18 BULLETIN 227, U. S. DEPARTMENT OF AGRICULTURE, 
In addition to the possibility, in some instances, of chemical 
combinations between the preservative and the media, there will also 
necessarily be a slight change in concentration, due to the drying 
out of the media when held in Petri dishes for six to eight weeks. 
Likewise, during this interval of time certain volatile constituents, 
particularly the lighter oils, may escape from the media. 
In recording observations of the behavior of the fungi toward the 
preservative, rapidity and amplitude of growth, together with any 
other peculiarities in appearance, were noted, inspection being made 
about once a week. 
One very interesting feature of the tests was the development of 
a "halo" around either the living or the dead transfers, or in advance 
of the fungous growth on the check cultures. These halos differed in 
appearance on the different preservatives, sometimes being lighter, 
sometimes darker, than the surrounding medium. In order to deter- 
mine whether the change was due to advance submerged hyph.se, 
several transfers were made from the halos to fresh sterile agar, but 
as no living organisms were demonstrated by this test or by micro- 
scopical examination to be present it appears to be an advance 
physicochemical change in the media, arising, perhaps, from the dif- 
fusion of enzyms from the transfer, as Kellerman (15) has recently 
demonstrated for cytase produced in fungus cultures. 
DEVELOPMENT OF FOMES ANNOSUS AND FOMES PINICOLA. 
IN NONTOXIC CHECK CULTURES. 
In the check cultures, Fomes annosus produces a rather compact 
creamy growth (PL II, fig. 20, and PL IV, fig. 48), forming an abun- 
dance of the characteristic conidia described by Brefeld. F. pinicola 
(PL I, fig. 4; PL III, fig. 31; and PL IV, fig. 40), on the other hand, 
develops a fluffy, deep, white mycelium of considerably more rapid 
growth than F. annosus. At 25° C, F. jpinicola develops a radial 
growth of 24 rnm. in 9 days (average of 14 tests) and covers the plate 
in 15 days (15 tests); F. annosus develops 15 mm. in 8f days (19 
tests) and covers the plate in 20| days (12 tests). 
IN CULTURES CONTAINING TOXIC SUBSTANCES. 
The rate of growth of each of these two fungi on toxic media is 
usually considerably retarded, in many cases strongly so, as com- 
pared with check cultures. In some instances where very low con- 
centrations of certain substances are used, a stimulating effect is ob- 
served, but this condition is reversed with increased concentrations. 
The stimulated growth on zinc-chlorid concentrations when heated 
in the presence of the culture media (such concentrations often being 
far above those necessary to kill when not so heated together) is 
