124 
ALBAN STEWART 
disorganized. In some instances it was found that the cells sur- 
rounding these cavities had begun to divide and had sent short fila- 
ments of cells into the cavity. 
Summary 
1. Knots arise primarily from the infection of the stem by spores^ 
and secondarily by the spreading of the fungus through the tissues of 
the stem from a knot already formed. 
2. The changes induced in young stems of the current year's 
growth, as a result of primary infections, are usually apparent into or 
nearly to the pith at the distal end of the infection, while at the proxi- 
mal there is a zone of sound wood surrounding the pith. 
3. The vicinity of the leaf gaps is the seat of unusual disturbance 
in knots whether resulting from primary or secondary infections,, 
owing to the large amount of parenchyma present in such areas. 
4. Normal wood of the choke cherry contains uniseriate rays, and 
multiseriate rays two to four cells wide, the last of which may become 
further broadened as a result of injury to the stem. 
5. As a result of the stimulating action of the fungus the multi- 
seriate rays still further broaden in infected stems, simulating the 
structure of compound rays. 
6. The production of the usual xylem elements is greatly inhibited 
during the first season's growth following infection, but there is a 
correspondingly greater production of parenchyma in the xylem. 
7. The parenchyma cells formed in the xylem as a result of such 
stimulation greatly increase in size the year following their formation, 
and the rapid growth of the knot at this time is due largely to this 
fact. 
8. There is an increase in the rate of division of the cambium cells 
on the side of the stem in which invasion occurs, and there may also 
be an increase in the rate of division on the opposite side of the stem 
when the fungus is apparently absent from its tissues on this side. 
9. The cambium retains its normal position between xylem and 
phloem throughout the development of the knot, except opposite the 
broad rays. 
10. Wedge-shaped masses of xylem are produced by the cambium 
during the second season's growth of the knot. 
11. Isolated xylem elements, or groups of such, in the cortical 
portion of the knot, are formed entirely from the misplaced and 
