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. 

 (PL LVIII, fig. 5.) * 



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. UV, middle.) In white oak the vessels are 

 completely closed by tyloses, as shown in Plate LIU, figures i and 2, 

 or Plate LIV, #3. 



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. LH, fig. i ; and PL 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. I, VIII of all conditions of open and closed horizontal resin canals 

 were taken from sapwood material. 



