RESISTANCE OF CONIFERS TO CREOSOTE INJECTION. 3 



MICROSCOPIC STRUCTURE. 



The structure of wood is illustrated diagrammatically in figure 1, 

 while Plates I, II, and III are photographs of magnified sections 

 seen in transverse, radial, and tangential planes, respectively. 



TEACHEIDS. 



The fibers called " tracheids," forming the greater portion of the 

 wood, are between one-twentieth and one-fifth of an inch long, 

 or from 40 to 100 times their radial diameter. They are tapered 

 and closed at their ends. (Pis. II and III.) In the springwood 

 the tracheids have thin walls and large cell spaces, and are polyg- 

 onal or rounded in form; in the summerwood they have thick walls 

 and small cell spaces, and are flattened radially. (See fig. 1 and 

 PI. I.) Located on the walls of the tracheids are numerous circlet- 

 like structures, the " bordered pits," which are plainly visible in 

 Plates II and III. These pits or pores are partitioned off by the 

 " pit membrane," the central portion of which is thickened, and is 

 known as the torus. Owing to the peculiar overhanging contour of 

 the border, the orifice of the bordered pit, where it opens to the cell 

 cavity, is of much smaller diameter than where it abuts on the pit 

 membrane. (Fig. 1, a.) When the membrane is forced toward one 

 of the two cell lumina, which it separates, the thickened portion or 

 torus is capable of blocking the narrow opening, and thus acting as a 

 valve. (Fig. 1, h.) In the heartwood the torus often is permanently 

 displaced from its central position and is sometimes cemented to one 

 side or other of the pit, effectually blocking it. 



MEDULLARY RAYS. 



The cells of the medullary rays are smaller and much shorter than 

 the longitudinal tracheids and lie with their long axis at right angles 

 to the latter. These ray cells are distinguished by their abrupt ends, 

 thin walls, numerous simple pits, 1 and by their short length, only 

 8 to 10 times their diameter. They are seen as bands on the radial 

 face (Fig. 1 and PI. Ill), and as pores on the tangential face. 

 (PL II.) Although quite small in the conifers they can be seen 

 readily without a magnifier on the radial surface. 



RESIN CELLS. 



Many of the conifers contain what are known as resin cells. These 

 resemble the ray cells in general appearance, but, except for those in 

 the medullary rays themselves, have their long axes parallel with the 

 major axis of the tree. 



* Simple pits do not have the circlet-like border. They are simple openings in the cen 

 walls closed only by the pit membranes. 



