General Plant taxonomy and the evolution of plant morphologies

2.2.2 General Plant taxonomy and the evolution of plant morphologies

It is important to recognize that the location of biologically active compounds within the plant depends on plant morphology—the division of the whole plant into organs such as roots, stems, and leaves—which in turn depends on plant taxonomy and the events that have occurred during plant evolution. Mosses, for instance, are primitive life forms that do not have true leaves (in which cells are differentiated into tissues specialized to perform different functions) but rather simple leaf-like structures consisting, essentially, of a sheet of general-purpose photosynthetic cells. Liverworts, the most primitive of all land plants (Sanderson et al., 2004), have no leaves and consist of a simple “thallus” or a flattened structure consisting of photo-synthetic tissue sandwiched between two layers of epidermal cells, with single-celled rhizoids acting in the same capacity as roots. Indeed, land plant evolution has been characterized by a series of increasingly sophisticated changes in morphology. This can be summarized by the general sequence of plant evolution laid out in the following paragraphs. Our aim in this section is not only to highlight the development of plant morphology through time, but also to provide a general framework that will clarify the taxonomic relationships between the major groups of plants, such as mosses, ferns, conifers, and flowering plants. It is essential to understand the relationships between groups of plants because, as we shall see, many of the relatively primitive plants are now being investigated for biologically active compounds and are yielding startling results.

First, it should be acknowledged that a range of organisms such as the fungi and blue-green “algae” are traditionally viewed as plants in many botany text, but actually belong to different lineages and thus have different morphologies and physiologies. Three main kingdoms of life are currently recognized: the eukaryotes, eubacteria, and the archaea (Madigan et al., 2003). Fungi are nonphotosynthetic species that form an ancient eukaryotic group and are classified together with plants and animals though they do not belong to any of these. Blue–green algae are not eukaryotes but eubacteria (specifically, cyanobacteria) that carry out photosynthesis in essentially the same manner as plants and thus share some characteristics, particularly with plant chloroplasts. However, cyanobacteria are prokaryotes (i.e., do not have membrane-bound genetic material, nor organelles), and many aspects of their biology are thus quite distinct from those of plants. Lichens are a symbiotic mixture of fungi with either algae (mainly green algae) or cyanobacteria and thus have a rather odd taxonomic status.

We have already defined plants as “a lineage of eukaryotic organisms exhibiting cell walls and chloroplasts.” Indeed, plants form a single lineage of eukaryotes that include, at their base, the marine and freshwater algae. The earliest known fossil alga, known as Grypania, had a morphology equivalent to that of a modern green alga (Chlorophyceae: Charales), and although other algal groups such as the red algae and the brown algae have persisted to the present day, it is only the green algae from which land plants evolved (Willis and McElwain, 2002).

The first plants to live on land were simple green algae that probably lived along humid riverbanks on thin soils formed by microbial activity. Around 477 million years ago (m.y.a.), this situation allowed the evolution of the first plants that were truly adapted to live on land: the liverworts (Marchantiopsida). These usually had (and still have) a flattened thallus with rhizoids on the undersurface and simple pores on the upper surface. This simple morphology reflects life at the interface between soil and air and is efficient for maximizing surface area-to-volume ratios for efficient gas exchange, water uptake, and for intercepting sunlight.

The first stem-like structures were photosynthetic but their main function was to hold spore-bearing structures aloft for spore dispersal and can be found in liverworts and their close relatives the hornworts (Anthocerotopsida). Structural support depended on a central core of hydrostatically pressurized cells encased in a rind of photosynthetic tissue. Simple stem-like structures are also a feature of the mosses (Bryopsida), which evolved around 45 million years after the liverworts. Mosses are able to capture rainwater and dissolved mineral nutrients, soaking them up among their “leaves,” and this accumulation of nutrients accelerated soil development and is thought to have fueled an explosive burst of plant evolution around 432 m.y.a. (Pierce et al., 2005).

This explosion resulted in the evolution of specialized lignified (strengthened) mechanical tissues and ultimately vascular bundles (in which lignified water- conducting tissues are associated with mechanical tissues) in the first true vascular plants (Tracheophytes), known as the Rhyniopsida. These were capable of the internal transport of water, nutrients, and the products of photosynthesis over much larger distances, and plants started to become large. None of these early vascular plants has survived until the present day and are only relevant to our discussion in terms of understanding how true stems came about and are structured to perform their role as supporting organs.

Differentiation into multicellular tissues allowed the production of long roots that anchored the plant more effectively and “foraged” for resources such as soil water and inorganic nutrients and, eventually, also acted as storage organs for carbohydrate reserves. True leaves, like true roots, are differentiated into specialized tissues, and probably evolved sometime between 390 and 354 m.y.a. (Willis and McElwain, 2002). Club mosses (Lycopodiaceae), Selaginella and Isoetes, are surviving examples of an ancient group of vascular plants that have microphyll leaves (i.e., leaves that have only a single strand of vascular tissue and are attached directly to the stem). Megaphyll leaves have many vascular strands and may be much broader and are characteristic of horsetails (Sphenopsids), ferns (Filicophyta), and seed plants (Spermatophyta). Around 330 m.y.a., ferns and early seed plants developed particularly broad, laminar leaves because the widespread vegetation had removed substantial quantities of CO2 from the

global atmosphere. This meant that plants needed more stomata (adjustable pores) over a greater surface area than was available on their photosynthetic stems, in order to assimilate enough CO2 to perform photosynthesis. Greater numbers of stomata also allowed greater evaporation and thus more efficient cooling, meaning that for the first time large photosynthetic structures exposed to direct sunlight did not dangerously overheat (Beerling et al., 2001).

