Abstract <p>Heteroplastic grafts of conifers are characterized by the phenomenon of delayed incompatibility, when scion trees that have been developing well and forming cones for decades begin to show signs of incompatibility and die. Studies of adult trees are generally limited to comparing the xylem increments of the scion and rootstock; the phloem component and fusion zone have hardly been studied; therefore, the purpose of this paper was to study the processes occurring in adult homo- and heteroplastic grafts of conifers in the contact zone of the scion and rootstock. We have studied the anatomical structure of the fusion zone of scions of <i>Pinus sibirica</i> Du Tour and <i>P. cembra</i> L., grafted onto the rootstocks of <i>P. sylvestris</i> L. To isolate phenomena directly related to incompatibility, we have divided the resulting data according to the width of the xylem increments and compared with the data for successful homoplastic grafts of Scots pine and Siberian pine and with data on non-grafted trees. In addition to the xylem study, special attention has been paid to the structure of the bark in general and the conducting phloem in particular, including albuminous cells and accumulation of callose on sieve fields. In addition to the mismatch of xylem growth of the scion and rootstock and reduction of growth rates in general, we have found new periderms formed closer to cambium in the fusion zone than those above and below the trunk at the first stages of the development of delayed incompatibility, as well as an increase in parenchymatization in the fusion zone and reduced cell specialization. The callose content in the fusion zone was higher in the studied successful grafts than in the nongrafted trees; the reduction in callose reserves in the fusion zone, in particular, on the rootstock side, may indicate an impaired transport of assimilates and be a symptom of incompatibility.</p>