Anatomical Changes Which Occur in Cuttings of Agathis australis (D. Don) Lindl 2. The Initiation of Root Primordia and Early Root Development

Cuttings of Agathis australis undergo complex anatomical changesin the sub-base and base. These changes include wound responsesin addition to the processes leading to adventitious root production.Although the root pnmordia form in the mid cortex the firstevents are associated with divisions in the interfasicular regiona few millimetres above the base of the Cutting. This is followedby differentiation into tracheids and phloem which then areoutwards and downwards into the mid cortex. When the inducedvascular strand is only a few cells wide, conditions at theadvancing front are most favourable for primordium formation.If sheets of vascular tissue occur, there is neither the spacenor the focal point for primordia to initiate. In cuttings fromolder material there are abundant resin canals, sclerenchymaand branch traces. These may reduce the amount of parenchymatissue to such a low level that potential primordial sites areno longer present and root formation is prevented. Organization is not observed until over 1500 cells are presentand at about this stage the beginning of organized cell arrangementcan be seen at the site of the apex of the primordium. Untilthis time the progress towards a primordium could not be saidto be ‘determined’. Although the lag phase before any morphological or anatomicalchanges are observed is variable in duration, the time takenfor the period of tracheid development and then for primordiumorganization and outgrowth is fairly constant, taking about2 weeks for each of the two phases. Evidence suggests that thevariation between species is probably in the duration of thelag phase and in the precise site of origin and pattern of theearly events. Once the primordium has formed the events leadingto root formation are probably similar for most species bothfor adventitious and lateral roots. Agathis australis (D. Don) Lindl, kauri, cuttings, wound responses, vascular connections, root primordia, root anatomy