574 BULLETIN 347 
this case the limits of the canker are easily distinguished on the surface; 
in rainy seasons the outline at first is not sharp, but if the canker is cut 
into with a knife a sharp line of demarcation will be found between the 
brown, diseased bark and the white, healthy, inner bark. 
When the spores have germinated in a wound, the germ tube thrives 
on the injured and dead cells until it has produced a mass of mycelium, 
Then, gradually accumulating strength as it increases, the mycelium 
en masse pushes out into the living tissue of the bark. Single threads 
do not seem to possess the power of penetrating alone among the living 
cells, but the invasion is accomplished by the force of mass action. Start- 
ing from a narrow point, the hyphz grow out in ray-like bundles, com- 
pletely destroying parenchyma and collenchyma and cambium cells as 
they go. The réle of toxins or enzymes in this process has not been 
investigated. All the rays starting from a single point are contiguous, 
forming the fan-like mats previously mentioned. 
Rate of growth of mycelium.— The rate of growth of mycelium under 
natural conditions in the tree can be roughly measured by the increase 
in size of the cankers. The growth varies with the temperature, being 
the most rapid during the summer months; but the mycelium does not 
always remain dormant during the winter, as will be shown. For twelve 
consecutive months at Charter Oak, Pennsylvania, cankers were outlined 
with a painted white line at the end of each month and the average rate 
of growth was computed. A second experiment of a similar nature was 
conducted at Napanoch, New York. The data on these two experiments 
are given in Tables 4 and 5. These tables give only the rate of growth 
TABLE 4. MontTuity RatTe or GrowTH IN DIAMETER OF CANKERS AT CHARTER 
Oak, PENNSYLVANIA 
Average 
Number | growth 
Month of per month 
: cankers | (in centi- 
meters) 
31 1.88 
200 2.78 
186 2.83 
140 1.85 
1.92 
November a 
December 27 = 
pee deine pthnah-e 4 dedeen gone hl aha argaey BAS We SHE AEG ta cand auc key Banas 89. 0.51 
BDMMALY 6 a. dy ag gitvsiea yd less Seas} tule Redo inane aqieuiind dnoted tance 89 0.00 
Matehivs scdait us as boxe ae 3326S RS MOA Gil nent gid Hike Aa wr be 84 0.7 
Ap ral vce tres sana tas. Ae k Gm sociasersiig e.sorseuavn awe Sbie'a @ vanicnnel ave eonaee’ 21 1.1 
INN sient nA tn ay ain dan ine aed ce Sess ccl ete at  ore Ee 41 2.4 
