8 



PL'XGI AND PL'XGICIDES 



FIG. 5. POTATO 3IILDEW 



wind. When one of tliem falls upon a moist potato 

 leaf it germinates by a very peculiar method : The 

 inside of the spore is composed of a granular substance 

 which at this time divides into several individual parti- 

 cles, that escape through an opening in the smaller end 

 of the spore wall, as seen in h. Fig. a. Each of these 

 particles now develops two minute hair-like projections, 

 called cilia, as seen at c, by means of which it swims 

 around in the drop of water ; these are called swarm- 

 spores, or zoospores. 

 After a short time these 

 swarm-spores come to 

 a rest, absorb the cilia, 

 and change from the 

 oval shape to one more 

 spherical (d) \ each 

 then germinates by 

 sending out a little 



a, conidinni; b, same, with contents escaping: rrprminnfino' fnhp 

 c, swann spores; d, same, come to rest and ociuiiuu-uiiii^ i>u uc, 



germinating. Something as a kernel 



of corn sends out its germinating radicle, as shown in 

 the two fiofures beside d. 



In case these swarm-spores germinate on the surface 

 of a potato leaf on or near to one of the breathing pores, 

 or stomata — the peculiar openings with which both sur- 

 faces of most leaves are provided — the germ tube passes 

 to the interior of the leaf through it, as shown at a, 

 Fig, 6. If no breathing pore is in the immediate vicin- 

 ity, the germinating spore bores through the cell wall, 

 as shown at c. 



After the jiarasitic fungus has thus entered the 

 inside of the leaf it develojos rapidly at the expense of 

 the tissues of the latter. It j)ushes its threads, or 

 JiypJice, about between the cells, choking up the cavities 

 which occur in the healthy leaf, and absorbins^ the con- 

 tents of the cells. In many species of Peronospora the 



