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 PI. 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 



