LEAVES
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LEAVES Leaves produced by mosses and liverworts are not true leaves because they do not have a vascular strand. Vascular plants have evolved 3 generalized plant forms those that are leafless (aphyllous); those with small needle - like leaves generally have a simple vascular strand (microphyllous) and those with elaborate large leaves with a complex of branched vascular strands (megaphyllous) - found in flowering plants (most familiar). Of course, in order to confuse taxonomists, there are always some plants which used to have megaphylls, but which have reduced back to the aphyllous condition. There are also species with large microphylls and others with small megaphylls. So we need a definition. In plants with megaphylls there is a break in the vascular tissue of the stem - above the point of leaf attachment. - called a leaf gap. Stems of microphyllous plants do not have a leaf gap - and their vascular tissue is generally very simply arranged.
The origin of branching systems and megaphylls can be explained through telome theory. Although based on many ideas and hypotheses, telome theory was largely developed by Zimmerman between 1930 and 1965. The theory has steadily gained favour. The basic unit in the telome concept is the telome, described as that part of the branching system between the last dichotomy and the distant end of the branch. It is thought that telomes become modified through several basic processes -
1. OVERTOPPING AND REDUCTION. One dichotomous branch grows beyond the other. - Overtopping. If reduction also occurs a sympodial branching system results. Further reduction and overtopping of the axis produced the usual monopodial axis - which is clearly the best adopted mode of growth able to give strength for trees to reach 100m tall.
2. PLANATION AND WEBBING. Planation is the evolution of a flattened blade in the plane from an original organism of dichotomous branches which required twisting of branches. Second process = webbing which is the filling-in of the space between planated branches. There are many examples in the fossil record to show that planation and webbing were trends in a number of distinct lineages. It seems that as the megophyllous leaf gradually developed through elaboration of the terminal branching systems, so the photosynthetic function was transferred from the axis to the more specialized blade areas (leaves). There are no concrete answers as to why this happened - but it seems to be because leaves provide more efficient photosynthesis. It might have occurred as plants started to develop secondary thickening and bark - clearly making stems unsuitable for photosynthesis.
In most plants the leaf is flattened with definite upper and lower surfaces and it is commonly divided into two parts - the leaf blade or lamina and the petiole - an elongated stem connecting it to the stem at the node. The function of the blade is to absorb as much light and CO2 as possible and the function of the petiole is to hold the blade out into the light and prevent self-shading. The petiole is also often flexible which has a number of advantages: it allows the leaf to move in the wind, helping to cool it, bring in fresh air (reducing the boundary layer) and making it more difficult for insects to land. The description of the shape of leaves is technical and well developed as it is the leaves which are often the most obvious feature of the plant by which to identify it. We can identify them into essentially two types: simple and compound. A simple leaf has a single blade although the margin may be toothed or lobed. A compound leaf has 2 or more blades each attached to the rachis by a petiolule A subunit of a petiolule and small blade is called a leaflet. Leaves can be palmately compound - all leaflets attached to same point or pinnately compound with leaflets attached individually along the rachis.
The epidermis is often different on the upper and lower surfaces of the leaf, with stomata usually concentrated on the lower surface. Either surface may have trichomes capable of providing a number of functions described in the lecture on plant surfaces. The ground tissue or mesophyll within the blade is the main site of food production for the plant. It is usually made up of two distinct layers both containing chloroplasts. The upper layer is made up of elongated cylindrical cells, arranged perpendicular to the leaf surface ; known as the palisade layer. The cells adjacent to the lower epidermis are lobed and irregular in shape and form. They make up the spongy mesophyll and have large intercellular air spaces which are continuous with the air spaces next to the stomata.
The xylem and phloem of the leaf are confined to the vascular bundles of veins. They are all interconnected and eventually connect with the main bundles in the stem. Each vascular bundle is surrounded by a bundle sheath which act as intermediaries in the transfer of organic solutes from the mesophyll. In the bundle the phloem is always situated towards the lower side of the leaf and the xylem towards the upper.
Leaves are commonly modified in various ways, the most common being to form spines or tendrils. Spines often represent highly reduced leaves where the lamina is no longer present and the central midrib has become hard and sclerenchymatous. In some species we can see a gradual change from leaves to spines along the stem. Tendrils have various origins, but the all function as holdfasts by coiling and twining around objects they touch. They are often found at the tips of leaves and in these cases are formed from the modification of the leaf. These are seen in peas when genetic mutations can modify leaf morphology, some plants produce tendrils and others produce leaflets.
