Fig. 15.2-11. Transverse section of pine wood. In conifers, all cells derived from the fusiform initials of the vascular cambium develop into tracheids, and all tracheids have more or less the same size. Consequently, the tracheids form extremely regular patterns of uniform radial rows of cells, every tracheid in a radial row being the daughter cell of the same fusiform initial. Consequently, the pattern of tracheids is a record of the activity of the fusiform initials. Notice that radial rows 1, 2 and 3 are regular, each being a single file of tracheids. But look at the arrow’s tip: at that point, a new row has appeared. When the vascular cambium was at that location, a fusiform initial divided such that it produced a second fusiform initial (this is called a multiplicative division). With the extra cell in the vascular cambium, the cambium could then begin making an extra row of tracheids. The fusiform initial that produced the extra initial might have been the one producing row a or it could have been the one producing row b. See pages 304 to 308 in Plant Anatomy by Mauseth, and especially see Fig. 14.10 on page 307 for a diagram of excellent research on this by M. W. Bannan.