Chapter 1: The Characeae Plant

15
1.4.1 Systematics and the concept of species
The superficial resemblance of characeae to flowering plants has led, in the past, to confusion
about their place in botanical systematics (Fritsch 1948). Early botanists discussed whether
they were the simplest of flowering plants or the most complicated of algae. Recent
phylogenetic studies (Karol et al. 2000) have shown that they are the closest living relatives
of the ancestors of land plants. They have evolved an analogous morphology to aquatic
angiosperms, in the same environment. The common and ancient ancestry of characeae and
land plants means that many of their cellular processes and mechanisms are the same as those
of land plants, a fact that makes the study of characeae physiology directly relevant to the
physiology of land plants, including plants and processes of economic interest.
Characeae have been recognised as fossils in sediments dating as far back as the Silurian
(Grambast 1974) and have been through various changes in diversity and abundance over the
millennia. Living characeae were among the species first described in Linnaeus’ Species
Plantarum in which he sought to develop a consistent taxonomy. Characeae were not
recognised as being substantially different from other plants, and most workers at that time
saw them as a watery Equisetum or horsetail. It was later that their unique differences were
recognised, and they were set apart in Division Charophyta.
Although a number of characeae species are regularly used in physiological studies,
physiologists seeking to have their plant identified to species can obtain erroneous or
conflicting answers. There are a number of reasons for this. Firstly characeae morphology is
affected by the environment for growth, and there is variation in plant size and appearance
depending on the light availability and water chemistry of the habitat. Secondly, although
characeae have been well known for a long time, the taxonomy of characeae is undergoing a
revolution, based on recent advances in the understanding of variation and speciation in the
group and genetic data about relationships among species and species groups.
The taxon level called ‘species’ is not the same in all groups of organisms. In birds and
mammals relatively small genetic differences are present between ‘species’. In plants there
are often larger genetic differences. In algae the genetic distances among ‘species’ can be
larger still. The detection of characteristics that allow us to reliably discriminate among
different taxa are often based on historical precedent or subjective assessment (i.e. what the
first taxonomist used) rather than objectivity or experimentation (Proctor 1980), and linking