Stevens: Polystictus versicolor 



269 



annual ring, and in the age of the wood when tyloses first appear. 

 In normal catalpa wood tyloses are found almost exclusively in 

 the larger vessels of the spring wood, and even in these they are 

 more or less scattering, many of the vessels containing no ty- 

 loses and others being only partly filled. Tyloses first appear 

 in the second ring from the bark, that is, they are found to a 

 very limited extent in two-year old wood. They are more nu- 

 merous, of course, in wood which is three- or four-years old, but 

 do not occur in sufficient numbers to hinder very seriously the 

 flow of water. 



When the outer portion of a trunk which is rotten at the heart 

 is examined, however, tyloses are found filling practically every 

 vessel of the spring wood and occurring to a considerable extent 

 in the vessels of the summer wood. Moreover, there are prac- 

 tically as many tyloses in the two-year old wood as in the five- or 

 six-year old wood. This is the case even when the rotten por- 

 tion of the trunk is at some little distance from the two-year old 

 wood, often as much as four or five annual rings. Such a condi- 

 tion means, of course, that even the two-year old wood in badly 

 diseased trees is of practically no use for water conduction. It 

 seems probable that the presence of these tyloses and the conse- 

 quent obstruction to the flow of water would result in the earlier 

 death of the parenchyma cells in these outer annual rings. It 

 seems possible also that these outer rings would then be more 

 readily attacked by the normally saprophytic Polystictus versi- 

 color, and that the formation of tyloses may play some part in the 

 invasion of the sap wood by the fungus. 



It would, of course, be unsafe to assume without further proof 

 that this markedly increased development of tyloses is due to the 

 fact that part of the trunk is decayed. But Hartig (4, p. 235) 

 observed that "when the wood of a dicotyledonous tree is ex- 

 posed by a wound the vessels become completely plugged up by 

 tyloses," and there is considerable resemblence between this con- 

 dition and that found by the writer in partly rotted catalpa 

 trunks. If, as is generally believed (5), the formation of tyloses 

 occurs when the pressure in the vessels becomes less than that 

 of the adjacent parenchyma cells, then anything that reduces the 

 pressure in the vessels while the parenchyma cells are alive 



