The role of hydroplasmic pressure in stolonic growth movements in the hydroid, <i>Bougainvillia</i>

Abstract

Abstract The effects of intrastolonic pressure on growth activity of the stolon have been ascertained by simultaneously observing the direction of hydroplasmic flow and movements of the stolon tip at close time intervals over many consecutive growth cycles. During each growth cycle (4.4–5.8 minutes long) the tip passes through three distinct stages of inherent retractile potential and associated responsiveness to hydroplasmic pressure. (1) Beginning abruptly at the crest, it displays a strong retractile propensity: low pressure (from proximal flow) elicits a retraction of up to 5.5 μ per 0.2 minute, until the onset of distal flow (increasing pressure) causes it to immediately re‐extend. Thus the depth of retraction is directly determined by the degree and duration of low pressure following the crest. (2) After 1.8–2.0 minutes this retractile tendency becomes suddenly diminished. The tip, if still retracted, now re‐extends regardless of the intrastolonic pressure, and proximal flow during the middle third of the cycle causes only a delayed and shallow retraction to define an intermediate peak. (3) During the final third of the cycle the tip is nonresponsive to hydroplasmic pressure: even under circumstances of lowest pressure, it extends forward to a new crest. It is suggested that this varying retractile capacity may be associated with the pattern of changing thickness of the epidermal layer at the stolon tip. Certain features of the hydroplasmic flow and stolon growth cycles appear to be interrelated, most notably the consistent cessation of proximal flow in relation to a crest or trough. Thus some degree of control seems likely between the stolon tip and the contractile zone responsible for flow.