Biological Sciences
Plant Leaves
Plant leaves are the primary site for photosynthesis, where they convert sunlight, water, and carbon dioxide into energy. They also play a role in gas exchange, allowing for the uptake of carbon dioxide and release of oxygen. Leaves are typically broad and flat to maximize surface area for light absorption and are supported by a network of veins for transport of water and nutrients.
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9 Key excerpts on "Plant Leaves"
eBook - PDF- Richard N. Arteca(Author)
- 2014(Publication Date)
- Cengage Learning EMEA(Publisher)
© Richard and Jeannette Arteca. Copyright 2015 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 92 INTRODUCTION TO HORTICULTURAL SCIENCE Leaves The main function of a leaf is to manufacture food for the plant through photosynthesis . Photosynthesis refers to a series of chemical reactions in which carbon dioxide and water are converted in the presence of light to carbohydrates (sugar) and oxygen. The simplified equation for photosynthesis is as follows: 6CO 2 + 6H 2 O → → → → C 6 H 12 O 6 + 6O 2 Both light and chlorophyll are essential to photosynthesis. The two major photo-synthetic enzymes found in plants are ribulose-bisphosphate carboxylase (Rubisco) and phosphoenolpyruvate carboxylase (PEP-Carboxylase). Carbon dioxide and other gases enter and exit the leaf through tiny pores in the leaf’s surface called stomata (Figure 5-5). Another important function carried out by the leaf is transpiration , which is the loss of water from the leaf in the form of water vapor. Respiration is another important func-tion carried out by the leaf; this process uses sugars made during photosynthesis and breaks them down into simpler molecules (such as H 2 O and CO 2 ) that are used as energy for plant growth and development. Parts of Leaves Leaves consist of several basic parts that help identify them. The major parts of a simple dicot leaf are listed here (Figure 5-6): ■ Petiole. The leaf stem or stalk that attaches the leaf to the stem. ■ Blade. The flat thin part of the leaf. ■ Midrib. The largest vein located in the middle of the leaf.
eBook - ePub- Patricia Barnes-Svarney, Thomas E. Svarney(Authors)
- 2014(Publication Date)
- Visible Ink Press(Publisher)
M any houseplants, annual plants, and fruit trees exhibit a phenomenon in which the terminal bud produces hormones inhibiting the growth of axillary buds. This allows the plant to grow taller, increasing its exposure to light. Under certain conditions, the axillary buds begin to grow, producing branches. This can occur when the terminal bud is pruned (“pinched back”) on certain plants and fruit trees, stimulating the axillary bud growth and producing bushy, full-looking plants.What are leaves?
Leaves are the main photosynthetic organ for plants; they are also organized to maximize sugar production while making sure little water loss occurs. Thus, they are also important in gas exchange and water movement throughout the whole plant. Leaves—outgrowths of the shoot tips—are found in a variety of shapes, sizes, and arrangements. Most leaves have a blade (the flattened portion of the leaf), a petiole (the slender stalk of the leaf), and leaflike stipules (found on some leaves and located at the base of the petiole where it joins the stem). Cross-sections of a leaf also show a variety of features, including the cuticle (the outer covering to minimize water loss), veins (also called vascular bundles, which carry water and nutrients from the soil to the leaf and also carry sugar), stoma (plural stomata; water escapes through the stoma; they open and close), guard cells (modified epidermal cells that contain chloroplasts and control the opening of the stoma), and palisade and spongy mesophyll cells (for photosynthesis).A cutaway of a typical leaf shows its interior structure.What is a cuticle in a plant leaf?
The cuticle contains a waxy substance, called cutin, that covers the parts of the plant exposed to the air: the stem and leaves. It is relatively impermeable and provides a barrier to water loss, thus protecting the plant from desiccation.What is the purpose of the stomata?
