Abstract does not appear. First page follows.

Introduction

An intensive investigation of the phloem of Vitis vinifera L., the grapevine, was previously reported in this Journal (Esau, 1948a).3 That study was centered on the development of the tissue, especially with reference to seasonal changes. It served as a background for the subsequent study of the effects of Pierce’s disease virus on grapevine (Esau, 1948b). In connection with the earlier study, samples of bark were collected from thirty species of Vitis, other than V. vinifera, growing in the experimental vineyard of the Department of Viticulture at Davis. The first part of the present paper reports on a survey of phloem of these species.

After an electron microscope was installed on the Davis campus, the ultrastructure of phloem tissue became one of the important research projects in the Department of Botany at Davis. Vitis phloem was included in the studies and a considerable amount of information on this tissue was obtained during 1960-1962. This information constitutes the major part of the present paper.

Methodology in submicroscopic research is still new and is changing rapidly. The ultrastructural data reported here are based on relatively early techniques. Nevertheless, these data expand the understanding of the structure and seasonal changes in the conducting element, the sieve element, and further differentiate this element from the associated nucleate cells. The present study has confirmed the interpretation of some previously recognized features but has also introduced new problems.

Early ultrastructural investigations on plant material have emphasized meristematic or relatively undifferentiated parenchyma cells, especially those derived from root tips (see, for example, (Whaley et al., 1960), and the submicroscopic structure of such cells is relatively well understood. Interpretation of the fine structure of mature and highly specialized cells must therefore be made by comparison with the less-differentiated cells. Part of the present paper deals with the sieve element in the root tip, which was studied against the background of information on more or less meristematic parenchyma cells of the same parts of the root. These background observations are reported in full because they help to single out the specific characteristics of the sieve element.

The literature pertinent to the subject matter is not reviewed under a separate heading. Certain references are mentioned in connection with individual observations in the main text, but the comprehensive comparison of data obtained with those reported in the literature appears only in the discussion. The specific terminology concerning the phloem tissue is explained in the previous Hilgardia article on Vitis phloem (Esau, 1948a), and that serving for the description of ultrastructural details has been reviewed by (Esau and Cheadle (1965)).