The dynamics of stolon elongation in the hydroid, <i>Campanularia flexuosa</i>

Abstract

Abstract Stolon elongation in Campanularia has been shown to occur in cycles, each consisting of the following phases: (a) extended and resting, (b) retraction, (c) retracted and resting, (d) re‐extension to the length in phase (a), (e) resting, and (f) extension. Three features of this cycle — cycle duration, magnitude of extension and magnitude of retraction — have been evaluated to determine their inherent constancy and their modification by environmental and other factors. In the strain studied, cycle duration was a highly reproducible 6.15 ± 0.05 minutes at 20° C, consistent both from cycle to cycle in a given stolon, and among different stolons. This time is most markedly affected by temperature but is also genetically regulated within moderate limits and is affected to a small degree by the organic content of the sea water. Extension per cycle averages 20 μ (about 5.0 mm daily) in most older stolons, though individual cycles vary randomly from this mean by ± 20%. This average varies considerably among different genetic stocks, is less in newly‐initiated than in older, establised, stolons and is markedly reduced at temperatures exceeding 22° C. It is remarkably resistent in established stolons to all but severe starvation. The magnitude of retraction is highly variable (1 to 28 μ) from cycle to cycle, though this variation occurs in a predictable fashion. Evidence is presented that the extent of retraction is determined by the degree of internal hydroplasmic pressure behind the tip. Isolated stolon tips 300–500 μ long (diameter about 200 μ) display all essential features of the normal growth cycle. Such rhythmic activity is therefore intrinsic to the cells at or within 0.3 mm of the tip. Effects exerted by more distant regions may modify but do not basically change this pattern. Synchronized with this elongation cycle is a cycle of thinning and thickening of the epidermal layer at the stolon tip. Thinning is interpreted as a passive response to the forward thrust of the gastrodermis.