SUMMARY OF THE ALGA-LIKE FUNGI 247 



disease is carried over from one year to the next in diseased 

 potatoes that are planted. The potato blight came originally 

 from South America (perhaps Peru) and first appeared in Europe 

 in 1845, probably introduced from North America. The disease 

 spread very rapidly, causing local famines in various countries, 

 notably in Ireland. It is now, however, largely held in check 

 by spraying the plants with Bordeaux mixture, which contains 

 copper and is poisonous to the fungus. 



The grapevine blight, or downy mildew. This genus (Plas- 

 mopara) develops conidia 011 hyphse outside of the host plant, 

 as in the potato blight, but they germinate by tubes instead of 

 forming zoospores. The disease had its origin in America, but 

 our vines are not generally very seriously injured by it. How- 

 ever, when it was accidentally introduced into Europe it proved 

 a terrible menace to the vine-growing industries there. The 

 European varieties of grapes are largely grafted upon American 

 rootstocks because the latter resist the attacks of a very destruc- 

 tive insect pest called Phylloxera. But the American grapevine 

 blight was for a time more injurious than the insect, until means 

 were discovered of keeping it in check by spraying the .vines 

 with Bordeaux mixture. 



The interesting genus Pythium, which causes the " damping 

 off " of seedlings, and is sometimes very destructive in green- 

 houses, is related to the blights. 



264. Summary of the alga-like fungi. The chief points of 

 resemblance of the Phycomyeetes to certain algse lie in the 

 coenocytic structure of the fungal filaments and the develop- 

 ment of zoospores in terminal sporangia. The sexual organs are 

 likewise similar to those of algae in that they are developed 

 terminally, but there are important modifications because motile 

 sperms are not generally formed. However, motile sperms are 

 known for one type (Monollepharis). The conidia are plainly 

 modified sporangia, which become detached from the parent fila- 

 ments and are distributed as special reproductive spores. The 

 algae which most resemble the larger filamentous Phycomyeetes 



