Fine Structure of Established Callus Cultures of Taraxacum officinale Weber

Abstract

The nodular, brown callus cultures of Taraxacum officinale grow slowly on a modified White's medium. A section of nodule reveals a meristematic layer bounded both internally and externally by parenchymatous tissue. No other types of tissue occur within the callus. The cells of the inner parenchyma are often compressed and senescing, whereas in the outer tissue localized de-differentiation apparently contributes to the development of new nodules during callus growth. Fine-structural observations of both meristematic and parenchymatous tissues show the normal complement of higher plant cell organelles except for the apparent absence of cytoplasmic microtubules. The rough endoplasmic reticulum cisternae are often aligned parallel to the cell wall or in whorls and may show pores, thus resembling annulate lamellae. Numerous lipid bodies, up to 7 μm wide, occur and these are sometimes invested by arrays of apparently membranous material. The mitochondria are frequently highly branched and often show a scalariform arrangement of cristae. The plastids show few internal membranes despite culture of the callus under continuous illumination. Lomasomes are very common in all cells and in the parenchyma tissue membranous wall bodies also occur. The latter bodies are much larger than lomasomes and consist of wall overgrowths in which vesicular, myelin-like or isolated membranous elements are enmeshed in fibrillar material. It is suggested that membranous wall bodies may originate from the amalgamation and subsequent proliferation of several adjacent lomasomes.