140 X. THE CONIFEROUS STEM. 



A cross-section extending through wood, cambium and bast, 

 shows us the tracheides arranged in very definite radial rows. 

 As we trace such rows outwards here and there they are seen 

 to double, or fork, to accommodate themselves to the increased 

 periphery of the stem. In section the tracheides appear mostly 

 four-sided, now and then five- or six-sided. As we pass along 

 a row outwards, they are seen to become progressively narrower 

 in their radial diameter, and their walls more thickened, up to 

 a point at which the cavity is quite flat; then abruptly, with- 

 out transition (Fig. 53, on p. 144 at g), we note a very large 

 tracheide, followed by another and another until the progressive 

 diminution sets in again. In each radial row the abrupt change 

 in dimension takes place, so that a clearly distinguishable curved 

 line can be traced in the section. This line marks the limits 

 of a year's growth, and is visible to the naked eye upon a 

 section of a stem or on the end of a plank of pine-wood. During 

 the summer the tracheides which are successively formed by the 

 activity of the cambium are more and more flattened and thicker 

 walled the most flattened and thickest walled being those which 

 are last formed in a season's growth ; while the tracheides which 

 are formed when growth is resumed in the ensuing spring are 

 of the largest size, and have the thinnest walls. We can distin- 

 guish in this way between wide-cavitied spring wood and narrow- 

 cavitied summer wood, the line of demarcation representing the 

 position of the cambium during a winter's rest. Hence, if we 

 were to take a section of a twig cut off during winter, the wood 

 in contact w r ith the cambium, on its inner side, would be composed 

 of the narrowest cavitied tracheides (such a section is shown in 

 the subjoined Fig. 51) ; and a fairly shrewd judgment as to the 

 time when material is cut for preservation could be made by 

 comparing the tracheides adjoining the cambium with those 

 found in the annual rings next within. Interpolated amongst 

 the elements may be found occasional small cavities, represent- 

 ing sections through the tapering ends of tracheides. Parallel to 

 these radial rows of tracheides run similar radial rows of narrow 

 elements, elongated radially, sometimes containing starch and 

 protoplasm, sometimes only water, the rows usually only one 

 cell broad but occasionally more. These are the medullary rays, 

 On the radial walls of the tracheides, i.e., the walls parallel with 

 the radii, and therefore facing the medullary rays, we shall see 

 the remarkable and characteristic bordered pits, If the section 



