The Control of Cell Proliferation within Root Meristems
It is useful for some purposes to consider roots as having a subdistal meristem extruding cells towards the base to lengthen the root and towards the tip to form the root cap. No meristem, however, is as simple as this. A complex spatial distribution of rates of cell growth and cell division is necessary because the cells are generated in two opposing directions and are smallest near the boundary between the cap and the rest of the root. In the meristem cells are arranged differently in different species and so the pattern of rates of cell growth and division differs from species to species. A morphologist can in fact predict what the relative rates of growth and mitosis in different regions of the meristem will be by analysing the arrangement of cells. In the roots of Zea, for example, it was possible to predict from the cell pattern that there is a quiescent centre to the meristem where rates of cell growth and division are negligible compared with those of the surrounding cells4. This prediction has been confirmed by labelling dividing cells and by direct measurements of rates of mitosis. In other species with different arrangements of cells there is also a quiescent centre to the meristem. Its shape may also be predicted from the cell pattern, but, in some species, more extensive knowledge of how meristems operate is needed before its existence can be deduced with certainty. The “open” type of meristem such as Vicia or Pisum possess has a quiescent centre which is shaped like a disc or a concavo-convex lens. Its distal surface may, in some species, become active for brief periods in giving rise to new sets of cap initials during certain stages of development. In spite of this its mean rate of mitosis is still about one tenth of that of the surrounding cells and the distal margin of the quiescent centre has at most times a visibly distinct type of cell wall separating it from the cap initials10.
KeywordsMaize Depression Carbohydrate Thymidine Dine
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