Origin of Epidermis and Development of Root Primordia in Pistia, Hydrocharis and Eichhornia

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Abstract
All three floating plants have roots bearing laterals derived from both pericycle and endodermis. In Pistia and Eichhornia laterals arise within the meristem of the mother root; in Hydrocharis they arise from mature tissue. In Pistia and Hydrocharis the epidermis becomes anatomically discrete between cortex and cap: in Pistia it is derived from the endodermis of the mother root, in Hydrocharis from the pericycle. The epidermis is not discrete in Eichhornia and is derived from the pericycle of the mother root with the cortex. Stathmokinetic data were used to construct timetables of development which show how the differences arise. In Pistia the first periclinal division of the endodermis-derived tissue individualizes the epidermis and occurs early, before a quiescent centre forms. In Hydrocharis the epidermis also becomes discrete before the pole of the meristem becomes quiescent, but it does so by a periclinal division of the pericycle-derived tissue. In Eichhornia periclinal divisions occur in the outermost layer of the pericycle-derived tissue long after quiescence has set in at the pole and after the fourth periclinal division in the endodermis derived cap. Its epidermis therefore never becomes anatomically discrete though it becomes functionally discrete because its polar cells stop dividing as in the other plants. The involvement of the endodermis of mother roots in the formation of laterals is discussed in relation to the state of differentiation at sites of primordium formation, discreteness of the epidermis and subsequent fate of primordia.