416 Witson: North AMERICAN PERONOSPORALES 
which infests a tree, although a few species occur on shrubs. 
Perhaps this unusual habitat accounts for the rarity of the species 
in herbaria. 
On ULMACEAE: 
Celtis occidentalis L., District of Columbia,* Waite 556, 557. 
Type LocaLity: Washington D. C., on Celtis occidentalis L. 
Distripution: Maryland and District of Columbia. 
Icon: Jour. Myc. 7: pl. 17. f. 1-16. 
NEw York BoranicAL GARDEN. 
