Senescence or the lack of it in modular organisms

There is also often no programed senescence of whole modular organisms - they appear to have perpetual somatic youth. Even in trees that accumulate their dead stem tissues, or gorgonian corals that accumulate old calcified branches, death often results from becoming too big or succumbing to disease rather than from programed senescence. This is illustrated for three types of coral in modules within modules

60 50 40

o 30



Acropora U Porites Pocillopora

10-50 Colony area (cm2)

Figure 4.2 The mortality rate declines steadily with colony size (and hence, broadly, age) in three coral taxa from the reef crest at Heron Island, Great Barrier Reef (sample sizes are given above each bar). (After Hughes & Connell, 1987; Hughes et al., 1992.)

the Great Barrier Reef in Figure 4.2. Annual mortality declined sharply with increasing colony size (and hence, broadly, age) until, amongst the largest, oldest colonies, mortality was virtually zero, with no evidence of any increase in mortality at extreme old age (Hughes & Connell, 1987).

At the modular level, things are quite different. The annual death of the leaves on a deciduous tree is the most dramatic example of senescence - but roots, buds, flowers and the modules of modular animals all pass through phases of youth, middle age, senescence and death. The growth of the individual genet is the combined result of these processes. Figure 4.3 shows that the age structure of shoots of the sedge Carex arenaria is changed dramatically by the application of NPK fertilizer, even when the total number of shoots present is scarcely affected by the treatment. The fertilized plots became dominated by young shoots, as the older shoots that were common on control plots were forced into early death.

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