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The microevolution of structural change Hydrostasis and the evolution of the upright plant The evolution of plant transport hydrostasis and evolution of the upright plant Cell Mechanics some problems to overcome • One of the most perplexing questions that need to be asked and answered in relation to plant growth, is how do terrestrial plants stand up? Unlike vertebrates, land plants have no obvious hard skeleton to help resist the pull of gravity. Land plants do not benefit from the buoyant effect of water as do aquatic plants. We therefore need to question the nature of the structures and physical principles that allow land plants to withstand gravity. The answers lie in understanding cellular biomechanics the aquatic habit • Comparisons with aquatic plants are necessary. • Many of these organisms consist of a single cell, and some may attain large size and complexity. one giant cell for all? Leaf-like appendage An example here would be Caulerpa, which has a one-celled thallus. The thallus is without cross walls, but has numerous trabeculae of callose and pectic materials. Mechanical support of the erect shoots is due to turgor and thickness of the cell wall. Specialisation for a holdfast area interlinked with a horizontal rhizomatous unit, linking this structure to other ‘leaf like’ structures. “Any plant body that is not divided into true leaves, stems, and roots. It is often thin and flattened, as in the body of a seaweed, lichen, or liverwort, and the gametophyte generation (prothallus) of a fern.“ http://www.tiscali.co.uk/reference/encyclopaedia/hutchinson/m0007018.ht ml the multicellularity of land plants It is very important to realise that all successful terrestrial plants are multicellular. The primary benefit of multicellularity is internal compartmentalization of the plant body, thus facilitating physiological specialization. The second benefit is often overlooked -multicellularity confers structural support (which is so crucial to land plants). Multicellularity thus allows for a more rigid form, through the alteration of the thickness, chemical composition, layering and spatial distribution of cell wall materials within tissues. land transition Land transition meant that unicellularity was lost. •Sheer size would preclude a single cell from attaining any mass. •The force of gravity as well as environmental issues such as solar radiation, desiccation and the plethora of physiological conditions attendant upon these, would negate the possibility or probability of a massive single cell. •They would simply flatten out, like a plastic bag full of water due to loss of support! hydrostasis • In their operational state, living plant cell are hydrostatic units. • The cellular aggregates that comprise land plants, act as composite hydrostats, capitalizing on the spatial configurations of cell wall • Concept of hydrostasis first discussed it in the 19th century. Defn: hydrostasis: concerned with fluids that are not in motion example: hydrostatic pressure hydrostats 1 By definition, a hydrostat is, a structure whose mechanical behaviour depends on internal water pressure. Osmotic potential is generated because of plasmamembranes (noun a device that detects the presence of water ) Since all living cells contain water, each can be considered to be a functional hydrostat. The unicellular thalli of aquatic plants such as Caulerpa and Bryopsis are thus, single-celled hydrostats in which the cell wall operates like an inflatable, semi rigid sac; within which the protoplast exerts an internal pressure that in part maintains the shape of the cell wall. hydrostats 2 • While the cell wall contributes to the rigidity of the organism by virtue of its own material properties, turgor pressure creates "hoop" tension in the internal fibrous network of the cell wall, further strengthening the whole cell. The cell wall can vary its response to changes in turgor pressure (Preston 1974). In growing cells, the wall has a metabolically controlled ability to yield. This allows for the differential expansion or elongation, which is essential to cell morphogenesis. hydrostatic behaviour The hydrostatic behaviour and composite nature of cell walls are as essential to unicellular hydrophytic plants such as Caulerpa and Bryopsis as they are to growing multicellular plant systems. For aquatic plants, water is not a limiting resource, and turgor pressure varies little. On land, the availability of water may change continuously, and a unicellular hydrostatic structure can become a liability. Terrestrial plants do not rely solely on this mechanism. the need for hydrostatic mechanisms 1 • The variability of water availability changes continually, and an all-encompassing hydrostatic structure would be a liability in land plants. Terrestrial plants do not & cannot therefore, rely solely on turgor pressure for their principal mechanical support. Cell wall infrastructure incompletely partitions the protoplasts of terrestrial plants. This allows cellular compartments to differ in size and shape. heterogeneity in cellular compartment geometry is ensured by the deaths of some cells during developmental programming. the need for hydrostatic mechanisms 2 Differences in size and shape of cell walls within living tissues and organs and differences in orientation of cellulosic fibrils within cell walls provide a system in which shearing, tensile and compressive forces can be preferentially resisted. cell wall mechanics The cell wall is composed of a fibrous network of cellulosic strands is embedded in a plastic and ductile matrix. Research has shown that preferential yielding is the result of the composite nature of the cell wall matrix itself. conclusions • • Cell walls impart a degree of rigidity wall pressure (changing solute concentration) imparts hydrostatic force • leads to a less [or more] rigid structure the evolution of plant transport engineering a plant Engineering theory leads to a blend of interrelated conceptual synthesis, which provide insight into the ecology and evolution of plants, as well as indicators of the the construction materials that need to be used. multicellularity leads to the formation of remarkable structures increasing complexity – domains and transport complexity related to function 1 Unicellular