452 MIRANDA ET AL.Figure 3 Cross section of a lilac (Syringa vulgarisL.) source leaf showing a minor vein. C, companion cell;
S, sieve element; X, xylem; MC mesophyll (photosynthetic) cell. The companion cell in this species is a type
1 (intermediary) cell. (Paraffin section slide courtesy of D. A. DeMason.)
transferred into the cell wall space (the apoplast) of the SE-CC complex and are taken up across the
plasma membrane of the SE-CC complex for export in the phloem [17,18]. Leaves with this type of mi-
nor vein configuration, therefore, use a membrane transport mechanism for phloem loading. Plant species
with type 2a companion cells are almost exclusively herbaceous annuals of temperate origin [16]. Most
of the major crop species of agronomic importance fall into type 2a.
Type 2b companion cells (Figure 4C) are similar to type 2a in that they lack extensive symplastic
connections to the photosynthetic tissues [6–8]. However, the cell walls of type 2b companion cells are
characterized by extensive wall ingrowths, which greatly amplify the plasma membrane surface exposed
to the apoplastic space. This type of companion cells is referred to as a “transfer cell” [18], for it has ap-
parently been modified to facilitate the transfer of assimilates from the apoplast into the SE-CC complex.
Type 2b companion cells are again typical of temperate herbaceous crops [16] and are a particularly com-
mon characteristic of legume species [19].
III. LONG-DISTANCE TRANSPORT
The vascular system of higher plants can be regarded as a series of parallel conduits of xylem and phloem
tissue, which run the length of the plant body from root to shoot and permeate all major plant organs. De-
spite the co-occurrence of xylem and phloem tissues in the vascular strands, these tissues are functionally
quite distinct.
The main function of the xylem is the transport of water and dissolved mineral nutrients to the
shoot following uptake from the soil by the roots. Xylem transport is unidirectional (upward from roots
to shoots) and is driven by the water potential gradient created by evaporation of water from leaves
(transpiration) [20]. Rates of transport can be of the order of 10 cm min^1 [20]. The transport pathway
is formed by the cell walls of the xylem vessels and tracheids, which are dead at maturity [20]. Xylem
transport therefore does not require cells with a living protoplast. Xylem sap consists primarily of wa-
ter, with low levels of dissolved solutes (Table 1). Most noticeably, sugars are absent from xylem sap
(Table 1).