462 
Journal of Agricultural Research 
Vol. I, No. 6 
never have split apart to form a canal opening. This was demonstrated 
by the writer by means of serial sections following the course of a num¬ 
ber of horizontal resin canals from the bark into the heartwood. Second, 
the canals once open may be closed completely by the growth of certain 
of the epithelial cells, as before explained. This closing is not produced 
by the equal action of all the cells which first split apart to form the 
canal, but only by the later growth of certain of these which possessed 
plasma and the growth potential for a longer period than their neighbors. 
(PI. LVIII, fig. 5.) 1 
PRACTICAL SIGNIFICANCE OF TYLOSES 
TYLOSES AS A NATURAL “FILLER” 
A good instance of the part played by tyloses in the structure of wood 
is in the case of red oak and white oak. These two species have prac¬ 
tically the same structure, yet the red oak can not be used for tight 
cooperage stock because the vessels are open tubes through which air or 
liquid can escape. (PI. LIV, middle.) In white oak the vessels are 
completely closed by tyloses, as shown in Plate LIU, figures 1 and 2, 
or Plate LIV, R3. 
In cabinetmaker's parlance, tyloses behave to some extent like a 
natural “filler.” On a radial-cut surface the large vessels in the spring 
wood of a red oak appear like hollow grooves, while those in the white 
oaks are partly filled by the network of the tylosal cells which catch and 
hold paint, for example. (PI. LII, fig. 1; and PI. LIU, fig- 2.) 
TYLOSES A FACTOR IN DURABILITY 
It is of interest to note the presence of tyloses (or sometimes of gums) 
in the large vessels of those hardwoods which are particularly valued for 
their durability. Many factors, such as the chemical composition of 
the wood, its rate of growth, and hardness, are, of course, important in 
determining durability, but the effect of tyloses should not be disregarded. 
Moreover the vigorous growth of parenchyma, which in some cases 
manifests itself by causing tylose formation and in others by producing 
tannins, essential oils, etc., appears to be a fundamental characteristic 
of naturally durable woods. White oak, in which tyloses are abundant, 
is, for example, more durable than red oak, in which they are almost 
wholly absent. The tylose walls present an added obstruction to the 
advance of fungous hyphse and tend to make the vessels impenetrable 
to air and water. They are especially effective in woods that have been 
dried. 
Although sapwood contains tyloses, it is usually less durable than 
heartwood. The latter fact, however, holds true also for woods without 
tyloses and can probably be explained by the condition of such materials 
1 The illustrations reproduced in PI. LVIII of all conditions of open and closed horizontal resin canals 
were taken from sapwood material. 
