Mat 28, 1915] 



SCIENCE 



781 



inflicted, while in other varieties it is not 

 be^n for forty-eight hours or more. 

 From this it is clear that the former may 

 withstand infection better than the latter. 

 By means of these experiments the rela- 

 tion between the structure of the plant 

 and its bacterial resistance has been estab- 

 lished beyond doubt. A similar relation, 

 however, does not exist in the case of 

 fungous diseases, as the fungi may pene- 

 trate the newly formed cork. 



All the instances cited illustrate the influ- 

 ence of mechanical means of protection. 

 But the plant also often escapes disease by 

 means of rapid growth. A microscopic ex- 

 amination of seedlings attacked by smut 

 shows that a number of seedlings may be 

 infected, and yet only a few of the plants 

 will show the disease, proving that the in- 

 fection has been suppressed in many eases. 

 In this connection attention is called to the 

 fact that in the case of both stinking and 

 loose smut the infection originates in the 

 seed. The fungus mycelium grows in the 

 seedling, but by rapid growth the latter 

 may outstrip the fungus, which remains in 

 the base of the plant and is harmless. 



There are still other factors in plants 

 which may influence resistance but which 

 are not perceptible through the microscope. 

 They may be found by physical or chemical 

 research because they are based on the 

 difference of contents. Probably these 

 factors are of far greater importance than 

 those already discussed. But till now these 

 questions are far from being treated in an 

 adequate manner. The foremost reason for 

 this may be that here we have to deal with 

 chemical substances such as albumens, 

 tannins, etc., and there are few botanists 

 who possess the necessary chemical knowl- 

 edge to undertake such experiments. A 

 bridge, therefore, must be built between 

 botanists and chemists, and the latter 's in- 

 terest in this question awakened. 



One of the best investigations made in 

 this direction up to this time is that of 

 Miinch on the immunity and susceptibility 

 of trees. He has shown that susceptibility 

 of woody plants to fungous diseases depends 

 on the quantity of water and consequently 

 on the quantity of air in the wood. This is 

 in accordance with the writer's experiments 

 with Rhizoctonia and Fusarium which have 

 shown that these fungi also have a 

 high air requirement. In the United 

 States, with its large areas of irrigated 

 land, this fact is of great importance. It 

 is possible that the influence of both of 

 these fungi may be diminished by thorough 

 regulation of water conditions. 



A glance at sugars and acids shows that 

 these substances also exert an influence in 

 disease resistance. The presence of benzoic 

 acid in Vacciiiium vitisidwa is supposed to 

 be the cause of its resistance to fungous dis- 

 eases. In the same way the tannins have 

 a relation to resistance. This was shown 

 by Behrens in his work on fruit decay and 

 confirmed by Cook and Taubenhaus. On 

 the other hand, sugar favors the growth of 

 fungi, as is shown clearly in the case of 

 apples and pears. Henneberg even claims 

 immunity for some varieties of potato from 

 certain diseases on account of their high 

 sugar content, but this has not been es- 

 tablished beyond doubt. . 



Finally the enzymes exert a definite in- 

 fiuence on immunity, the oxydases taking 

 the lead. These ferments work directly or 

 indirectly by producing resistant chemical 

 substances. 



This paper, it is believed, gives sufficient 

 idea as to how, in the opinion of the writer, 

 the problem of disease resistance should be 

 dealt with in the future. The present 

 methods should by no means be abandoned, 

 for practical experience and happy acci- 

 dents may help a great deal, but in addi- 

 tion to carrying out these methods an ef- 



