Mar. 25,1&14 
Tyloses in American Woods 
447 
the wood produced by felling the tree may have an important bearing on 
the presence of tyloses in the outer rings of a log, where the parenchyma 
cells are still living and capable of growth. It is possible to find in these 
rings young or old, or large and small, tyloses together in the same vessel. 
(PI. LIV, i?3<) Although exceptions have been noted, the idea that a 
considerable number of the outer rings are entirely free from tyloses 
has, however, been very generally accepted (Strasburger). 1 The data 
obtained from the present study show that there is a very considerable 
formation of tyloses in the outer rings of the sapwood. The question 
then arose as to whether these sapwood tyloses were of normal origin or 
whether they were due to some wound stimulus, such as the felling of 
the tree. It was finally concluded that they were normally formed 
tyloses, because their development throughout the vessels was very 
uniform instead of being sporadic or irregular, as in the case of tyloses 
associated with wounds (PI. LIV, Ri and R2), and because an exami¬ 
nation of branches from living trees of Rhus, the sumach, Catalpa, and 
Robinia, the black locust, made immediately after cutting, confirmed the 
other observations of the relatively early formation of tyloses in many 
species. In material which was not received for examination until 
several weeks after it was cut, thin, irregularly distributed tyloses were 
often found in the outer vessels, though the latter must have been func¬ 
tioning in sap condition at the time the tree was felled. 
It is noteworthy also that in this study tyloses were found to reach the 
most remarkable development in ring-porous woods, such as oak, hickory, 
black locust, or osage orange. (PI. LIU, figs. 1 and 3, and PI. LVI, 
fig. 2.) In woods where tyloses are few and scattered there is consider¬ 
able variation from specimen to specimen in the actual number of tyloses 
present. This tendency is clearly shown in the woods of the diffuse 
porous group. (Table II.) It is also noticeable that in the two or three 
rings surrounding the pith in a diffuse porous wood tyloses are often 
much more abundant than elsewhere in either the heartwood or sapwood. 
EFFECT OF THE DISTRIBUTION OF PARENCHYMA TISSUE 
Since tylose formation depends upon the presence of parenchyma cells 
either in the form of wood parenchyma or medullary rays in close prox¬ 
imity to vessels or tracheids, the variation in position, abundance, and 
vitality of these cells affords at least a partial explanation of the irregular 
development of tyloses in different species of wood. Parenchyma tissue 
is considerably developed in the following families and their respective 
genera. 2 This study has shown that in these families are a large number 
of native woods exhibiting ^tyloses. 
1 Tyloses are . . . instrumental in closing the water courses of the heartwood. . . . These are intrusive 
growths from living cells which penetrate the cavities of the adjoining tracheal elements during the transi¬ 
tion of sapwood into heartwood. 
2 Solereder, Hans. Systematic Anatomy of the Dicotyledons . . . v. 2, p. 1143. Oxford, 1908. Certain 
other woods with abundant parenchyma frequently produce gummy substances rather than tyloses. 
