56 MISSOURI BOTANICAL GARDEN. 
grating action on wood must be, cannot be due to mechan- 
ical causes, such as difference in the character of the wood 
cells, or the presence of obstructing layers. In the first 
place all wood cells are disintegrated, whether they be of 
the spring or summer wood, and the bounding lines of a 
cavity are not influenced by the harder summer wood as 
might be supposed. Then again no resistant layers, as 
such, excepting the harder summer wood, exist in the heart- 
wood. The isolated resin cells seem to have no bearing in 
this connection. The only remaining explanation is to 
attribute the local decay to chemical influences, which pre- 
vent the decay from spreading beyond well-defined limits. 
It is a well-known fact that certain kinds of wood are 
more durable, i. e., resist the destructive influences of fungi 
longer than others. Willows, for instance, are more 
easily destroyed than oaks or cedar. Frank * and Temme ft 
have shown that in dicotyledonous trees a certain preserv- 
ative gum is found. This is formed in all wounds open 
to the air, and occurs normally in all heartwood. This 
wound gum fills the vessels similarly to thylloses, and ren- 
ders them impassable to air and water. The wound gum 
is insoluble in alcohol, ether, HeSO4, KOH, but soluble in 
hot HNOs. These authors do not explain what causes the 
formation of this preservative material, beyond the fact 
that it forms when healthy wood is wounded and exposed 
to the air. In the Coniferae this substance is not present 
and Frankf holds that the infiltration of resin takes the 
place of the gum. Hartig § finds a yellowish-brown mass 
in the cells adjacent to wounds in trees. This mass is 
usually much cracked. He calls it dried brown solution, 
* Frank, B. Uber die Gummibildung im Holze u. deren physiologische 
Bedeutung. (Ber d. deut. bot. Ges. 18 Juli, 1884). 
+t Temme, F, Uber Schutz und Kernholz, seine Bildung u. seine phy- 
siologische Bedeutung. (Landw. Jahrb. 14: 465. Taf. 6,7. 1885. 
} Frank, B. Die Krankheiten der Pflanzen 1:41. 1895. 
§ Hartig, R. Zersetzungserscheinungen etc. 66. pl. 11. fig. 7. 
34 