The most complex modifications of leaves are seen in the carnivorous plants. Some modifications relate to the functioning of glands on the surface of the leaves as already discussed. In other cases we see extensive modification of the leaf itself, pitcher plants are an obvious example - not only is a cup formed from the leaf, but the plants will often have mechanisms designed firstly to attract the insects and then ensure they remained trapped once they have entered the pitcher. The Venus fly trap is another good example of highly modified leaves - form two wings with long marginal teeth. The surface of the leaf has trigger hairs which respond to an insect causing the two halves of the blade to fold together quickly.
Light absorption is of course necessary for photosynthesis. It also affects phototropic responses - the movements that occur in response to light. If we study the upper surfaces of some leaves, we find that the epidermis cells have a characteristic shape. The lower wall is usually flat while the upper is more or less papillose.(small sharp projections) Each cell therefore acts as a conducting lens. The leaves tend to velvety to the touch and are often found on plants of tropical rain forests e.g. Begonia. In some species the cells don't only function as lens, but are constructed in the same way - Petrea volubilis has a disc of silica embedded in the outer tangential wall.
In plants which have the special type of metabolism called C4 photosynthesis, we also see a different sort of leaf anatomy. This is known as Krantz anatomy - Krantz is the German word for wreath. C4 plants lack the distinct palisade parenchyma and spongy mesophyll cells. Instead they have a sheath of cells around the vascular bundle. In turn, these cells are surrounded by large mesophyll cells with many chloroplasts. This very different form of anatomy is directly linked to the photosynthetic process seen in C4 plants and the need for compartmentalization of the reactions. Without describing C4 metabolism - we see the formation of malate in the mesophyll cells and pyruvate in the bundle sheath cells.
Plants produce new leaves and shed old ones throughout their life. All leaves (except Welwitschia) eventually become old and die. In deciduous trees this is an annual process and in evergreen trees leaves may remain attached to a plant for several years. Leaves usually abscise cleanly at the node leaving a leaf scar with small vascular bundle scars. The place where the petiole and node part company is called the abscission zone. The cells along this zone are structurally weak with few lignified cells, and as the leaf senesces, the cell walls begin to separate due to increased enzyme activity responding to increased hormone activity. When the weight of the leaf can no longer be supported, it breaks away cleanly and the stem tissue that is exposed quickly becomes suberized to protect it from the environment.
Compound leaves are essentially lots of small leaves on a deciduous stem (effectively a small stem which is also lost). Why not just have small leaves? - In habitats with a pronounced dry season, small branches/twigs will continue to transpire, creating significant water loss. It may therefore be better to lose these fine twigs and reproduce them again the following year than risk the whole tree dying. Once compound leaves are lost, it is only secondary wood covered in bark that remains. What is the evidence for this theory? In warm, seasonally arid environments, the characteristic type of tree is compound leaved and deciduous. Compound leaves are also common in lowland tropical rainforests. they are usually found on trees of the upperstories as it is here that the tree is more prone to desiccation and intense sunlight. Compound leaves produce more turbulence across photosynthetic area - Turbulence brings in CO2 and removes excess heat. Large leaves can be prone to tearing in the wind. Can be prevented by either making the leaf small and tough or effectively pretorn ie. compound. Large simple banana leaves tears in special lines of weakness to a compound condition. Compound leaves are also thought to be an adaptation allowing effective rapid vertical growth. Certain tree species specialize in competing for new gaps in a forest, they need a high rate of vertical elongation if they are going to reach the canopy. However, this raises a conflict: the plant doesn't want to put energy into branching, because it wants the energy for increasing height. But, without branching, it cannot capture sunlight to provide with the energy to do anything. Large compound leaves arranged around the stem is the ideal solution because: they act as cheap throwaway branches - not as costly to build as a woody branch and the green stem can also photosynthesize. Compound leaves can cover a large area where a simple leaf or leaves to fill the same area would require much greater support tissue.
Therefore, compound leaves appear to be adaptive in at least two sorts of environmental contexts: in warm seasonally arid situations that favour the deciduous habit, and in light gap and early successional vegetation where rapid upward growth and competition for light favour the cheap throwaway branch. Among small and medium sized leaves simple leaves are common - perhaps because greater percentage of compound leaf non-photosynthetic parts i.e. rachis, petiolules and edges.
Shape - usually long, tapering. Lack petiole, base of leaf wrapped around stem forms the leaf sheath. Not all monocots - palms, bird of paradise plants appear to have petioles- evolved from grass-like leaves, "petiole" is modified lamina. Initiation - Monocot leaves are produced only though the activity of shoot apical meristem. develop from leaf primordia.
