Vascular transfer cells in Angiosperm leaves A taxonomic and morphological survey

A study of the fine structure of minor veins of mature leaves of 975 species and 242 families of Angiosperms shows that transfer cells are widespread amongst herbaceous Dicotyledons, are much rarer in woody Dicotyledons, and are virtually absent from the Monocotyledons. The evolutionary significance of the distribution of the cells amongst and within orders, families and minor groupings is discussed. Four types of transfer cell are recognized in minor veins, all possessing irregular ingrowths of wall material protruding into their protoplasts, and all being regarded as modified parenchyma of the minor vein. Two types occur in phloem. One (the A-cell), with ingrowths distributed right round its periphery, is associated specifically with the sieve elements. The other (the B-cell) occurs more generally throughout the phloem and has zones of wall ingrowths oriented towards sieve elements and their associated companion cells or A-cells. Two other types (C- and D-cells) occur in xylem parenchyma and bundle sheath respectively, and have ingrowths only on walls in contact with or in close proximity to vessels or tracheids. Each species has a characteristic combination of types of transfer cell. The variations encountered in the survey are classified. Consistent differences in the frequency and form of ingrowths are to be found between the different types of transfer cell of a single species, and between different species in respect to a particular type of transfer cell. The functional significance of transfer cells in minor veins is discussed in relation to the loading and unloading of the conducting elements and to the retrieval of extra-cytoplasmic solutes from the mesophyll and the transpiration stream.