Lateral meristems, successive cambia and their products: a reinterpretation based on roots and stems of Nyctaginaceae

Anatomical studies of stems and roots with lateral growth from eight species of seven genera of Nyctaginaceae provide material for analysis of meristematic activity and histological products of that activity. Ideas about occurrence of meristems that achieve lateral growth in Nyctaginaceae are reviewed. The interpretation offered for the family differs from those of other workers, although the new interpretation is clearly implicit in Solereder's figure of secondary growth in Pisonia. A lateral meristem produces secondary cortex to the outside. To the inside, it produces conjunctive tissue (both parenchymatous and fibrous), true rays (except in Bougainvillea and Heamerliodendron) and a succession of vascular cambia. As each vascular cambium is produced, the lateral meristem outside of the vascular cambium tends to become quiescent, returning to activity when the vascular cambium internal to it has become less active. Quiescence of the lateral meristem at these points coordinates the amount of tissue produced to the inside in zones without vascular cambia with that produced in zones with cambia. Heimerliodendron is rayless; Bougainvillea has minimal differentiation between conjunctive tissue parenchyma and ray parenchyma. Vascular cambia do not produce rays in Nyctaginaceae (lateral meristems do), although vascular cambia produce vessels, axial parenchyma and sometimes fibres to the inside and indefinite amounts of secondary phloem to the outside (earlier increments of phloem are crushed). Conjunctive tissue is held to have both parenchymatous and fibrous components and is readily distinguished from products of vascular cambia. Nonbordered perforation plates characterize Nyctaginaceae, as they do many families of Caryophyllales. The anatomical plan of Heimerliodendron is markedly different from that of Pisonia s. s., and Pisonia sect. Prismatocarpus (which includes Heimerliodendron in some treatments) may merit generic recognition. The lateral meristems of all Nyctaginaceae studied are storied (stratified), as are products except for fibres, which undergo such extensive intrusive growth that a storied pattern is not achieved at maturity. The terms 'included phloem' and 'interxylary phloem' must be abandoned for descriptions of vascular tissue in families of the order Caryophyllales. (C) 2004 The Linnean Society of London.