[Vol 1 



386 ANNALS OF THE MISSOURI BOTANICAL GARDEN 



damage serious enough to be recognized externally as a dis- 

 ease. This is the so-called "period of incubation," during 

 which the parasite is growing and its enzymes and toxins are 

 becoming active. The microscope shows it to be present in the 

 tissues, but the latter have yielded only a little in the immediate 

 vicinity of the bacterial focus. This time is short or long 

 depending on whether the parasite or the host has the first 

 advantage. If the host is growing rapidly it may either en- 

 tirely outstrip the parasite, or be only so much the more sub- 

 ject to it. All depends on whether the parasite finds the initial 

 conditions entirely suited to its needs, or by means of its 

 secretions and excretions can quickly make them so, and con- 

 sequently can from the start make a rapid growth, or must 

 first slowly overcome obstacles of various sorts, such as in- 

 hibiting acids and resistant tissues. The plant may show 

 signs of infection within as short a time as one or two days 

 after inoculation (various soft rots), or it may be as long a 

 time as one to two months before they appear (Cobb's disease 

 of sugar-cane, Stewart's disease of sweet-corn). In the latter, 

 infection generally occurs in the seedling stage and the maize 

 plant may be three months old and six feet tall before it finally 

 succumbs. Of course, as in case of bacterial animal diseases, 

 the greater the volume of infectious material the shorter the 

 time. I have seen many instances of that law. In general, 

 the period of latency may be said to vary from one to three 

 w r eeks (yellow disease of hyacinth, black rot of cabbage, black 

 spot of plum, cucurbit wilt, pear blight, angular leaf-spot of 

 cotton, sorghum leaf -stripe, etc.). 



RECOVERY FROM DISEASE 



Mention has already been made of the self -limited spot 

 diseases and blights. As the actively growing season draws 

 to a close such diseases cease their activity. 



Also in some plants well developed signs of vascular dis- 

 ease may be suppressed (squash, maize, sugar-cane) or re- 

 main in abeyance for a longer or shorter period, according to 

 the varying fortunes of the host and the capabilities of the 

 parasite. The tomato plants inoculated with Bacterium Sol- 