domains Single vegetative cells (as in this post cell division example) contain all components needed to sustain growth and metabolism. Cell division will occur when the cell surface area to volume ration becomes unfavourable. Yet, within this cell control and regulation system occur, due to compartmentation (akin to domains). complexity related to function 2 – multifunctioning domains RC RC Complex multicellular and supracellular organisms contain many differing domains. In higher plants there are domains involved in transport, storage, retrieval or metabolism. This is exemplified in this illustration of a cross section of a young pine stem, where resin canal cells (RC) occur in proximity to tracheary elements as well as radial parenchyma. transport in hydrophytes Transport in hydrophytes is in some instances, simpler than that in their land plant counterparts. Yet, in others, being immersed in water presents a series of complex physiological problems, mostly to do with ion exchange processes (such as Na+ : K+ pumping) or aeration, to prevent anaerobic processes becoming dominant. Energy expenditure (ATP use) will be high and respiration outputs less efficient?) the two transport domains In lower plants, including some of the giant Kelps, we find evidence of transport systems composed of hydroids (water-conducting elements) and Leptoids (carbohydrate-conducting elements). Hydroids (left, above) occur associated with Leptoids (left, below) in the central region of the stem and evolved simultaneously. trumpet cells Trumpet cells are found in many of the large brown algae – here we see an example of a three files of trumpet cells, within the stalk of the algae. Below this, a scanning electron micrograph showing the details of the porous sieve plate platelike end wall structure. These are thought to be nearly as advanced as Angiosperm sieve tubes! evolution of vascular tissues vascular systems First, simple central xylem core protostele siphonostele eustele atactostele Hollow-centred Vascular bundles (young Gymnosperms and Dicotyledons) Spiral arrangement Poaceae multifunctioning transport domains In this example, one can see a multifunctional transport system. The xylem (an apoplasmic domain) is adjacent to the phloem (a symplasmic domain). Both have different functions, yet they co-exist in the same plane and axis. domain control & regulation domain control requires communication pathways. These are usually provided by plasmodesma such as these, seen in a scanning electron microscope image of a surface view showing plasmodesmal fields in the bundle sheath interface in Themeda triandra. domains - summary • A single celled organism has one DOMAIN;. • A multicelled organism may have two to many DOMAINS; • DOMAINS have different functions in the land plant, • DOMAINS can regulate activities of other DOMAINS simple beginnings Amongst the earliest vascular plants, there was little distinction between the arrangement of the water conducting (xylem) and carbohydrate-conducting (phloem) in stems and rhizomes or roots. the protostele The fossil record has many examples to offer of these early cellular arrangements. These first-formed structures were called protosteles (first-seen or formed) are evidenced in the seedless vascular plants, which arose some 400 to 350 million years ago. Today, these structures are evidenced in groups such as the division PSILOPHYTA. (Psilotum, Tmesipteris) Psilotum is unique as it lacks roots and leaves. Tmesipteris is an epiphyte of tree ferns. Both have protostelic vascular tissue. the protostele 2 The LYCOPHYTA (Lycopodium) 200 species; vascular system is more advanced with a dissected protostele. The xylem in the example below is dissected into three pieces, with four “points” to the xylem, thus four groups of phloem exist between the xylem poles. the plectostele further division, into a plectostele (plate like) structure, see the xylem and phloem occurring in alternating sheets. Note large-diameter sieve cells in close proximity to the tracheary elements. the actinostele The vascular tissue is more dissected, with some of the xylem strands showing almost complete dissection from one another. Phloem form finger-like projecting mass between the xylem dichotomy and anastomoses - prelude to leaf and branch traces Fusion of separate dichotomous strands of vascular tissue led to anastomoses of originally separate vascular strands (individual protosteles). From this simple beginning, the evolution of the megaphyll reduction in complexity, cheaper structures Diagrams illustrating the difference between a hollowcentred (A) and solid (B, pithcontaining) core in which vascular bundles are scattered – This is an Atactostele. A B microevolution - 1 •Engineering theory relevant to the design of plant stems, can be used to predict which anatomical configurations are most likely to succeed in different environments and to test aspects of (successful) evolutionary theories. •For example, genotypes that maximize their net carbon gain are favoured through natural selection over those less efficient. microevolution - 2 •Plant ”economics” suggests that that metabolic investment made in the construction of an organ should be minimized provided its functions are not impaired. •One would expect a reduction in the relative density of tissues or organs involved in vertical growth for example ‘pannelboard’ organs, hollow sterns, and hollow leaves. microevolution – 3 •The ratio of the living protoplasm to the living and dead cell wall infrastructure in an organ would be expected to be greater in environments where the tissue’s depravation of water is less likely, as hydrostatic support mechanisms are energetically "cheaper" than those based on thick cell walls and little or no protoplasm. •Thick cell walls prevent collapse microevolution - 4 In contrast, plants growing in habitats with cyclical or unpredictable episodes of water stress would be expected to "invest" in thick walled cells or waterinsensitive tissues, such as wood or lignified fibres.. microevolution - 5 An advantage of a living tissue with thin cell walls and turgid protoplasts is its resistance to deformations due to rapid water cycling out of cells - it takes time for water to move out of cells. Thus, "quick" stresses are resisted whilst "slow" stresses produce "slow' strains end