SECRETARY'S REPORT, 13 



corn is not certain to ripen in the northern part of this State. If the tem- 

 perature around a plant becomes much less than its constitution requires, it 

 eventually perishes. 



UndtT the most f\ivorable circumstances, vitality in plants is limited in 

 duration. In annuals it continues about sis months ; in biennials it lingers 

 in the root through one winter, but cannot survive a second, while in the oai 

 it endures for centuries. Wheat in Scotland requires six months to mature 

 its grain, but in Venezuela only three. Vital action in deciduous trees and 

 shrubs is nearly dormant every winter as it is in some animals. 



A disease in a plant may be defined an alteration of one or more of its 

 functions, which tends to interfere with the due play and performance of some 

 of these conditions of vitality. 



The causes of disease may be grouped in two classes, predisposing and excit- 

 ing. Of the individual plants exposed to the ravages of any particular dis- 

 ease, some will be attacked, while others may escape. In the one ttiere must 

 be a tendency to be affected which does not exist in the other. This tendency 

 is termed a predisposition, while the agents which may call it into action are 

 denominated exciting causes. 



Amoiig predisposing causes may be mentioned hereditary tendency. As 

 certain families among men transmit certain peculiarities of form and feature 

 from generation to generation, and are liable to certain constitutional dis- 

 eases, so many kinds of plants inherit all the peculiarities of structure and 

 constitution of their parents with a predisposition to the same maladies. It 

 is owing to this law of nature that the difierent varieties of cultivated crops 

 continue unchanged. It is not the disease but the hereditary proneness to it 

 tltHt is transmitted Thi« mav long remain latent, but favoring cirou Mstances 

 •will develop it. This predisposition ia manifested in such aflPections as the 

 potato disease, and disorders of assimilation, pdrticularly in cereal crops; 

 hence the importance of using only good seed of healthy origin. 



A plant that has once had a disease is more liable to it again. An exces- 

 sive amount of sap and nutriment, and much exposure to heat is another pre- 

 disposing cause. On the other hand, a deficiency and deterioration of the 

 required food from want of soluble salts of the necessary mineral substances 

 combined, with too little heat and light during a cloudy wet season, occasion a 

 state of debility, and powerfully predispose to mildew, ergot, smut, and vari- 

 ous other chronic affections. 



Miasm or malaria, which produces such deleterious effects upon mankind, 

 is believed to contribute to the nourishment of plants. It certainly has no 

 bad influence upon them, for malaria causes essentially diseases of the ner- 

 vous system which vegetables do not possess, though some have a remarkable 

 sentsiljiliiy. Dr. CarUvright states that an aquatic plant (Jussicua grandi- 

 Jlorn,) which grows abundantly in the stagnant waters in the southern part 

 of Louisiana, completely prevents the miasmatic diseases peculiar to that 

 region. It derives its sustenance wholly from water, making tlie foulest 

 sweet and pure by consuming the products of vegetable decomposition as fast 

 as they are formed. 



Cuhtaoion and epidemic influences of the air are efficient exciting causes. 

 A disease is said to be contagious which is capable of producing by contact, 

 by inoculation or through the medium of the atmosphere the same malady in 

 other plants, propagating itself from its source in rapid succession to the sur- 

 rounding plants, and gradually extending from one field to another; while a 

 diseas-e excited by some peculiar condition of the air which suddenly prevails 

 siniultaneously throughout a more or less extensive range of country, and 

 diff.TS in character, progress and disappearance from the ordinary complaints 

 of the region, is called an ejddemic. It may also be contagious, of which we 

 have instances in the Asiatic cholt-ra and potato disease. 



In respect to the action and influence of insects and parasitical fungi as 

 exciting causes, there is a difference of opinion among scientific men. Some 



