Pattern in root meristem development in angiosperms

Abstract

Roots of angiosperms differ in the behaviour of their apical meristems, and this diversity has been studied through the patterns of differentiation in a range of families. Except for three families of the Nymphaeales, all dicotyledons derive the epidermis wholly or partially with the root cap. In closed meristems the inner cell layer of the cap complex forms the epidermis; in open meristems, where cortex and cap are intermittently of common origin, linkages between the epidermis and the cap predominate. Roots of most monocotyledons derive the epidermis as the outer layer of the cortex. Nymphaeaceae, Cabombaceae and Barclayaceae resemble monocotyledons in this respect, and this applies even in the species with open meristems, in which a cleft forms between the cap and the rest of the apex. This feature fits with the close relationship between the Nymphaeales and monocotyledons revealed by other data. But none of the other Magnoliidae with close links to Liliidae has this type of epidermis. The Nymphaeales are also almost unique among dicotyledons in having trichoblasts in a vertical pattern. This results from transverse divisions of epidermal cells giving two conspicuously different daughters. The smaller and most densely staining one later bears a root hair. This is the way trichoblasts develop in many monocotyledons. The only other dicotyledons found with trichoblasts in vertical patterns are in the Piperaceae, though their closed meristems derive the epidermis in the normal dicotyledon pattern. All other dicotyledons that bear trichoblasts develop them in radial patterns probably related to the radial disposition of cells across the cortex. This pattern is more widespread than was previously known, though it can occur only in roots with closed meristems. In plants of both subclasses with vertical patterns of trichoblasts, it is usually the proximal daughter of the initiating mitosis which becomes the trichoblast. But it is the distal daughter in Juncaceae, Restionaceae, Cyperaceae and Poaceae. Vertical patterns differ further as to whether the atrichoblast daughters divide after initiation, thus changing the alternation of trichoblasts and atrichoblasts along a file of epidermal cells. Monocotyledons and the Nymphaeaceae can produce trichoblasts in open or closed meristems. In some embryos and root primordia the epidermis becomes discrete from cortex and cap and remains so in the meristems. This has now been found in six aquatic genera of the Liliidae, but reports of it occurring elsewhere are probably due to misinterpretation of sections. Discrete epidermises differentiate into trichoblasts and atrichoblasts in Hydrocharitaceae, but not in Araceae or Lemnaceae.