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Rates of mitosis in each of the four tiers of the cap meristem of primary roots of Zea mays L. were measured at uniform root length by a stathmokinetic method and compared with rates for other regions of the apices in seedlings growing at between 15 and 35 ⚬C. The highest elemental rate of cap cell production in the proximal tier and in the cap meristem as a whole occurs at 26 ⚬C and the proportion of cells supplied by the proximal tier rises from 48% at 15 ⚬C to 90% at 35 ⚬C. The number of cells in the whole cap and in its meristem decreases with increasing temperature. This is due to the general reduction in the width of root apices with age and growth in length being faster at the higher temperatures. The slimming of the apex is effected by a decline in the proportion of divisions that are longitudinal to the files of cells at the quiescent centre. This is enhanced by a slight but continuous increase in the rate of mitosis in the quiescent centre the higher the temperature (in contrast with the rest of the root whose maximal rate is at 28 ⚬C). The decline results in an increase in the elemental rate for longitudinal divisions in the cap initials from 0.02 to 0.17 cells per cell per day between 15 and 35 ⚬C and a consequent change in the cell pattern in the cap.
The New Phytologist © 1988 New Phytologist Trust