Origin of the epidermis in root meristems

Abstract

A survey of root meristems shows that in general, closed meristems, those with separate regions generating outer and inner tissues, derive the epidermis with the cortex in monocotyledons and with the cap in dicotyledons. Open meristems, where the cells between the stelar pole and cap divide transiently producing an unstable boundary between the cap and the rest of the root, also show a distinction between the classes, monocotyledons displaying greatest affinity between epidermis and cortex, dicotyledons between epidermis and cap.Exceptions in monocotyledons occur where there is a discrete epidermal meristem, an anticlinally di\iding, monostromatic sheet of cells between cortex and cap. This condition has been reliably found only in a few Boating plants and it is associated with the root being derived from both pericycle and endadermis of the mother organ. This is uncomnKin as a permanent feature In angiospcrms though not confined to those with discrete epidermises. The mother tissue producing the epidermis depends on the species. In many other monocotyledons with closed meristems the epidermis is functionally discrete, for the branching of the epidermal cell files from the cortical files is seen within the quiescent centre, i,e, it occurred during the meristem's initiation.Exceptions in dicotyledons nccur in the Xymphaeales. In their closed meristems the epidermis is part nf the cortical complex and in their open meristems its affinity with the cortex is manifest by early separation of the cap creating a cylindrical cleft in some genera. Differentiation of epidermal trichoblasts is also by a mode more characteristic of monocotyledons. Nelumbo roots, previously described as lacking an epidermis, as in gymnosperms, have closed menstems of dicotyledonous type and epidermises vvithoui distinctive trichobiasts, supporting Nelumbo's transfer fronn the Nymphaeales.The survey has corrected many errors of observation and interpretation in the literature enabling better use to be made of the mode of origin of r(K)t epidermises in discussing e\'olution and the new reel as si fi cation of angiosperms.

Pattern in root meristem development in angiosperms

“…No dicot has been found to have a discrete epidermis in this survey, so Voronkina's (1974) report that this can occur in Barclaya longifolia as well as in Nuphar lutea is probably a misinterpretation of the sections, like that previously discussed (Clowes, 1994) concerning Nelumbo which was formerly placed in the Nymphaeales on superficial characters.…”

“…those that form the outer cell layer of the root with its root hairs) are those of the outer layer of the cortex and not the inner layer of the cap as might be expected in a dicot. Other members of the Nymphaeales show this more clearly, the two Nymphaeas in Table 1 because there is a cleft between the cortex and the cap in the open meristems, and Cabomba because it has a closed meristem as well as a similar cleft (Clowes, 1994).…”

“…Several plants, including Nymphaea and Nelumbo, were thought to have initially a single cotyledon that later developed two lobes (Lewis, 1904). Nelumbo can now be excluded from any close relationship with either Nymphaea or monocots on its pollen structure and other features including its quite different root meristems (Clowes, 1994).…”

“…The genera of the Nymphaeales previously examined all display the vertical pattern with trichoblasts alternating with their atrichoblast sister cells or their progeny along a vertical file (Clowes, 1994).…”

Cluster root development in Grevillea robusta (Proteaceae). I. Xylem, pericycle, cortex, and epidermis development in a determinate root

“…We have used both transverse and longitudinal serial sections in an attempt to build a true picture of events, as recommended by Clowes (1994). Cluster rootlets in Grevillea robusta are determinate (Purnell, 1960, Skene et al, 1996.…”