Ontogeny and structure of collenchyma and of vascular tissues in celery petioles

Abstract

Abstract does not appear. First page follows. Introduction Efforts to obtain stringless varieties of celery (Apium graveolens L.) drew attention to the nature of the so-called celery strings. The two structures that constitute these strings, the vascular bundles and the collenchyma strands, differ greatly from each other in their development, their histology, and their physical properties. This paper deals with the ontogeny and structure of the tissues that make up the strings and gives some information regarding their relative strength. The present work treats of the histological part of the problem in considerable detail and adds to our knowledge of histogenesis and tissue differentiation. It compares the mode of origin of collenchyma with that of the vascular tissue and describes in detail the phloem and its transformation, in the final stages, into the collenchymatous bundle cap. Material and Methods In preparing the material for histological study, the procedure was as follows. Pieces of petioles were killed and fixed for 48 hours in Karpechenko’s chrom-acetic-formalin solution (Rawlins, 1933,3 p. 13). The fixed material, washed in three changes of pure dehydrated dioxan (Graupner and Weissberger, 1931), was placed in a paraffin oven in a mixture of dioxan and paraffin. The latter was changed four times to expel the dioxan. Three to four days after placement in the oven the material was embedded in paraffin. Without being soaked in water, the embedded material was cut 10 microns thick on a rotary microtome. Instead of a microtome knife, Gillette razor blades, clamped into a Spencer razor-blade holder, were used. Very little difficulty was experienced in cutting the material, even the collenchyma and xylem of the oldest petioles. The protoplasts, however, commonly shrank throughout the material, though no attempt was made to determine at what stage of the process this shrinkage occurred.