Stomata (singular “stoma” from the Greek term stoma , meaning “mouth”) are specialized pores in the leaves and sometimes in the green portions of the stems, as well as flowers and fruits. Carbon dioxide (CO2 ) enters the plant through the stomata, while water vapor escapes through the same pores. The guard cells that border the stomata expand and contract to control the passage of water, carbon dioxide (CO2 ), and oxygen (O2
eBook - PDFIntroduction to Crop Husbandry
(Including Grassland)
- J. A. R. Lockhart, A. J. L. Wiseman(Authors)
- 2014(Publication Date)
- Pergamon(Publisher)
Most plants consist of roots, stems, leaves and reproductive parts and need soil in which to grow. The roots spread through the spaces between the particles in the soil and anchor the plant. In a plant such as wheat the root system may total many miles. The leaves, with their broad surfaces, are the main parts of the plant where photosynthesis occurs (see Fig. 1). A very important feature of the leaf structure is the presence of large numbers of tiny pores (stomata) on the surface of the leaf (see Fig. 2). There are usually thousands of stomata per square cm of leaf surface. Each pore (stoma) is oval-shaped and surrounded by two guard cells. When the guard cells are turgid (full of water) the stoma is open and when they lose water the stoma closes. The carbon dioxide used in photosynthesis diffuses into the leaf through the stomata and most of the water vapour leaving the plant, and the oxygen from photosynthesis diffuses out through the stomata. Carbon dioxide + water + energy chlor °P h y} 1 carbohydrates + oxygen nC0 2 nH 2 0 (light) (CH 2 0)n nO ? 1 INTRODUCTION TO CROP HUSBANDRY Water and mineral salt· FIG. 1. Photosynthesis illustrated diagrammatically. Upper leof surfoce (cuticle) Stoma FIG. 2. Stomata on leaf surface. Cells containing f Chlorophyll in tiny particles (chloroplasts) 7 Spongy cell tissue Lower leof surface Carbon dioxide cL·—-Water vapour S t o m a and oxygen FIG. 3. Cross-section of green leaf showing gaseous move-ments during daylight. Transpiration The evaporation of water from plants is called transpiration. It mainly occurs through the stomata and has a cooling effect on the leaf cells. Water in the cells of the leaf can pass into the pore spaces in the leaf and then out through the stomata as water vapour (see Fig. 3). The rate of transpiration varies considerably. It is greatest when the plant is well supplied with water and the air outside the leaf is warm and dry.
eBook - PDF- Rene Fester Kratz(Author)
- 2022(Publication Date)
- For Dummies(Publisher)
5 Going Green with Plant Biology IN THIS PART . . . Take a close look at plant body structure. Learn how different types of plants reproduce. Explore how plants regulate their water balance. CHAPTER 18 Studying Plant Structures 321 Studying Plant Structures A plant’s structure suits its lifestyle. After all, it has flat leaves for gathering sunlight, roots for drawing water up from the soil, and flowers and fruits for reproduction. Plants begin their lives from seeds or spores, grow to maturity, and then reproduce asexually or sexually to create new generations. In this chapter, I present the fundamental structures of plants and introduce you to their reproductive strategies. Peering at the Parts and Types of Plants Like animals, plants are made of cells and tissues, and those tissues form organs, such as leaves and flowers, that are specialized for different functions. Plants have two basic organ systems: » The shoot system, located above ground, helps plants capture energy from the sun for photosynthesis (see Chapter 4). » The root system, located below ground, absorbs water and minerals from the soil. Chapter 18 IN THIS CHAPTER » Understanding plant parts and their functions » Breaking down the tissues of herbaceous and woody plants » Following the steps of plant reproduction 322 PART 5 Going Green with Plant Biology The structure of each type of plant organ is tailored to match its function (see Figure 18-1): » Leaves capture light and exchange gases with the atmosphere while minimizing water loss. • Many leaves are flattened, so they have maximum surface area for light capture. • Tiny holes called stomata in the surfaces of leaves open and close to allow plants to absorb carbon dioxide from the atmosphere and release oxygen. (You can see a stoma in the leaf cross-section in Figure 18-1.) • Guard cells surround the stomata, ready to close them if water loss from the leaves becomes too great.
