2l6 



A J? ACE OF FLOWERLESS PLANTS. 



ious other diseases. Nearly all epidemics and contagious 

 diseases are known or believed to be caused by bacteria. 

 Small-pox iscaused bya bacterium (Fig. A 2.) The same 

 organisms produce the same disease in cows, but in a less 

 violent form. A person inoculated with small-pox from 

 a cow, /. f. , vaccinated, has a mild form of the disease, 

 after which he is supposed to be free from contagion. 



In Europe one of the most contagious diseases of stock 

 is known as charbon. In France, in 1881, an experiment 



*.-vr, 7 *■;■*:' ■-: 



, , ■. '^IN '// ' V ''-5'--' 



^-■^ IP? 

 9A° 



Co 



Fig. a. 



was made with sheep to find a mode of protection against 

 this disease similar to vaccination for small-pox. The 

 bacteria causing charbon (similar to Fig. A 14), were 

 for this purpose cultivated artificially in chicken broth, 

 a few of them being transferred to fresh broth every 

 few hours, where rapid multiplication took place in each 

 interval, till they had thus passed through many, say 

 thirty generations. By this method a cultivated variety 

 was obtained which was less virulent than the wild bac- 

 teria, if we may call them so. Then a flock of fifty 

 sheep was taken, and of these twenty-five selected at 

 random, inoculated with the cultivated germs, marked 

 and turned out with the rest. Ten days later, the entire 



fifty were inoculated with germs fresh from a diseased 

 animal. At the end of forty-eight hours the twenty-five 

 previously inoculated were alive and well, while the 

 twenty-five not previous inoculated were dead. 



The following illustrates the vitality of bacteria. A 

 widow living adjacent to the estate of a great land owner 

 in Germany cut clover upon his land to feed her cow 

 and goat. They became sick, and on investigation it 

 was found that at the spot where the clover was cut a 

 steer, dead from charbon, had been buried the year be- 

 fore, and that the germs of the disease had been brought 

 up by earth worms. Cases are recorded where bacteria 

 have been kept sealed up for years without losing their 

 vitality. They will often survive hard freezing and con- 

 siderable boiling. 



The discovery of bacterial diseases in plants is quite 

 recent and was made by Professor T. J. Burrill, of the 

 University of Illinois. In searching for the then un- 

 known cause of pear blight, he observed, first in 1877, 

 what appeared to be bacteria in the diseased trees and 

 their viscid exudations. In 1880 he began to inoculate 

 healthy trees by inserting small pieces of diseased bark 

 or some of the exudation by means of the point of a knife 

 blade or a needle. The blight developed in the inocu- 

 lated branches, and repeated experiments and tests estab- 

 lished the fact that the cause of blight is bacteria. The 

 name fire-blight expresses the prevailing idea that its de- 

 velopment is rapid, but this is not the case. Blight re- 

 sults from inoculations usually. in eight to twelve days, 

 but sometimes not so quickly, and i:s progress is slow. 



Apple trees were successfully inoculated with virus 

 from the pear, and the reverse, and the quince blight 

 was similarly proved to be the same. The disease fol- 

 lowed the inoculation in a majority of cases. In more 

 recent years these experiments have been repeated by 

 others, especially by J. C. Arthur, with confirmatory 

 results. 



Pear blight bacteria, when magnified one thousand 

 times, appear as in Fig. D (after Burrill). They usually 

 consist of two joints, but sometimes q 

 of one only. Twenty-five thousand rf^^'^^0~''^ 

 could lie side by side in an inch. ^^T^SS'^^n 

 Their effect on the tree is merely to cfS^ ^ ft 

 produce fermentation. Cells of a Qo^Oo^^ 

 certain layer in the bark are stored '^^§^^^^^&)^^0 

 full of small starch grains which the ^'^o^^ 

 tree has laid away in this customary Apjrp'^J)''-'^ 

 place to use as food in adjacent parts ^c®i ^ 

 when needed. The bacteria cause 

 this starch to ferment or putrefy in 

 much the same way that starch will " spoil " when left 

 standing, and with a corresponding bad odor ; but the 

 fermentation is of a special kind, its products being car- 

 bonic acid and butyric acid. Fig. B (after Burrill) 

 showed a thin section of bark, highly magnified, from a 

 healthy branch. The cells are filled full of small egg_ 

 shaped grains of starch. Fig. C (after Burrill) shows a 

 ^similar section from a diseased branch. The bacteria 

 have destroyed all the starch grains. And this destruc- 



FiG. D. 



