576 BULLETIN 347 
Development of pycnidium.— The pycnidial stage usually appears on 
the cankers in three to six weeks after inoculation. The very earliest 
stages in the development of the pycnidium are not readily found and 
studied in the bark, but when the fungus is grown in Van Tieghem cells 
in drops of agar the process can be ‘easily observed under the microscope. 
At room temperature it begins in less than a week, in the following manner: 
At a certain point on a hypha, short cells are formed by the laying 
down of new septa. These cells increase in diameter and in amount of 
contents and each one sends out short septate branches (Fig. 92, a and b), 
Fic. 92.— Development of pycnidium 
aand b, Very earliest stages observed in Van Tieghem cell cultures : 
c and d, Cross sections of pycnidium on agar before the beginning of the cavity 
¢, Cross section when the cavity first becomes apparent 
the individual cells of which in turn put out other short branches, until 
the whole structure has the appearance of a witches’ broom. Other 
hyphe, and more distant branches of the same hypha, grow toward it 
. and mingle with its branches. This tangle of hyphe soon becomes so 
dense that a surface view does not show what is occurring on the inside, 
and later stages have to be studied from serial sections after imbedding, 
sectioning, and staining. Cross sections at this time and for some days 
later show that the mass is merely an increasing solid ball of hyphe, 
which are all alike and densely interwoven but not anastomosed in any 
way (Fig. 92, c and d). A little later the hyphe at the center appear 
