\ 



[Vol. 2 



382 ANNALS OF THE MISSOURI BOTANICAL GARDEN 



the spring and the greater part of it occurs during a brief 

 period in the early summer, in which growth of roots, leaves 

 and shoots is proceeding rapidly and there are many young 

 and succulent parts. The cause of the disease may and often 

 does remain on the plant over winter in a latent or semi-latent 

 condition (walnut blight, pear blight, plum canker), but the 

 active period is limited to three months, more or less, of 

 actively growing weather in which developing tissues, subject 

 to infection, are abundant. With definitive growth and the 

 hardening of the tissues in late summer and autumn, the 

 disease is checked and disappears, or remains as a slow canker 

 to appear again on other parts the following spring. It is a 

 very instructive experiment to see, for example, inoculations 

 of Bacillus amylovorus on ripening fruits and shoots of the 

 pear wholly fail toward the end of July, which were eminently 

 successful on the same trees at the beginning of June. The 



b .. lllli , b 



difference in this case is not due to lessened virulence on the 

 part of the organism, but to changes in the host-plant, making 

 it non-susceptible. Similar changes leading to non-suscepti- 

 bility occur in the Japanese plum subject to Bacterium Pruni; 

 the young fruits are very susceptible, the maturing fruits 



cannot be infected. 



Other parasites on the contrary are able to attack, disin- 

 tegrate and destroy matured tissues, e. g., the pith of cabbage 

 stems, turnip roots, the ripened tubers of the potato, well de- 

 veloped roots of sugar beets, the bulbs of onions and hyacinths, 

 full-grown melon and cucumber fruits. 



In both of these types the action of the parasite is expended 

 chiefly on the parenchyma, although in some cases (the plum 

 disease, AppePs potato rot) there is more or less bacterial 

 invasion of the local vessels. Vascular occupation is not a 

 special characteristic. 



In the typical vascular diseases the case is reversed. Here 

 parenchyma is also destroyed, more or loss, but the most con- 

 spicuous and destructive action is on the vascular bundles 

 themselves, which are occupied for long distances, to the death, 

 or great detriment, of the whole plant. In maize attacked by 

 Bacterium Stewarti, it is not unusual, indeed one might rather 



