ON THE ULTRASTRUCTURE OF CAMBIUM AND ITS VASCULAR DERIVATIVES. III. THE SECONDARY WALLS OF THE SIEVE ELEMENTS OF PINUS STROBUS

The sieve elements of Pinus strobus have thick, lamellate secondary walls, which are composed predominantly of cellulose and lesser amounts of polyuronides and pectins. Eight to 10 lamellae may be present in walls 2-3 μ in thickness. Each lamella represents a plane of high cellulose density which results from intersection of two parallel sets of fibrils. Polyuronides and pectins are more or less evenly distributed in the wall, possibly with a greater concentration near the middle lamella and the inner surface. Resemblance of these walls to the so-called nacré walls is indicated, and it is possible that the two represent the same structure.