Roots were examined by transmission (TEM) and scanning (SEM) electron microscopy. For the light microscopy observations, roots and cotyledons of sterile plants were cut with a sharp blade, stained with 1% aqueous solution of toluidine blue, mounted on glass slides and observed microscopically. For the SEM, sterile seedlings were removed from the tubes using a large forceps and excess agar was carefully removed using a scalpel blade. All samples were placed on the surface of a copper specimen holder (16 29 1.5 mm). A small amount of the cryo-adhesive ‘Tissue Tek’ was applied to the mounting surface of the specimen holder prior to placement of the samples. Holders containing specimen materials were rapidly placed on the surface of a square brass metal tube pre-cooled in liquid nitrogen ( 196 C) within a styrofoam box. This process of contact freeze immobilization was performed on all samples used in the experiment. Numbered, frozen sample holders were placed into the same square brass tubing for storage in a liquid nitrogen storage dewar until removal for observation. Samples were transferred into a modified specimen carrier and transferred to an Oxford CT1500 HF cryo preparation system attached to a Hitachi S-4100 scanning electron microscope. Here the sample temperature was raised to 90 C for 10 min to etch surface water from the sample surface. The sample was then cooled to below 120 C and coated with 5– 10 nm of platinum metal, using a magnetron sputter coater, to render the surface electrically conductive and yield more secondary electrons when scanned with the electron beam during observation in the SEM. Samples were transferred to the cold stage in the SEM at 170 C and observed with an electron beam accelerating voltage of 2 kV. Micrographs were recorded on Polaroid Type 55P/ N film. Some samples were returned to the cryopreparation system after observation and fractured at 170 C, (in vacuo), using a cold scalpel blade ( 196 C), to visualize internal interactions of fungal hyphae with cacao cells. For the TEM observations, tissues from cacao plants inoculated as described above and control plants were fixed for 2 h at room temperature by immersion in 3% glutaraldehyde–0.05 M sodium cacodylate buffer, pH 7.0. They were then placed into a refrigerator at 4 C overnight. This was followed by washing 6 times over 1 h with the above-stated buffer, post fixed in 2% buffered osmium tetroxide for 2 h, dehyd rated in 100% ethan ol and infilt rated with Spurrs low-vi scosity embedding resin . Thin (10 nm) gold sections of the tissue wer e cut on a Riech ert/AO Ult racut microtome with a Diato me diamond knife and mo unted onto 200 mesh Ni grids. They were stain ed with 4% uranyl acetate and 3% lead citrate and viewed in an H-70 00 Hitach I TEM at 75 kV. Micrographs were recorded on a digit al camera.