[Vol. 2 



282 ANNALS OF THE MISSOURI BOTANICAL GARDEN 



may be attacked differently, which is supported by the evi- 

 dence of many observers. Not infrequently do we find tree 

 trunks in which only some annual rings have been infected, 

 or in which the same ring is diseased on one side and healthy 

 on the other. The decayed rings are always the broad ones. 

 The same varieties have a different air content in different 

 localities. In the neighborhood of water sprouts or vigorous 

 branches, the tissues are rich in water and poor in air, and 

 infections very often do not penetrate into such regions. 

 We know now that poorly fed and crippled specimens are 

 likely to be attacked; on the other hand, it seems clear that 

 fruit trees which are richly fed with nitrogen are very sus- 

 ceptible to canker. An abundance of nitrogen induces the 

 development of a very loose tissue, which during drought 

 is more subject to diseases than a firm tissue. We recog- 

 nize the periodicity in the occurrence of many plant diseases, 

 lor we know the fluctuations in the water content of a tree. 

 The air content of the healthy bark of beeches in winter-rest 

 is 19-20 per cent, and diminishes at the time of budding 

 to 11 per cent, rising afterwards. This is correlated with 

 Ihe fact that the canker, which in Europe is caused by Nectria 

 ditissima, does its damage from autumn until spring, while 

 this damage ceases during the vegetative period. This was 

 pointed out by Aderhold, who, however, failed to recognize 

 the cause. 



If once we know the absolute percentage of air necessary 

 for fungous growth in the different kinds of wood, we may 

 decide through direct investigation whether in certain locali- 

 ties the danger of infection is large or small. We may test 

 the different varieties and try to avoid the danger. By this 

 method the control is not directed against the fungus, but 

 against the conditions which make its growth possible. In 

 other words, we use instead of direct control, measures which 

 prevent the outbreak of epidemic diseases. 



You see by this example what an exactly planned scientific 

 investigation may do, and you can recognize the application 

 of these facts to the American conditions. In the irrigated 

 districts the fruit trees have but few die-back diseases due 



