Minnesota Plant Life. 47 



the leaf is destroyed, and thus the vigor of the whole vine is defi- 

 nitely impaired. When the mildew has accumulated in this 

 manner sufficient nutriment for its needs it puts forth a branch 

 which grows out through one of the air-pores of the leaf and 

 here, in the open air, spore cells are formed. These are sepa- 

 rated from the branch which produced them and are carried 

 away by wind-currents to other vine leaves. Moreover the mil- 

 dew within the tissues of the leaf breeds after its fashion, form- 

 ing little spherical eggs which after they have been fecundated 

 divide up internally into a considerable number of tiny motile 

 bodies, provided with lashes, so that, when the rotting mass of 

 the leaf has broken down after some rain, these motile cells can 

 be washed out and swim to fresh parts of the leaf or fall with 

 the rain drops to other leaves upon the same plant. Such egg- 

 cells of the mildew serve to bridge over the winter season, and 

 it is, therefore, important, if potatoes, vines, or lettuce should be 

 in the habit of mildewing, that all dead leaves in the autumn 

 should be burned. This diminishes for the following season the 

 danp-er from fresh infection. 