ROOTS Roots grow in much the same way as the shoot system. A root system is comprised of the main parent roots ( it is here where secondary thickening is possible in some species) and it is these thicker roots which form a sort of skeleton. Lateral or capillary roots then function to absorb water and nutrients. Many woody plants possess a characteristic pattern of root development. However, modification occurs depending on soil texture, water availability and overall nutrition. Even so, most trees have most of their roots within 3 feet of the soil surface. Whereas the stem becomes dormant in temperate trees in winter, the roots will grow whenever the soil temperature is favourable. Not all roots grow at any one time; while some are growing others are quiescent. Most dicots have a single prominent taproot larger than the rest with numerous small lateral (branch) roots coming out of it. Tap root develops from the radicle that was present in the seed, e.g. carrot, turnip etc. (small laterals are removed before market). Most monocots and some dicots have a mass of similarly sized roots - a fibrous root system. Radicle dies soon after germination, root primordia at base of radicle grow out and form first stages of fibrous root system. Dicot - mostly perennial, secondary growth in stem and root - increasing quantities of xylem (as wood) and phloem (inner bark) - enlarged conduction capacity - permits increase in number of leaves and fine absorptive roots. Monocots -no secondary growth (no vascular cambium). Once stem formed the number of vascular bundles, tracheary elements and sieve tubes is set. Can not supply water to extra leaves nor could such a shoot supply an ever increasing tap root.
The root apex differs in several respects from the shoot. First, it produces a root cap. Second, no lateral organs are produced near the apical meristem of the root. Extension growth is confined to a narrow (2-3mm) region and lateral roots arise from deep within the tissues some distance behind the meristem. They grow outwards by rupturing the exterior primary tissues (cortex)
Nothing unusual except the presence of root hairs which are modified epidermal cells designed to increase the interface with the surface of grains of soil.
A simple cortex lying between the epidermal and vascular systems.
In dicots, gymnosperms and ferns the xylem is a solid core with radiating arms. Centre metaxylem, outer arms -protoxylem. Alternating with these arms are strands of phloem. Within phloem strands inner metaphloem - outer protophloem.
In monocot roots -separate strands of xylem and phloem within the ground tissue.
The pericycle is a layer of parenchyma cells surrounding the vascular tissue - representing its limit.
Endodermis - the pericycle is in turn enclosed by a single layer of cells which is the innermost layer of the cortex. Each cell is cube shaped with a continuous band of suberin and lignin which makes up the Casparian strip - suberin -chemically similar to cutin and wax - extremely hydrophobic -makes endodermis impermeable to water through the non-living cell walls Water passing from the root hairs to the vascular tissue must pass through the plasmodesmata and cytoplasm of these cells, i.e. prevents transport through the apoplast, only allows transport through the symplast. This enables the plant to be selective - preventing harmful minerals from entering the transport system.
Lateral roots are initiated in the pericycle by mitotic divisions of the cells lying directly opposite the primary xylem. The basic structure of these roots is the same, and of physiological importance is the connection between the vascular tissue of the main root and the lateral roots.
Storage roots - Roots frequently provide long term storage of carbohydrates accumulated from summer in perennial plants. Advantage in root storage cf. stem storage More stable environment, less extreme changes in temp and humidity. stems easily visible to predators, some perennials most of stem is annual.
Prop roots - adventitious roots give extra absorptive capacity and extra stability e.g. Screwpine (monocot), banyan tree, mangroves.
Contractile roots - freq. in bulbs - roots extend into soil, become firmly anchored then top part of root contracts, pulling bulb down deeper into soil.
Haustorial roots of parasitic plants - penetrate to host's xylem.
Mycorrhizae.
Root nodules and nitrogen fixation.
As in the shoot, variation occurs in the type of apical meristem in roots, both among different taxonomic groups and within them.
Again, in generalized vascular plants and ferns (seedless) there is an apical cell. However, the structure of roots is different from shoots, so the apical cell must lead to different patterns of growth. Distal to the apical cell is the root cap. The root cap is a protective structure and its found in all roots. The apical cell therefore has to give rise to tissues which move forward and become the rootcap as well as others moving back to give rise to the ground meristem and procambium. The protoderm differentiates from the ground meristem.
In gymnosperms, a group of initials gives rise to a series of mother cell zones. One zone gives rise to the central cylinder which develops into vascular tissue. A second zone is in the form of a short column and gives rise to the columella of the root cap, which then produces the root cap itself. The third zone occurs as a ring around the root cap or columella mother cells. This gives rise to the ground meristem and protoderm - tissues which develops into the root cortex and epidermis
In angiosperms, there is also a group of initials, but unlike those in gymnosperms these give rise directly to the tissues and rootcap - very simple. The configuration structure of the apical meristem varies - in some the initials are found in one group - in others the initials are divided into several groups - but they all act in much the same way. In addition, in most angiosperm roots, the centre of the apex is occupied by a group of cells which divide at a much slower rate than the cells around them. This area is called the QUIESCENT centre. (dormant, sleeping) This seems to be the site of production of the initials again we see the multistep meristem theory in action.