Woodiness (secondary growth) evolved at around the same time because the evolution of large leaves resulted in intense competition for light, and height became a distinct advantage in many ecosystems. All plants possess the structural compound lignin, used to reinforce cell walls and thus tissues, and in woody plants, lignin is used to produce particularly sturdy vascular and structural tissues. However, in order to grow in height the stem must be stable, meaning that it must also have a mechanism for growing in girth, despite its solidity. The term secondary growth refers to this lateral expansion of woody stems, which is based on the laying down of fresh cells over the flanks of the stem by a cylinder of differentiating cells called the cambium, or lateral meristem, which lies just beneath the bark. This results in the formation of wood, which is a distinguishing characteristic of the early Eutracheophyta.

Also around 380 m.y.a., all sexually reproducing plants had at least one phase of the life cycle that depended on sperm cells swimming in the water films covering plants and soil, in order to fertilize the egg cells of neighbors. Thus, vegetation was restricted to humid environments, despite the fact that the plants themselves included large tree-like growth forms that formed extensive forests. The last link between plant life and humid habitats thus took place with the evolution of pollen (essentially sperm that is coated to resist drying, to travel on the wind) and, contemporaneously, seeds. Seeds are composed of an embryo protected by a tough seed coat and, typically, a store of reserves to fuel embryo growth during germination.

As forests invaded drier habitats, and thus covered most of the land surface, all of the basic ways of growing had evolved, leaving scope for only relatively minor variation around the theme of the general land plant body-plan. However, there was still room for the improvement of gymnosperm reproductive systems, and the evolution of flowers and fleshy fruit, probably around 217 m.y.a. (Smith et al., 2010), allowed early flowering trees to survive in forests already dominated by large tree life forms, where they languished for around 75 million years before becoming common.

The innovative and defining feature of flowering plants was not actually the flowers, but the fact that the egg was completely enclosed and protected within an ovary (it is from this that we get the name Angiosperm; angio meaning cup, representing the ovary, and sperm meaning seed). It is the ovary that develops, alongside the seeds, to form the surrounding fruit, which may be fleshy and attractive to animals that disperse the seeds and thus important for animal nutrition. Petals are simply modified leaves that advertise the presence of pollen: the first simple flowers offered no nectar reward for pollinators; it was the pollen itself that visiting beetles ate, some of which was accidentally transported from flower to flower in a kind of pollination known as mess-and-spoil due to the general defacement sustained by the plants. Another theory of flower evolution suggests that the volatile secondary metabolites produced to protect delicate regenerative organs against disease organisms were similar to many insect hormones, resulting in the use of gymnosperm reproductive organs as sites for insect mating, with pollen of lesser importance as an attractant (Harrewijn et al., 1995)—although the bright yellow color of pollen is undoubtedly attractive to many modern insect pollinators. Nectar evolved later as part of symbio- ses with particular insects such as bees and butterflies.

We have, so far, described the general sequence of events leading to the evolution of the different parts of plants: thalli and rhizoids, stems, roots, leaves, wood, pollen, seeds, and ultimately fruits and flowers. We have also seen that the evolution of these morphological features defines the taxonomy of the major plant groups. Thus, it should also be evident that not all plants will possess all of the organs that we may typically associate with plants and that the morphologically primitive taxa that still exist today offer a more limited range of growth forms and organs that can be exploited for medicinal purposes.

However, most plants possess photosynthetic tissues, and it is usually these that are exposed to attack and are thus relatively rich in secondary metabolites (Harborne, 1997). Among the vascular plants, buds and storage organs that must persist in hostile surroundings, such as tubers and rhizomes (underground stems), may also be particularly rich sources of secondary metabolites, as are young, expanding organs which do not yet enjoy the protection of highly lignified (and thus hardened) cell walls. Flowers, which are usually relatively ephemeral, are typically poor sources of secondary metabolites involved in defense, as they require relatively little protection (Harborne, 1997), although they may be unique sources of volatile fragrance compounds. Fruits are also poor in defense compounds, being rich in secondary metabolites such as pigments (particularly anthocyanins and carotenoids) in order to attract animal dispersal vectors, but the seeds themselves may be rich in tannins to deter ingestion (e.g., grape seeds). The pigments are antioxidants and are thus an important part of the diet.

Soure: Giacinto Bagetta, Marco Cosentino, Marie Tiziana Corasaniti, Shinobu Sakurada (2012); Herbal Medicines: Development and Validation of Plant-derived Medicines for Human Health; CRC Press