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Summary • To assess the influence of mechanical impedance on cell fluxes in the root cap, maize (Zea mays ) seedlings were grown in either loose or compacted sand with penetration resistances of 0.2 MPa and 3.8 MPa, respectively. Numbers of cap cells were estimated using image analysis, and cell doubling times using the colchicine technique. • There were 5930 cells in the caps in the compact and 6900 cells in the loose control after 24 h growth in sand. Cell production rates were 2010 cells d[sup -1] in compact and 1570 cells d[sup -1] in loose sand. • These numbers represent accumulations of 4960 and 3540 detached cells d[sup -1] around the cap periphery following the two types of treatment. The total number of detached cells was estimated as sufficient to completely cover the whole root cap in the compact sand, but only 11% of the root cap in the loose sand. • In conclusion, mechanical impedance slightly enhanced meristematic activities in the lateral region of the root cap. The release of extra border cells would aid root penetration into the compact sand. [ABSTRACT FROM AUTHOR]

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 chant4e in the cell pattern in the cap. [ABSTRACT FROM AUTHOR]