MECHANICAL-PROPERTIES OF THE LACUNAR GAS IN EGERIA-DENSA PLANCH SHOOTS

The effects of varying internal and external pressures on the lacunar gas system were examined for the submerged macrophyte Egeria densa Pla nch. Application of 30 kPa to cut shoots caused gas flow in accordance with the Poiseuille equation, once the meniscus formed by surface ten sion and hydrostatic pressure was broken. Surface tension averaged 1.8 5 kPa for the internodal canals and 41.10 kPa for the pores of the nod al diaphragms. According to calculations based on hydrostatic pressure effects, broken shoots will not release gas in water deeper than 2.4 m in the light, and water could infiltrate internodal canals at 2.9 m and nodal pores at 6.8 m. In the dark, when internal suction assists i nfiltration, the meniscus would break and the internodal canals flood at the surface, and at 4.4 m depth for the diaphragm pores. Yet the no dal diaphragms were remarkably resistant to flooding. Water applied at 50 kPa failed to penetrate them, owing to their high resistance to wa ter flow (130 000 MPa s(-1) m(-3), cf. 540 MPa s(-1) m(-3) for the int ernodal canals) and hydrophobic lining at the pores. Infiltration thro ugh broken surfaces is therefore unlikely to cause extensive natural i nfiltration in Egeria; this probably results from condensation from th e surrounding cells. External pressures slightly compressed the lacuna e; the lacunar system lost 2.3% of its initial volume at 50 kPa applie d pressure. The elastic modulus of the shoots was only 1845 kPa, sugge sting that the lacunae are well protected from external pressures by t he turgor pressure and wall resistance of the surrounding tissues. The lacunar system did not expand when it was internally pressurised, so lacunar pressurisation does not add buoyancy or mechanical strength in this species.