eBook - PDFScience and the Garden
The Scientific Basis of Horticultural Practice
- David S. Ingram, Daphne Vince-Prue, Peter J. Gregory, David S. Ingram, Daphne Vince-Prue, Peter J. Gregory(Authors)
- 2008(Publication Date)
- Wiley-Blackwell(Publisher)
Variations on a theme The basic form of the leaf described above, a flat plate with a high surface area to volume ratio, roughly oval to lanceolate in shape, cov-ered with a cuticle and an epidermis pierced by stomata and with the cells of the various tis-sues arranged to maximise the internal surfaces available for gas exchange, is a magnificent compromise. It has evolved in response to the conflicting environmental and physiological constraints imposed most notably by the need to collect the maximum amount of light energy and atmospheric carbon dioxide for pho-tosynthesis, whilst at the same time minimising heat gain and water loss. However, such a com-promise is only effective under conditions of an equable climate and adequate supplies of water, light, carbon dioxide and mineral nutri-ents. Any deviations from these optimal condi- Know your Plant 21 tions reduce the effectiveness of the leaf as a photosynthetic organ. Many plants have evolved, in response to a variety of environ-mental conditions, a number of variations on the basic leaf form (Fig. 1.7), some of which are so extreme (as with the spines of cacti, Fig. 1.11), that it requires the experienced eye and knowledge of the botanist to assure us that what we are looking at is really a leaf at all. Variations in the shape, form and colour of leaves One of the commonest variations has been in the evolution of divided leaves, perhaps minimising wind and rain damage by posing less resistance, as in the maples (Acer spp.). In other species the leaf may have holes, as in the cheese plant (Monstera deliciosa), caused by the death of certain cells as the leaf develops, or drip-tips as in the weeping fig {Ficus bejamina), to facilitate run-off of water in tropical climates.
eBook - PDF- Mary Ann Clark, Jung Choi, Matthew Douglas(Authors)
- 2018(Publication Date)
- Openstax(Publisher)
Figure 30.23 Leaves may be simple or compound. In simple leaves, the lamina is continuous. The (a) banana plant (Musa sp.) has simple leaves. In compound leaves, the lamina is separated into leaflets. Compound leaves may be palmate or pinnate. In (b) palmately compound leaves, such as those of the horse chestnut (Aesculus hippocastanum), the leaflets branch from the petiole. In (c) pinnately compound leaves, the leaflets branch from the midrib, as on a scrub hickory (Carya floridana). The (d) honey locust has double compound leaves, in which leaflets branch from the veins. (credit a: modification of work by "BazzaDaRambler"/Flickr; credit b: modification of work by Roberto Verzo; credit c: modification of work by Eric Dion; credit d: modification of work by Valerie Lykes) Leaf Structure and Function The outermost layer of the leaf is the epidermis; it is present on both sides of the leaf and is called the upper and lower epidermis, respectively. Botanists call the upper side the adaxial surface (or adaxis) and the lower side the abaxial surface (or abaxis). The epidermis helps in the regulation of gas exchange. It contains stomata (Figure 30.24): openings through which the exchange of gases takes place. Two guard cells surround each stoma, regulating its opening and closing. 920 Chapter 30 | Plant Form and Physiology This OpenStax book is available for free at http://cnx.org/content/col24361/1.8 Figure 30.24 Visualized at 500x with a scanning electron microscope, several stomata are clearly visible on (a) the surface of this sumac (Rhus glabra) leaf. At 5,000x magnification, the guard cells of (b) a single stoma from lyre-leaved sand cress (Arabidopsis lyrata) have the appearance of lips that surround the opening. In this (c) light micrograph cross-section of an A. lyrata leaf, the guard cell pair is visible along with the large, sub-stomatal air space in the leaf.
eBook - PDF- Russell Monson, Dennis Baldocchi(Authors)
- 2014(Publication Date)
- Cambridge University Press(Publisher)
Plants with large leaves and high transpiration rates could maintain leaf temperature close to air temperature (or even below air temperature), but in this case by increasing the potential for transpiration and latent heat loss. The cooler leaf temperatures that are achieved by high rates of transpiration in these species are close to the biochemical optimum for photosynthesis and thus they permit high rates of CO 2 assimilation. It would be possible to dedicate an entire chapter to the topic of diversity in leaf structure. We will focus on only one type of leaf – the bifacial leaf. Bifacial leaves exhibit heteroge- neous anatomies in cross section, with tightly packed cells just beneath the top epidermis and loosely packed cells just above the bottom epidermis (Figures 8.1A and 8.1B). The bulk of the cells between the two epidermal layers compose a tissue known as mesophyll. Leaf mesophyll cells contain chloroplasts and are specialized for conducting photosynthesis. In many bifacial leaves the mesophyll tissue is organized into two distinct zones: the tightly packed upper zone known as palisade mesophyll, and the loosely packed lower zone known as spongy mesophyll. In conifer needles, the mesophyll tissue is not differentiated into palisade and spongy layers, but rather occurs as tightly packed cells surrounding a vascular bundle (Figure 8.1C); a form that results in relatively low surface-to-volume ratios. The vascular system of the leaf enters through the leaf petiole; the “stalk” of the leaf. Vascular bundles in the petiole and veins of the leaf contain xylem, the tissue of water transport, and phloem, the tissue of carbon transport (sugars and amino acids). (Organic molecules can also be found dissolved in xylem water, but the principal path of carbon transport is in the form of sugars transported through the phloem.) Xylem water transport is from roots to leaves, although water can also be moved among leaves if driven by appropriate water potential gradients.
