I 
4 
out and the structure and development of the fungus were much more 
easily studied than on the host itself. 
A medium composed of a 1 per cent solution of agar-agar in sweet 
potato broth was the most satisfactory. This was made by adding to 4 
grams of agar steeped for 5 hours in 300 cubic centimetres of water 100 
cubic centimetres of sweet potato broth. The sweet potato broth was not 
essentially different from ordinary potato broth, and was made by 
steeping the slices from one large potato for several hours in enough 
water to cover them. This medium was used both in the form of plate 
and slanting test-tube cultures, but proved most satisfactory in the lat¬ 
ter on account of the ease with which the tubes were handled. 
Besides the sweet potato agar upon which the main study of the fun¬ 
gus was made, several other media were tried. Slices of sweet potato 
cooked and uncooked in test tubes, similar slices of Irish potato, and 
ncihrlosung agar made by adding 2 grams of agar to 200 cubic centi¬ 
metres of a strong decoction of fresh horse dung, were used, but showed 
no special points of interest. Ordinary potato broth proved entirely 
successful and did not reveal the presence, in this species, of sprouting 
or yeast forms. The fact that sections of the white pine taken from 
living trees and first sterilized in test tubes by intermittent boiling, 
grew the parasite in profusion is at least suggestive. Sections from 
the willow similarly treated failed to nourish the fungus. 
Because of the difficulty of obtaining conidia of the fungus with 
which to start the growth in the hardened culture media the inocula¬ 
tions of sterile media were started from the mycelium itself. Small 
particles of the diseased tissue were carefully removed with a glass 
hook from the border line between the healthy and diseased portions 
of the potato, and sufficiently deep beneath the surface of the skin to 
render contamination improbable. These particles were at once inserted 
into the media and almost without exception produced a pure growth 
of the fungus. Around portions of tissue thus inserted appear in 24 
hours the radiating mycelial hypliae, and in a few days the first form of 
conidia. After the appearance of the first or hyaline conidia upon the 
surface of the culture, inoculations with the pure spores were at once 
made upon the various media above referred to. The cultures grew 
with great rapidity and maintained their vitality for months. Inocula¬ 
tions made on May 26 from tubes started March 18 show the pycnospores 
to be still alive. 
Van Tieghem cells and hanging drop cultures were employed to 
ascertain a number of the details of growth and by the use of sweet 
potato agar in hanging drops on the under side of cover glasses, the 
growth of certain pycnidia was satisfactorily followed from day to 
day. 
Three days after sowing pure conidia upon sweet potato agar an 
abundant thallus is formed which has a number of characteristic fea¬ 
tures. 
