Early stages of bordered pit formation in radiata pine

Summary The development of bordered pit sites in the cambium of Pinus radiata D. Don was studied using light and electron microscopy. No primary pit fields occur in the fusiform initials and no plasmodesmatal connections were seen in radial walls of undifferentiated cambial cells or differentiating tracheids. Thin areas bounded by a thick rim appear in the radial walls of these cells once enlargement is under way. They appear to result from a redistribution of wall material which, it is suggested, is brought about by some agent which penetrates the primary wall. The thin areas, which are the sites of bordered pits, are common to the cells joined by the affected wall, and the rim which protrudes symmetrically into each cell provides a template on which the border is subsequently formed. This mechanism would explain how symmetrical pit pairs can be formed in the absence of plasmodesmata.