Razumovskaya A.V. Cytology of the minor-vein phloem in 320 species from the subclass Asteridae suggests a high diversity of phloem-loading modes. Frontiers in Plant Science. 2013, V. 4, Article 312.

Batashev et al. Minor vein phloem in Asteridae FIGURE 1 I Structural diversity of companion cells in minor veins in representative species according to classification in Table 1. L, leucoplast; arrow points on plasmodesmal fields between companion cells and bundle sheath cells at several subfigures. (A) Swertia obtusa, "ordinary cell" ОС-a: plasmodesmal fields and protuberances of the cell wall are absent; plastids are leucoplasts. (B) Vinca minor, "ordinary cell" OC-b: neither protuberances of the cell wall nor plasmodesmal fields are present; plastids are well-developed chloroplasts. (C) Onosma gmelinii, "transfer cell" TC-a: multiple protuberances of the cell wall are present while no plasmodesmal fields are formed; plastids are well-developed chloroplasts. (D) Dracocephalumperegrinum, "transfer cell" TC-b with leucoplasts (not shown) in the abaxial sieve element-companion cell complex (two other sieve element-companion cell complexes in this minorvein contain intermediary cells). (E) Gentiana aquatica, "transfer cell" TC-c: multiple protuberances of the cell wall of unusual flange morphology are present, plasmodesmal fields are absent, plastids are leucoplasts. (F) Hamelia patens, companion cell presumably (as no data on RFO synthesis are available) of "intermediary cell" type with large plasmodemal fields, leucoplasts and a number of fragmented vacuoles; the detail in F1 shows a plasmodesmal field with branched plasmodesmata, x 15,000. (G) Catesbaea spinosa, companion cell ICL with large plasmodemal fields; plastids are leucoplasts containing starch; the detail in G1 below shows a plasmodesmal field with branched plasmodesmata, x 15,000. (H) Amsonia tabernaemontana, companion cell CC-b with well-developed plasmodesmal fields (the details in H1 and H2 below, x 15,000) and well-developed chloroplasts; no cell wall protuberances are formed, and one (shown) or a few (not shown) relatively large vacuoles are present. (I) Asarina barclaiana "modified intermediary cells" MIC-a containing small plasmodesmal fields and small cell wall ingrowths. (J) Rhinanthus minor, companion cell MIC-b with well-developed plasmodesmal fields, cell wall ingrowths, and leucoplasts; detail in J1 shows fragment of the cell containing both cell wall protuberances and a plasmodesmal field with magnification x 10,000. Magnification: x8000 if not otherwise indicated. www.frontiersin.org August 2013 | Volume 4 | Article 312 | 5