136 
W. J. DoWSON, 
On two species of Heterosporium particularly 
Heterosporium echinulatum. 
By 
W. J. 1)0WSON, B. A. Cambridge. 
(Schluß.) 
Experiment VII. 
The plant was one of those used in the previous experiment; but 
was turned round through an angle of 180° so that leaves not previously 
inoculated were presented to the experimenter. 
On February 5 th 1912 the plant was inoculated. A watery suspen¬ 
sion was made from infected Dianthus plants collected from one of the 
nurseries at the beginning of the month. The conidia were brushed off 
from three diseased areas by means of a small paint brush into a watch 
glass containing a little tap-water. A drop of this liquid under the low 
power of the microscope showed it to contain numerous spores of H. 
echinulatum. 
The lower leaves of the plant were painted over the whole of the 
upper surface with the water suspension of spores, and were marked with 
small copper-wire loops. Two days later, namely upon February 7 th one 
inoculated leaf was cut off close to the stem, and taken up to the labor¬ 
atory where it was laid over night in a petri-dish kept damp by means 
of a filter paper saturated with water. Free hand sections were made 
of the end portion of the leaf about 2 cm from the leaf-tip. The sec¬ 
tions were killed in lacto-phenol, and stained in Bleu coton G 4 B. Two 
or three sections were obtained which showed spores lying upon the 
upper epidermis; the spores, however, showed no sign of germination. 
Sections were cut daily from this leaf to find out when the spores ger¬ 
minated, and when the penetration of the host took place. Although the 
leaf was servered from the parent plant, yet until the mycelium of H. 
echinulatum was found within its tissues, it would be useless to look for 
penetration in those leaves still on the parent plant. Thus this leaf 
afforded a time-limit for the infection to take place. The time required 
for the infection of the leaves not removed from the plant might be longer 
than that for the isolated leaf; but it could not be shorter. 
On the 12 th February 1912 sections were obtained, which, however, 
showed no spores, but mycelium which had penetrated the host tissues 
by passing direct through the cuticle and epidermis into the central tissue 
of the leaf. The mycelium or infecting hypliae had taken a very nearly 
straight course as far as the middle portion of the leaf, and was as yet 
unbranched. No infection by way of the stomata could be observed. The 
infection in this case took place in from six to seven days. Some of the 
leaves on the plant itself were now cut off, and examined in the same 
way. Penetrating hypliae were found, but spores were not seen, and the 
actual mode of penetration, except in one case was not observed. In 
this one case a fairly thick section was mounted obliquely, so that a sur- 
