The fine structure of Collenchyma cells in Heracleum Sphondylium L

Abstract

The study of Heracleum collenchyma follows as a natural consequence of the previous study of sclerenchyma. The cells are elongated with pointed ends much like sclerenchyma cells, but are distinguished from these by their typical collenchymatous thickening, the early onset of thickening, and the heavy impregnation with pectin in the apparent absence of lignin. The thickened wall consists of a series of lamellae (commonly 4-7) running completely round the cell, alternately cellulose-rich pectin-poor and cellulose-poor pectin-rich. The innermost layer of all is almost free of pectin. The X-ray diagram indicates an orientation of the molecular chains of cellulose approximately longitudinal with an angular dispersion of some ± 14 degrees, and this is fully supported by determinations of the major extinction position. The remarkable uniformity of the m.e.p. suggests that the angular dispersion noted in the X-ray photograph occurs among the micelles in a wall and not among the cells in a bundle used, and the optical discontinuity observed in transverse sections under the polarizing microscope is explained on these grounds. It is suggested that the innermost wall layers are bright in transverse section between crossed nicols because they consist of cellulose micelles with an angular dispersion greater than that in layers deposited earlier. The remarkably constant orientation thus revealed in the cellulose-rich layers suggests that either the comparatively few cellulose chains in the layers alternate to them suffice to impose on subsequent layers their orientation or, more probably, that there is in the protoplasmic surface an orienting mechanism itself affected by elongation. The indications are that the longitudinal orientation of the chains in the mature wall, and the more perfect alignment of micelles in the earlier layers, are a consequence of the enormous elongation which the wall has undergone. Swelling of the wall in a variety of treatments is readily explained in terms of the high pectin content of the central region of the wall and of the greater angular dispersion of the micelles in the innermost layer. During growth of the cells wall thickening commences long before elongation has ceased, and it is therefore suggested that some modification is necessary of the scheme of nomenclature proposed by Bailey and Kerr. Finally, at no stage has a wall been observed with cellulose chains oriented even approximately in the transverse direction.