UBC Theses and Dissertations
Morphogenesis in micrasterias Lacalli, Thurston Castle
The morphogenetic process responsible for elaboration of cell wall shape in dividing Micrasterias rotata cells is described and the associated ultrastructure discussed. The process is dissected experimentally into its contributing parts; laser microbeam studies reveal the growth sites responsible for morphogenesis, and autoradiography, the associated synthetic patterns. Tip growth similar to that occurring in fungal hypae, root hairs and pollen tubes makes a major contribution to cell shape. Tip growth, together with branching and broadening of the growths produced,accounts for the patterns of morphogenesis exhibited in M. rotata and M. radiata, two species having distinctive shape differences. The ability of dividing cells to impose a biradiate symmetry on the developing cell wall is discussed and is attributed to a template. Evidence is presented that for both the morphogenetic process and the formation of this template, appropriate spatial information and organization is embodied and maintained in the cell cortex and is not imposed by the cytoplasm or nucleus. Both nucleus and cytoplasm play only an indirect, supportive role; cytoplasmic polarity and specific structures such as microtubules are also not of primary importance. Template formation and tip growth are not reduced to their biochemical mechanisms, but are instead discussed in terms of known cell wall microstructure and physical properties. A discussion of the nature of scientific explanations is included to demonstrate that the explanations employed in this thesis are scientifically satisfying even though devoid of exact molecular mechanisms.
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