eBook - PDFTrees
Their Natural History
- Peter A. Thomas(Author)
- 2014(Publication Date)
- Cambridge University Press(Publisher)
(1979) Plant Physiology (2nd edn). Macmillan, New York, and Fitter, A.H. & Hay, R.K.M. (2001) Environmental Physiology of Plants (3rd edn). Academic Press, London. Chapter 2: Leaves: the food producers 17 around the edges of the holes than in the middle so many small holes lose more water than would one big one of the same area. Because of these con- straints, stomata in tree leaves are similar in size and number to other plants: typically around 0.01–0.03 mm long and one third that across with densities as low as 1400 per square centimetre (cm -2 ) in larch to 25 000 cm -2 in apple and over 100 000 cm -2 in scarlet oak (Quercus coccinea) and almost this many in some cultivated varieties of red maple (Acer rubrum). Most leaves fall into the range 4000–35 000 cm -2 . Even with such high numbers the stomata still usually cover less than 1% of the leaf area. Despite the small area, the stomata will lose around 98% of the water taken up by the roots. Usually the stomata are evenly spread over the leaf but this is not always so. In European ash (Fraxinus excelsior) stomata are most plentiful in the middle of the underside while in horse chestnut (Aesculus hippocastanum) they are predominantly around the edge of the broadest parts of the leaflets. Sun and shade leaves With the large number of leaves held by a tree it is almost inevitable that some will shade others. How the tree gets round this problem is dealt with in detail in Chapter 7. Part of the solution is that the leaves of most trees, even those with open canopies such as birch, can be divided into sun and shade leaves. Shade leaves are larger and thinner with a thinner cuticle, darker green (more chloro- phyll per weight), and less lobed, with half to a quarter the density of stomata. They can work more efficiently at lower light intensities than sun leaves but cannot handle bright light for long periods.
eBook - PDFPlant Anatomy
An Applied Approach
- David F. Cutler, Ted Botha, Dennis Wm. Stevenson(Authors)
- 2009(Publication Date)
- Wiley-Blackwell(Publisher)
Chapter 6 74 components – the mechanical and supply systems, the tissue in which pho- tosynthesis is carried out, and the outer skin or epidermis. Leaf structure Many leaves are flattened dorsiventrally. The leaf of Ilex aquifolium, in Fig. 6.3 in transverse section and surface view, serves to illustrate general dicotyle- donous foliage leaf anatomy. In this example, the epidermis forms the bound- ary between the atmosphere and the underlying mesophyll and vascular and non-vascular tissues. Its cells are specialized for this function. Note that the adaxial epidermal cells (Fig. 6.3c) have thickened outer walls. The epidermal cells are covered by a thin cuticular layer. In this dorsiventrally compressed leaf, the upper and lower surfaces are different, as can be seen from Fig. 6.3(a– c). Stomata occur among the cells of the lower (abaxial) surface only; the leaf is thus hypostomatic (see Fig. 6.3d). The mesophyll consists of chlorenchy- matous, palisade-like cells on the adaxial side with few intercellular spaces, and of more loosely arranged spongy cells with larger intercellular spaces on the abaxial side (Fig. 6.3c). Part of the vascular system is shown, including the large midrib bundle (Fig. 6.3a) and in more detail in Fig. 6.3(b). A smaller sec- ondary vein is illustrated in Fig. 6.3(c). In all leaf blade vascular bundles, phlo- em occurs to the abaxial and xylem to the adaxial side of the leaf. The larger and many of the smaller vascular bundles frequently have a cap of sclerenchy- ma cells associated with the phloem pole only. In general terms, all leaves have similar features – an epidermis with sto- mata, mesophyll and vascular tissue. However, the arrangement of these components is, to a large extent, dictated by the physical environment such as water availability, light intensity, ecological niche and herbivores. Through selection pressure, it is the interplay of these environmental parameters which serves to drive the evolution of leaf structure.
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