32 MISSOURI BOTANICAL GARDEN. 
comparatively small, The diseased wood is darker in color 
than the normal wood, has no tenacity, and when crushed, 
turns into fine powder. These properties lead one to sus- 
pect profound morphological as well as chemical changes. 
A radial section through a rotted hole and the adjacent 
sound wood is represented on Pl. 5, fig. 10. The normal 
wood cells (a) show the constituent lamellae of the 
cell-wall plainly. The first noticeable change is in the bor- 
dered pits, which look as if they were corroded, like starch- 
grains in process of solution. This appearance is due to 
drops of resinous oil, which arrange themselves in this 
peculiar manner (PI. 5, fig. 3) on the walls and within 
the cavity of the pit. When treated with turpentine the 
resin is dissolved and the pits then look smooth. The walls 
at the same time are perforated in many places by colorless 
hyphae; no preference is shown for the pits. As one pro- 
ceeds toward the decayed spot the pits look fragmented ; fin- 
ally peculiar spiral breaks appear extending from the pits and 
passing upward from left to right. The breaks of two adja- 
cent walls cross one another at the pits (e), the lower one 
apparently extending from right to left. The whole cell- 
wall becomes striated, the striae all extending in a spiral line 
from left to right around the wall. Before long the circu- 
lar disc of the pit drops out, leaving a hole with jagged 
outline, which gradually increases in size. The number of 
breaks in the walls has increased, and finally the whole wall 
breaks up into innumerable pieces. The wood, when once 
the disintegration sets in, becomes so very brittle that it is 
very difficult to get good sections. Imbedding the same in 
soft paraffine was found very useful. The longitudinal 
walls grow thinner because of the shrinkage of the middle 
lamella. This gradually disappears, and gives rise to the 
breaks in the wall already spoken of. The shrinkage and 
solution finally has gone so far that only the primary 
lamella is left, which breaks into many pieces. The drop- 
ping out of the more resistant walls lining the pits is char- 
10 
