Topography of microsymbionts in apogeotropic roots of the cycads Cycas revoluta Thunb. and Encephalartos horridus (Jacq.) Lehm.

Morphological peculiarities of the apogeotropic (coralloid) roots of the cy cads Cycas revoluta and Encephalartos horridus and the distribution of micr osymbionts in them were studied. In the peridermal intercellular spaces and the external layer of the cortical parenchyma of the coralloid root apex, numerous bacterial cells of various morphological types were found (cocci a nd long and short rods). Localization of the dominant microsymbionts, cyano bacteria, was different in coralloid roots of different cycad species: in E . horridus, cyanobacteria occurred only in the intercellular space of the s lime ring, whereas, in C. revoluta, they also occurred inside the cells of cortical parenchyma. It is suggested that two different cyanobacterial spec ies serve as microsymbionts in C. revoluta. Peculiarities of the localizati on of microsymbionts in cycad coralloid roots correlated with the morpholog ical peculiarities of the latter and the amount of secondary metabolites sy nthesized in them, suggesting that the infection process and distribution o f microsymbionts in apogeotropic cycad roots is under the control of the pl ant host.