

 Europe, though, as in many parts of this 

 country, it does not assume the highly ma- 

 lignant form which it sometimes takes in 

 this country. Ed. 



LEAVES AND ROOTS— THEIR FUNCTIONS AND STRUCTURE. 



BY DR. JAMES PAUL, TRENTON, N. J. 



To the practical and scientific cultivator, 

 a knowledge of the organs of plants, their 

 uses and functions, must not only be highly 

 interesting, but most important. When we 

 look at a plant, no matter how minute and 



delicate, or how gigantic and magnificent 

 its proportions, we behold a thing endowed 

 with life, in which are various organs ne- 

 cessary to the sustentation and production 

 of its several parts. Let us, for the pre- 



