Sloughing of Root Cap Cells

Root cap cells of Zen mays sloughed into water have been countedat short intervals and after 24 h. The results have been comparedwith rates of cell production calculated from the use of thestathmokinetic agent, colchicine, on the four active tiers ofthe cap meristem. More cells are produced by the cap meristem and sloughed bythe cap when roots are grown in low densities in water or whenthe water is changed at more frequent intervals. The range atbetween 250 and 50 roots per litre is from 3000 to 7000 cellsper root per day for seedling primary roots grown continuouslyin water at 23 °C for 24 h The results are discussed in relation to previous estimatesof cap cell proliferation and the possibility of periodic sloughingof cells and slime.     

The Fine Structure of the Quiescent Centre and Neighbouring Tissues3

The quiescent centre of the root meristem of Zea provides asource of undiffer-entiated, non-meristematic cells whose finestructure we have compared with that of the meristematic, non-differentiatedcells of the meristem and that of the differentiated, non-meristematiccells of the cap. We have shown an association between the developmentof the endoplasmic reticulum and the development of cell walls.Differentiation in the cap is accompanied by a decrease in thenumber of mitochondria and plastics per unit volume of cytoplasmand an increase in their number per cell. There is an even greaterincrease in the number of Golgi bodies per cell. The structureof the mitochondria becomes more elaborate in differentiatingcells especially in the cap and the Golgi increase their sizein meristematic and differentiating cells. These differencesare discussed in relation to some of the views about radiosensitivityand organization.     

Alteration by dinitrocresol of pathways for glucose oxidation in eggs of arbacia punctulata

1. The C¹⁴O₂ production by Arbacia eggs and embryos from glucose-1-C¹⁴, glucose-2-C¹⁴, and glucose-6-C¹⁴ has been measured without and with dinitrocresol in the incubation medium. In the absence of the dinitrocresol, the C¹⁴O₂ production from glucose-1-C¹⁴ is more rapid than from glucose-2-C¹⁴ and much more rapid than from glucose-6-C¹⁴; this, together with previous findings, indicates that glucose is utilized in Arbacia eggs predominantly via the TPN shunt rather than via the aldolase step of the glycolytic pathway. In the presence of the dinitrocresol, C¹⁴O₂ from glucose-6-C¹⁴ approaches that from glucose-1-C¹⁴, indicating that, in the presence of this reagent, glucose utilization is diverted from the shunt to the glycolytic pathway. 2. Incorporation of C¹⁴ from glucose labelled in the 1-, 2-, or 6- positions into other metabolic products of the eggs and embryos is also inhibited by dinitrocresol, particularly incorporation into the acid-insoluble fraction containing nucleoproteins.     

The Difference Between Open and Closed Meristems

An open and a closed root meristem have been compared by investigatingthe cell kinetics of small regions of the apices of Helianthusand Zea. The cells of the stelar pole are quiescent in both and thereis no exchange of cells between stele and cortex or stele andcap. The immediately distal cells in the closed meristem (Zea)are also quiescent and the few divisions that do occur can betransverse or longitudinal. In the open meristem (Helianthus)these cells are not quiescent, but they go out of cycle transiently,prolonging the potential cell-doubling time. Their divisionsare transverse. It is a consequence of these differences thatclosed meristems form root caps discrete from the cortex whereasopen meristems force instability in the boundary between theperipheral part of the cap and the cortex. Another consequencein roots with open meristems is a succession of columella complexestransversely displaced from each other by the state of fluxin the meristem during the non-cycling phase of the proximaltier of cells, those immediately distal to the stelar pole. The results are discussed in relation to the ontogenetic onsetof quiescence and the evidence for switches between open andclosed operation of meristems. meristem, root apex, Helianthus annuus, Zea mays L.