Excretion is a key function of leaves and petals of higher plants. A new study of phytoremediation (sometimes called bioremediation or even phytobioremediation) has stemmed from this new model (Brian J ford Falling Leaves and Plant Excretion A New Approach to the Bioremediation of Contaminated Soils). Plants must excrete (this is one of the cardinal properties of all living things) yet the term excretion is virtually absent from the index of standard works on plant physiology. Leaves are being revealed as having a third function.
First, they capture sunlight;
Second, they act as the agent of transpiration, drawing water up through a plant;
Third, they are the plant's organ of excretion. The leaf as excretophore has been the subject of a key programme of research.
When trees shed their autumn leaves not due because of the damage that the leaves would suffer in the winter but as part of an excretory mechanism. All living things must undergo excretion it is an essential property of all life and plants are no exception. As the leaves turn yellow the plant the seasonal plant relieves itself of accumulated wastes: the leaf is an excretophore, as well as an energy trap. Chlorophyll is removed levels of many toxic materials - like heavy metals - increase sharply as the yellowing proceeds. And metabolic rates rise, rather than fall, during this phase.
Metal and toxin tolerant vegetation not only extracts poisons from damaged soils, but potentially offer valuable metals for recycling.