8 



FUXGI A XI) FUXGICIDES 



FIG. 



POTATO mildf;w 



wind. When one of them 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 oj^ening in the smaller end 

 of the spore wall, as seen in I, Fig. 5. Each of these 

 particles now develops two minute hair-like projections, 

 called cilia, as seen at c, bv 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 



rt, conidiuni: h, same, Avitli contents escaping; o-prminnfincr tnhp 

 <•, swarm spores; (/, same, come to rest and o ^ iii i li a u i ii g tiiuc, 



^^^'"^^^''"^- something as a kernel 



of corn sends out its germinating radicle, as shown in 

 the two figures 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 sj^ore bores through the cell wall, 

 as shown at c. 



After the j^arasitic fungus has thus entered the 

 inside of the leaf it develops rapidly at the exj^ense of 

 the tissues of the latter. It j^ushes its tlireads, or 

 liyplice, about between the cells, choking up the cavities 

 which occur in the healthy leaf, and absorbing the con- 

 tents of the cells. lu many species of Peronospora the- 



