The levels that compose biological hierarchies each have their own energetic, spatial and temporal structure. Indeed, it is the discontinuity in energy relationships between levels, as well as the similarity of sub-systems that support them, that permits levels to be defined. In this paper, the temporal structure of living hierarchies, in particular that pertaining to Human society, is examined. Consideration is given to the period defining the lifespan of entities at each level and to a periodic event considered fundamental to (...) the maintenance of that level. The ratio between the duration of these two periods is found to be approximately 2.5 × 104. A similar relationship is found when lower, non-living levels of molecules and atoms are considered. This suggests that there is a constant factor of amplification between analogous periodic events at successive levels of the Human hierarchy. (shrink)

Polarity is an inherent feature of almost all prokaryotic and eukaryotic cells. In most eukaryotic cells, growth polarity is due to the assembly of actin‐based growing domains at particular locations on the cell periphery. A contrasting scenario is that growth polarity results from the establishment of non‐growing domains, which are actively maintained at opposite end‐poles of the cell. This latter mode of growth is common in rod‐shaped bacteria and, surprisingly, also in the majority of plant cells, which elongate along the (...) apical–basal axes of plant organs. The available data indicate that the non‐growing end‐pole domains of plant cells are sites of intense endocytosis and recycling. These actin‐enriched end‐poles serve also as signaling platforms, allowing bidirectional exchange of diverse signals along the supracellular domains of longitudinal cell files. It is proposed that these actively remodeled end‐poles of elongating plant cells remotely resemble neuronal synapses. BioEssays 25:569–576, 2003. © 2003 Wiley Periodicals, Inc. (shrink)

There is a certain measure of perplexity concerning the significance of endopolyploidy. It seems that this results from a narrow frame of reference from which investigators view the phenomenon; that is, a predilection for emphasizing the specialized functional aspect of endopolyploidy as it operates in species at the present time overrides any consideration of the rôle that this state may play in the life of a species in its encounter with the forces of natural selection either in the past or (...) in the future.There does not seem to be any obvious relationship between the degree of endopolyploidy that a species can exhibit and either its basic DNA content or the structure of its nucleus. The significance of endopolyploidy may reside not so much in any specialized function that the condition can support, but rather in the properties that are consequent upon the endopolyploid condition itself and which are distinct from those that apply to diploid cells. Some of the properties of the endopolyploid state, and examples of their manifestation in plants and animals, are discussed. The conclusion is that these properties have a potential that opens possibilities for new paths of development and serves as a factor upon which natural selection can operate. (shrink)

The secondary vascular tissues (xylem and phloem) of woody plants originate from a vascular cambium and develop as radially oriented files of cells. The secondary phloem is composed of three or four cell types, which are organised into characteristic recurrent cellular sequences within the radial cell files of this tissue. There is a gradient of auxin (indole acetic acid) across both the cambium and the immediately postmitotic cells within the xylem and phloem domains, and it is believed that this morphogen, (...) probably in concert with other morphogenic factors, is closely associated with the determination and differentiation of the different cells types in each tissue. A hypothesis is developed that, in conjunction with the positional values conferred by the graded radial distribution of morphogen, cell divisions at particular positions within the cambium are sufficient to determine not only each of the phloem cell types but also their recurrent pattern of differentiation within each radial cell file. BioEssays 27: 533–541, 2005. © 2005 Wiley periodicals, Inc. (shrink)