4 BULLETIN 212, U. S. DEPARTMENT OF AGRICULTURE. 
flowers, it is believed that Peridermium cerebrum can enter young 
seedlings or the tender portions of more mature growth without first 
having the bark broken. Entrance in this manner must, out of 
necessity, be aided by sufficient moisture for germination and to per- 
mit a rapid penetration by the young mycelium. On the sandy 
plains of the Great Lakes region rain water disappears almost immedi- 
ately and the sand becomes heated about the isolated tree groups, 
causing a rapid evaporation from the surface of the trunk and 
branches and leaving the moisture content of the outer bark at a 
minimum. In whatever manner the fungus may enter its host, 
directly or through wounds, the number of galls and imperfect 
branches is usually much less on trees of the sandy barrens than in 
more moist regions. 
In swampy areas the jack pine grows in close stands. Here the 
percentage of infected trees is much greater. The trunks of the 6 to 
12 year old jack pines are often covered with swellings stunting the 
growth of the trees very rapidly (PL I, fig. 1). Trees so infected 
never reach maturity and may continue living for an indefinite 
period in a stunted condition, to be finally blown over by the wind 
or broken down by the snow. The 1 to 4 year old seedlings are quite 
often attacked. With these, as is often the case with larger trees 
which through mechanical injury may become infected at the ground, 
the gall is formed directly at the base of the main stem. When a 
seedling is infected there or higher up on the stem, it develops into a 
deformed growth after the manner of a witches '-broom (PL I, fig. 2) 
and never attains a height of more than 2 or 3 feet. The perennial 
mycelium of the fungus thrives in the cambial layer and in the living 
parts of the sapwood. Trees with a single infection on the trunk 
occurring at the age of 4 to 6 years are known to support the living 
mycelium of the fungus to the advanced age of 70 to 80 years. Usu- 
ally, however, the excessive production of resin in the infected 
tissues infiltrates the woody portion of the trunk, and the sap supply 
is cut off so that death results in a comparatively short time (fig. 2). 
This is especially true in young seedlings. Peridermium galls are 
frequently observed a foot or more in diameter. Trees supporting 
galls of this size had succumbed in every instance to the disease. 
Some knowledge of the damage done by Peridermium cerebrum to 
the jack pine may be obtained from notes of a pathological survey 
by the writer in the national forests of Michigan. Out of 100 trees 
of an average plat on dry sandy soil, not selected because of any 
pronounced diseased condition, 50 per cent were heavily infected, 
while only an occasional tree out of a second hundred on similar but 
moist er soil was absolutely free from the disease. Out of 100 trees 
taken from the swamp type, practically all were infected. Not all 
the trees were infected seriously. A tree bearing a single branch gall 
