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A Lindenmayer

Publications and source records attributed to A Lindenmayer.

9 recordsLinked to original sources
PubMed1977

A diffusion model for mesoderm induction in amphibian embryos.

In this paper we try to answer the question whether diffusion is a possible mechanism to explain mesoderm induction in Amphibians. First the embryological data are discussed and a hypothesis for mesoderm formation is set forth. The blastula being essentially a hollow sphere, we assume that the induction mechanism in an embryo at the blastula stage can be simulated by diffusion-reaction processes on spherical surfaces. A model is constructed for the simple case when the source is held constant with respect to time, the decay proportional to the concentration and the diffusion coefficient a constant. From simulation we find a (best) value for the decay constant to be 6 x 10(-5)/sec and for the diffusion constant to be 0.24 x 10(-6) cm2/sec. The relation between the parameters is derived from an analytic solution for the diffusion process on a spherical surface with a continuously producing point source and the concentration proportional to the decay. The form and regulative properties of the steady concentration gradient are discussed.

Amphibians↗
PubMed1977

Diffusion mechanism for phyllotaxis: theoretical physico-chemical and computer study.

The mechanism for leaf position determination by the diffusion of an inhibitor has been studied in relation to the geometry of leaf positions. A computer model has been constructed for the inhibitor-diffusion process on a cellular cylindrical surface. The behavior of the model has been analyzed mathematically. The main results are: (a) that our model generates most of the phyllotactic patterns observed in nature; and (b) that restraints have been found for permissible values of diffusion rates and decay rates of the hypothetical inhibitor.

Journal Article↗
PubMed1967

Selective effects of purine and pyrimidine analogues and of respiratory inhibitors on perithecial development and branching in sordaria.

The initiation of perithecia in the homothallic ascomycete Sordaria fimicola was completely suppressed, without seriously inhibiting vegetative growth, by growing the fungus on an agar medium containing one of the following additions: 1) 1 mum 5-fluorouracil, 2) 10 to 100 mum 6-azauracil, 8-azaguanine or 8-azaadenine, 3) 50 to 500 mum cyanide or azide, 4) 5% (w/v) casein hydrolysate. In contrast to the selective activity of the analogues of 3 RNA bases, whose inhibition could be reversed by the appropriate normal bases only, none of the analogues of thymine were active, neither were the thio-derivatives of RNA bases. Other inhibitors of RNA and protein synthesis, like actinomycin D, puromycin and cycloheximide, were also without selective activity, although the last of these inhibited perithecial maturation at 0.1 mum concentration but not initiation. Amino acid analogues were inactive, as were the metabolic inhibitors thiourea, 2,4-dinitrophenol and fluoride. The compounds which inhibited the formation of perithecia also lowered the branching frequency of leading hyphae, but not their linear growth rates. Consequently, the branch densities were diminished in their presence. Hypotheses to account for these findings are discussed in terms of inhibition of growth in general, of the synthesis of some specific messenger RNAs, and of RNA-mediated transport across membranes, the last of which seeming the most fruitful for further work.

Journal Article↗