IN synchronous cultures of bacteria, the rate of cell elongation 1 or of envelope synthesis2 seems to increase abruptly at a particular age. These and other findings suggest3-5 that bacterial envelope is formed at a constant rate, possibly by enzymes organised in zones, and that some event in the cell cycle leads to a discrete increase in the amount of enzymes or number of zones concerned. (The actual event involved has been variously identified with initiation of chromosome replication4, with termination1,5 and with the attainment of a critical cell length6.) Growth zones are known to occur in the filamentous fungi7. The rate at which these zones are able to add new envelope is limited, and since mass increases exponentially8, hyphal density eventually begins to rise; when it reaches a critical value, new zones are formed. Thus density (or indeed the concentration of any product synthesised exponentially9) could control the rate of surface growth, and we suggest that such is the case in bacteria.
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