Biology teaching is founded on the seven essential characteristics of living organisms: nutrition, respiration, growth, movement, sensitivity (= irritability), reproduction, and excretion. In botanical texts the last of the list is widely ignored, for the term excretion does not appear in the index to the standard volumes on plant physiology. It is usually taught that there are in plants no organs or systems evolved for excretion. There are some phenomena which have been linked with the process, including the deposition of material in bark, the laying down of oxalate crystals in cystoliths (Brocklehurst and Ward, 1977), even the release to the atmosphere of gaseous CO2 at night.
In primitive plants we can discern the origin of a thickened cell wall in terms of a metabolic sink for the products of photosynthesis (Ford, 1976). Green plants must photosynthesize when exposed to light, and the formation of the polymer cellulose would be a sensible way of routeing excess levels of photosynthetic energy. Clearly, since it is insoluble, laying down deposits beyond the cells vital constituents (i.e. as cell wall) would make evolutionary sense. The formation of tracheary elements thickened with lignin may be a further extension of this process, allowing the plant cell to site potentially burdensome metabolic products outside the cell body.
Aquatic plants such as the large phaeophyte algae, which can grow to a frond length of 200 metres or more, are able to rid themselves of metabolites through diffusion, in a similar manner to that by which they obtain their nutriment input; for these reasons conducting vascular tissues are not a feature of these types. But a plant that evolves to live on land can utilize the evolution of a thick cell wall as a means of gaining rigidity. The osmotic potential of a plant growing in the soil can be harnessed to provide turgor with which to gain structural resilience, whilst the tracheary components can form the basis of a vascular bundle system.
However, there is in this concept no provision for excretory organs. I now believe that we should examine the cyclical phenomena in plants, for in them we may discern an excretory mechanism that has not been previously recognized.
Objections have been raised to the need for an excretory system in plants because of their autotrophic nature, that is, they produce metabolites through photosynthesis. Though a contrast seems to exist here with animal metabolic behaviour, the dark phase of plant metabolism features a form of metabolism at night which has more in common with that of animals than it has in disparity, and in this catabolic phase we would also expect to find some evidence of an excretory process.
The turnover diagrams of Woolhouse and Jenkins (1983) exemplify the profound deficiencies in contemporary understanding. They show clearly how degradation products of protein cycles give rise to wastes that, in their treatment, are exported to other parts of the plant. Their summary of the hydrolytic breakdown of protein suggests that the eventual fate is through the phloem transport system mediating export from the leaf (Woolhouse and Jenkins (1983), fig. 15.7). Yet we only have such mechanisms of the passing out of excess water in guttation, or into intercellular spaces (as in Cucurbita) to offer in response (Strasburger et al., 1912); a single excretory mechanism that would apply to all higher plants remains elusive.
It is difficult to designate a leaf as senescent when the plant which bore it has many years to live. We require a term to connote the change of colour which is the hallmark of this process. What the leaf exhibits is metachromism, that is a change in colour. The metachromatic leaf is undergoing a precisely coordinated metabolic sequence that removes by translocation substances that are of value to the plant body, whilst consigning specified compounds to be shed. Thus I believe we should view the anthocyanins and tannins, along with the red and yellow leaf pigments so typical of autumn, as excretory products. The oxalates which develop in cystoliths (supra) and which increase during the metachromatic phase of maturation in land-borne species, are grouped together under the same category. Little is yet known of the enzyme systems that are involved in translocation, nor of the mechanisms which underlie the autumnal changes in deciduous leaves (Bonner and Varner, 1976), but it is known that protein inhibitors can delay the process (Rhodes, 1980), which is a clear indication that there is an active phenomenon at work, rather than a degenerative senescence.
Thus I propose that we view the leaf not only as the photosynthetic centre in which key biochemical changes take place, but also as the organ into which waste materials are located prior to leaf fall. The shedding of the metachromatic leaf is then seen as the plants mechanism of excretion. Recycling of the wastes and their processing into a form that can be of value to a future generation of plants and animals is then undertaken by the soil microbiota, many species of which have doubtless specifically evolved to perform this function.