Perforated ray cells

Abstract

In certain woods, whose rays have extensive uniseriate margins, an individual marginal cell may sometimes be modified, by the perforation of its side walls, to connect two vessel segments on opposite sides of the ray. This feature does not appear to have been previously described though it is by no means uncommon; it has been observed by the authors in woods of many widely separated families. It is limited to woods with particular types of ray and vessel structure, but its occurrence seems to be primarily determined by the manner in which the vessels develop. Large ray cells which appeared to have scalariform perforations in the side walls were first observed by the authors in the wood of Lacistema aggregatum (Berg). Rusby. (Lacistemaceæ); examination of macerated material, fig. 1, has shown that the side walls of these cells are exactly similar to the end walls of the vessel segments, and though it has not been possible to demonstrate the absence of a membrane, the authors are convinced from a careful study of the sections, that these walls are actually perforated, and that each forms a link between two vertical series of vessel segments. The course of the vessel is unusual; instead of continuing up and down between the rays, it periodically passes obliquely through a ray, and continues on the other side. The ray is usually uniseriate at the point at which it is crossed, and the ray cell involved becomes slightly swollen, rather in the manner of an oil or mucilage cell but without visible contents. The lateral walls are perforated, so that the cell resembles a very short vessel segment, and presumably functions as such. From the position and shape of these cells, however, it is clear that they have been derived from ray initials and not from fusiform initials. Attention was first drawn to this phenomenon by the presence of scalariform bars in the walls of cells which, from their shape, appeared at first sight to be oil or mucilage cells; simple perforations in such cells are obviously more easily overlooked, and it was not until a special search was made that they were found to be by no means uncommon, and to occur in woods of several different families.