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 f 

 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, H2SO4, 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 Frank J 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. liber Schutz und Kernholz, seine Bildung u. seine phy- 

 siologische Bedeutung. (Landw. Jahrb. 14 : 465. Taf. 6, 7. 1885. 



J Frank, B. Die Krankheiten der Pflanzen 1 : 41. 1895. 



Hartig, R. Zersetzungserscheinungen etc. 66. pi. ll.flg. 7. 

 34 



