236 BOTANICAL GAZETTE [October 



Van Tieghem rings in order to follow further the development of 

 the pycnidia. Growth in Petri dishes was observed periodically, 

 the intervals depending upon the rate of growth of the organism. 

 Pycnidia of each species were kept under observation during their 

 entire period of early development. For close detailed study and 

 for drawing, mounts were made by removing from Petri dish 

 cultures small squares of agar containing pycnidia in various stages 

 of development, mounting them either in water for immediate 

 study, or in glycerine for later study. Material from test tube 

 cultures was teased out and studied for comparison and verifica- 

 tion of many points. 



Following the usage of earher writers, DeBary (9), Bauke (5), 

 ZoPF (52), and VON Tavel (46), the word primordium is used 

 throughout this paper to designate a group of cells that have become 

 so differentiated that it is clearly evident that from them a pycnidial 

 or similar structure will arise. 



Genera and species studied 



Phoma (Fries) Desmazieres 



Phoma herharum West; isolated from its host. Polygonum 

 hydropiper L., at Urbana, Illinois, July 8, 1916. 



Pycnidia are produced in cultures in moderate abundance. 

 All stages of development, from the mature pycnidia on older 

 mycelium to beginning stages on the younger mycelium, can be 

 observed even in a small sector of a Petri dish culture. Develop- 

 ment proceeds typically from a single cell of a hypha. This cell 

 divides both transversely and diagonally (fig. i) into a few cells 

 which swell and enlarge. These cells by continued swelling and 

 dividing form an irregular mass with very few or no hyphal 

 branches. This mass shows distinctly its origin from a single hypha 

 (figs. ^1-3). As the mass continues to enlarge, one side protrudes 

 slightly as a short rostrum which becomes lighter in color 

 (fig. 4). This is the primordium which later becomes a pear- 

 shaped pycnidium with an ostiole from which spores are 

 discharged (fig. 5). The development is a typically simple meris- 

 togenous one. 



