Introduction

Some relatively simple observations suggest that there must be movement of nutrients within the hyphae of mould mycelium from the growing margin to the centre of the colony. When such mycelium is growing on agar, the concentration of glucose under the colony decreases markedly as one moves from the margin towards the centre and increasingly moves towards zero with time (Robson et al., 1987). If the central portion of the colony is to function normally, there must be movement to it (translocation) of metabolic substrates from those regions where glucose (and other nutrients) are being absorbed.

In making the above statement, one is making the assumption that the central portion of the colony is functioning normally. It is a reasonable assumption in the experiments referred to but it should not be taken as axiomatic, particularly in cultures that have been growing for more than a few days. Then it is possible that autolysis may take place in the centre of the colony. Much of what is known about changes taking place in a colony on agar comes from a variety of sources and a range of fungi. There is little doubt that metabolic activity declines as the colony ages – all the various observations support this (Trinci & Thurston, 1976). There are also morphological changes, particularly increased vacuolation (Buller, 1933; Park & Robinson, 1967) and septal plugging (Trinci & Collinge, 1973). However, there is no integrated picture of what is happening over a realistic distance from the colony margin.