242 INTRODUCTION TO BOTANY 



Among the other algae-fungi which cause common destructive 

 diseases are those which cause the cranberry gall ; the brown 

 rot of lemons and other citrous fruits ; the white, or downy, 

 mildew of shepherd' s-purse, the common radish, mustard, and 

 turnip; and the downy mildew of cucumbers, pumpkins, 

 watermelons, and lima beans. 



230. The sac fungi. Some of the more common sac fungi 

 are the cup fungi, the morel, the yeasts, and the mildews ; the 

 latter often appear upon leaves of plantain, smartweed, lilac, 



FIG. 188. Brown rot (Sderotinia) growing npon old plums 



At the right are some of the fruiting cups ; in the middle is a greatly magnified 

 portion of the cup, showing the spore-bearing areas ; and at the left is one of the 

 spore-bearing threads still more magnified, so as to show the spores. After Duggar 



and oak. More fungi belong to this group than to any other, 

 and since most of them are parasitic, it is evident that they 

 are of great economic importance. They have wide variations 

 in form and structure. Usually the parasitic sac fungi grow 

 upon instead of within the host plant. From this superficial 

 growth haustoria are sent into the tissues of the host plant. 



As illustrations of the damage that may be done by mem- 

 bers of this group we may cite the brown rot of peaches, 

 plums, cherries, and apricots. Old dried fruits are sometimes 

 found lying on the ground or still clinging to the trees. These 

 " mummies " are a result of the brown rot. Sometimes they 

 produce little brownish cups which are the spore-producing 



