OBSERVATIONS ON THE PHLOEM IN THE MONOCOTYLEDONEAE. I. THE OCCURRENCE AND PHYLOGENETIC SPECIALIZATION IN STRUCTURE OF THE SIEVE TUBES IN THE METAPHLOEM

Abstract

INTEREST IN both the structure and functions of phloem has heightened in the past decade, and consequently it may be timely to offer some supplementary information concerning sieve tubes in the Monocotyledoneae. Esau (1939) has recently reviewed the literature on the general subject of the structure of phloem elements in vascular plants and it is unnecessary to duplicate her work in the present paper. Some of the contributions, such as those of Hemenway (1911, 1913), MacDaniels (1918) and Crafts (1939), somewhat resemble the nature of the present work, which is in the form of a survey of the phloem in a large number of representative plants. Other articles, such as those of Schulze (1893), Ross (1883), Lange (1910), Bouvier (1915) and many more, only incidentally mention the structure of the phloem in the Monocotyledoneae. A source of general but brief information on the sieve tubes in this group of vascular plants is Solereder and Meyer (1928, 1929, 1930, 1933). Esau (1939) pointed out the confusion of terminology in the literature concerning phloem and suggested certain changes, especially with regard to sieve cells and sieve tubes. It will be profitable, therefore, to define the terms2 used in this paper before the objectives of the present work are presented. 1. A sieve area is an area in the walls of parenchyma cells, sieve cells, or sieve-tube members in which pores with connecting strands are clustered. In sieve cells and sieve-tube elements, the sieve areas are usually specialized through a more or less pronounced enlargement of connecting strands and a deposition of callose around each strand. 2. A sieve plate is a wall or portion of a wall of a sieve-tube member bearing one or more highly specialized sieve areas. There are two generally distinct types of sieve plates. One is the simple type in which the pores are uniformly distributed (fig. 6), and the other is the compound.type in which the pores are clustered in scalariform, reticulate, or any other complex manner (fig. 4 and 5). 3. A sieve cell is a cell, enucleate at maturity, in which the sieve areas of all walls are of the same degree of specialization. 4. A sieve-tube element or member is a cell, enucleate at maturity, in which certain sieve areas are