SUMMARYAerial sclerotia originated from intercalary cells of aerial hyphae. Initially only a single cell was involved. Repeated branching from the initiation point formed the sclerotium initial which was a more or less spherical mass of undifferentiated, radially arranged cells. Accumulation of glycogen in cells towards the centre of the initial marked the commencement of maturation. Cells of the central (medullary) region went through a differentiation process which involved first a heavy accumulation of glycogen; the glycogen was then mobilized, its reduction in concentration being exactly correlated with the formation of a thick, hyaline, secondary wall. As the medulla developed, differentiation of localized areas of cells just within the margin of the initial occurred to form the protective rind layer. Wall thickening in rind cells was not preceded or accompanied by glycogen accumulation. Strains which failed to produce sclerotia behaved as stable variants, sclerotium non-production segregating as a single gene. Four such genes were recognized and some linkage information obtained.

SUMMARYSclerotia were formed in both aerial and submerged parts of the mycelium. In addition a layer of cells with pigmented thick walls (called brown matting) which differentiated at the air/agar interface was interpreted as an aspect of sclerotial behaviour since it was regularly formed by strains which produced submerged sclerotia and was composed of cells of similar structure to those of the outermost layer of the submerged sclerotium. Apart from producing sclerotia and oidiospores the cells of the aerial mycelium remained undifferentiated. In contrast, cells of the submerged mycelium, though initially indistinguishable from those of aerial hyphae, became individually differentiated within about 5 days of growth producing two further novel cell types; inflated cells containing glycogen, and hyaline thick-walled cells. Submerged sclerotia were pale brown in colour, irregularly shaped and about 0.5-1.0 mm in diameter. The only structure which differentiated the submerged sclerotium from the submerged mycelium was the outer rind, a layer of cells with thick, pigmented walls. The central (medulla) region contained the same cell types in the same frequencies as the general submerged mycelium. In sharp contrast the aerial sclerotia were highly organized structures composed of distinct and compact tissues. Mature aerial sclerotia were dark brown to black spheroidal structures up to 0.5 mm in diameter. An outer layer of dead and moribund hyphae surrounded the main body of the sclerotium which was bilayered with an outer rind and inner medulla. The rind was multilayered and consisted of small cells with thick pigmented walls; intercellular spaces were cuticularized. The medulla was a closely packed tissue composed predominantly of hyaline thick-walled cells of the same type as were encountered in the submerged mycelium.

Mycoplasma-like organisms (MLO) associated with lethal yellowing disease of coconut palms exhibited a range of different morphologies even within the same sieve element. The morphologies were revealed by graphic reconstruction from ribbons of serial ultrathin sections containing 45 sections. Five different morphotypes were recognized amongst the 120 organisms reconstructed. MLO could be saccate, erythrocyte-like, cylindrical, moniliform or filiform. The different morphotypes showed differences in linear dimensions apart from their particular morphological characteristics. Saccate MLO were generally the smallest, the maximum length being 1 pm, whilst the filiform organisms could be more than 16 ,um long. The morphologies of the MLO were analogous to those now recognized as usual for Mycoplasma and Acholeplasma species in vitro during the later stages of development. Comparison of the individual profile shapes used in the reconstructions with those of MLO from other diseased plants suggest that many previous interpretations of MLO morphology may have been too simple. With the insight provided by the understanding of the relationship between profile shape and three-dimensional form, the ultrastructure of individual profiles is shown to be an important indicator of form. I N T R O D U C T I O NObservations of intact mycoplasmas taken from culture show that although they occur most frequently as simple spheres or filamentous bodies they are capable of becoming freely branched or even giving rise to mycelial complexes (Razin, 1978;Tully, 1978). Other modifications of form, such as rings and beaded chains, have also been described. The elucidation of morphology in vitro began nearly 70 years ago but the exceptionally detailed light microscopic studies of Turner (1 935) provide some of the best early illustrations. More recently, the use of similar or more sophisticated techniques has generally confirmed these early studies despite some controversy over the manner of reproduction and the significance of the numerous biological and physical factors which are known to affect form (

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f Present address: Plant Pathology Department, Rothamsted Experimental Station, Harpenden, Herts. AL5 254. 0022-1287/81/oooO-9560 $02.00 0 1981 SGM