of vascular tissues. This chapter emphasizes the phloem structure of the minor veins of leaves, the key in-
terface of the phloem transport system with the photosynthetic tissues.
A. General Feature of Phloem Tissues
- Sieve Elements
Phloem tissues in general consist of several structurally distinct cell types: sieve elements, companion
cells, parenchyma cells, and fibers [1]. The most characteristic cells are the sieve elements, which are
linked end to end to form the conduit for the long-distance movement of solutes (Figure 1). Unlike xylem
tracheids, which are dead at maturity, functional sieve elements are living cells. During maturation of the
sieve element, the tonoplast and nucleus degenerate and all ribosomes disappear. Mitochondria and plas-
tids assume a parietal position next to the plasma membrane. Plastids accumulate either starch or protein
inclusions. Proteinaceous strands (P protein) may also be present in the cell lumen [1–4].
The end walls of the sieve element are modified to form the sieve plate (Figure 1). Contiguous sieve
elements are interconnected to form a sieve tube through strands of protoplasm, which pass through the
plasma membrane–lined sieve plate pores. The side walls of adjacent sieve tubes may also contain sieve
areas connecting the protoplasts of the neighboring sieve elements [1–4]. The cytoplasmic compartments
of the sieve elements, therefore, form a continuum through which solutes can be moved.
- Companion Cells
Companion cells are associated with sieve elements and arise concurrently with the sieve elements by di-
vision of a common mother cell. Unlike the sieve elements, companion cells retain their nuclei and vac-
uoles and are characterized by densely staining cytoplasm containing numerous free ribosomes and many
highly differentiated mitochondria and plastids [1–4]. These structural features are indicative of high
metabolic activity, and it is thought that the companion cells act to maintain the structural integrity of the
sieve elements, which lose metabolic capability as a result of the structural changes that occur during mat-
uration. The protoplasts of the companion cells are connected to sieve elements by numerous branched
(on the companion cell side) plasmodesmata, providing cytoplasmic connection for metabolite exchange
450 MIRANDA ET AL.Figure 1 Longitudinal section of a squash (Cucurbita pepoL.) stem, showing the sieve elements (SE). Ar-
row indicates a sieve plate. (Paraffin section slide courtesy of D. A. DeMason.)