Vessel development in celery

Abstract does not appear. First page follows.

Introduction

Botanical literature contains comparatively little information concerning the development and breakdown of end walls in vessels, and, moreover, the available descriptions show a surprising lack of agreement with regard to the sequence of events in the establishment of continuity between vessel elements. This circumstance prompts the writer to report the observations on vessel end walls that were made incidentally to an earlier anatomical study (Esau, 1936)3 on petioles of the celery (Apium graveolens L.).

As in the previous investigation, the material was killed in a chromoacetic-formalin solution, was washed and dehydrated in dioxan, and embedded in paraffin. Heidenhain’s haematoxylin and a combination of aniline blue and safranine were employed in staining the sections.

Development and Breakdown of Vessel End Walls

Structure of End Walls in Very Young Vessels.—In the ontogeny of vessels the mother cells expand and vacuolate before the secondary thickenings develop on their longitudinal walls. Very young mother cells have uniformly thin walls (fig. 1, below). When the cell reaches its mature diameter but still contains a nucleus and cytoplasm, its longitudinal walls are thin, whereas the transverse end walls are characteristically thickened (fig. 1., above). This thickening, which in longitudinal sections of cells appears lenticular in shape, occurs only on that portion of the end wall which is to be removed during maturation of the vessel, whereas the rest of the end wall is thin.

In prepared sections the thickened portion of the end wall usually appears to be limited above and below by two sharp, dark lines enclosing between them a lighter-colored substance. Plates 2,A and 4,A illustrate this condition. They also show the thin marginal region of the end wall.

Not infrequently a line is perceptible in median position in the thickened portion of the end wall. It seems to separate the thickening into two layers (plate 4, K).