`Falling Apart' as a Lifestyle: The Rhizome Architecture and Population Growth of Salvinia Molesta

Abstract

(1) Colonies of Salvinia molesta ramets were sampled from populations at several different densities growing on waters high and low in nutrient concentration. A colony consisted of a group of ramets connected together by a single, intact, usually branched, piece of rhizome. (2) All rhizomes were planar and rhizome segments zigzagged between ramets enabling parent branches to be distinguished from side branches which were produced on alternate sides of successive ramets. (3) Ramets bore up to three ranks of side branches and the degree of branching was positively correlated with concentration of nutrients in the ambient water. Branching was not correlated with the degree of crowding. (4) Rhizomes from infertile water were tough and most contained no second or third rank side branches. Their oldest ramets were senescent and had a mean age of 12 ramet generations. These rhizomes appeared to fragment when their oldest segments degenerated. (5) Rhizomes from fertile water were brittle and most contained three ranks of side branches. Their oldest ramets were not senescent and had a mean age of 7 ramet generations. These rhizomes appeared to fragment when repeated branching forced segments to bend beyond the limits of their elasticity. (6) Fitness appeared to be maximized in infertile water by the retention of senescing ramets so that the nutrients they contained could be reallocated within colonies, and in fertile water by rapid proliferation of small colonies which could act as dispersal units. (7) Rhizome architecture could not be explained as an optimal solution to the problem of tesselating a plane to maximize light utilization. (8) A model of branching growth was found to give a good description of increase in the population of ramets under conditions optimal for branching. The population (yx) in generation (x) was given by