Structure and development of phloem in some species of Merremia Dennst. ex Endl. sensu lato (Convolvulaceae)

Abstract

Convolvulaceae is known for the occurrence of cambial variants but studies on phloem development are rare. In the present study, the structure and development of primary and secondary phloem in seven species of Merremia Dennst. ex Endl. s.l. (Convolvulaceae) were investigated. All the examined species were characterized by the presence of inter‐ and intraxylary phloem, except M. emarginata (Burm.f.) Hallier f. which showed an absence of interxylary phloem. The secondary xylem of all the species was characterized by the presence of phloem wedges and thin‐walled parenchyma strands/islands. Among them, all the species showed the formation of interxylary phloem from unlignified parenchyma cells located in the phloem wedges, except Distimake dissectus (Jacq.) A.R.Simões & Staples (syn: M. dissecta (Jacq.) Hallier f.) and M. emarginata. Its development was also observed in the parenchyma strands/pockets of M. hederacea (Jacq.) Hallier f. and Camonea vitifolia (Burm.f.) A.R.Simões & Staples (syn: M. vitifolia (Jacq.) Hallier f.), while in the rest of the species such pockets were present but no interxylary phloem was observed in them. In relatively thick stems, a quantifiable amount of intraxylary secondary phloem is accumulated by the activity of the intraxylary phloem (internal) cambium. The structure and dimensions of the sieve tube elements significantly vary in all three different types of phloem (i.e. regular external, inter‐ and intraxylary). Sieve elements may possess simple or compound sieve plates and may be wide or narrow in diameter within the same species. The presence of inter‐ and intraxylary phloem and phloem wedges is a characteristic feature of all the investigated species while the absence of interxylary phloem in M. emarginata may be associated with its habit as it spreads on the ground. More studies are needed on species that show the presence of wide and narrow sieve elements, which may contribute to understating the functional ecology and evolution of climbing habits. Further studies are recommended on thick stems of all the investigated species along with other species of Merremia to understand their taxonomic position because some of them were recently transferred to other genera.