BULLETIN OF THE BUSSEY INSTITUTION. 441 
extend completely or nearly completely round the branch, the portion 
above the knot dies. Frequently the upper part of the stem bends over 
so as to form a right angle with the lower part, and sometimes the portion 
involved in the knot forms an irregular coil. The surface of the knot 
is undulated, and flakes of bark not unfrequently adhere to it. In 
winter, it is more or less cracked and broken open, and the inside is 
seen to be worm-eaten and hollow, except the woody portion of the 
stem, which passes through, comparatively sound, generally on one 
side of the knot. 
Below and above the knot, unless the branch above have been 
completely killed, the stem is swollen for from half an inch to two 
inches, rarely for a greater distance. Under the microscope, sections 
show, that, although the bark has been cracked in several places by 
the expansion of the stem, yet a new layer of bark has formed over 
the exposed portions. An abundance of mycelial threads are seen 
extending in streaks from the cambium towards the cuticle. At their 
outer extremity the bundles of mycelial threads spread out into fan- 
shaped masses, as shown in Plate VI., Fig. 1. The threads are hyaline, 
very fine, .0007 mm. in diameter, and are most intricately twisted to- 
gether in bundles, so that it is almost impossible to say whether there 
are any cross partitions in them. Jn longitudinal sections, it is easy 
to see that they begin in the cambium, and radiate outwards. The 
threads are found only in the swollen part of the stem; and on the 
most careful search we have been unable to detect any in the stem, 
just below the swelling. 
Contp1a. — As spring advances, the mycelial threads increase in size, 
burst through the bark, and then form the dense pseudo-parenchym- 
atous tissue characteristic of the Pyrenomycetes when about to 
fructify. The knot grows larger, until, about the time the choke 
cherry flowers, it has nearly the same diameter as the knot of the year 
before, of which it is-an extension. It is not, however, black, but 
a very dark brownish green. It is solid, rather pulpy, and, on 
section, a number of radiating lines are seen, as shown in Plate IV., 
Figs. 2 and 3. With a hand lens, one can see small hemispherical 
protuberances, which are the beginnings of the perithecia. The 
whole surface of the protuberances is covered with filaments (Plate V., 
Fig. 2 6), about .04 mm. to .06 mm. in height, and .004 mm. in breadth, 
which are somewhat flexuous, and frequently divided by cross parti- 
VOL. I. 56 
