Weed Anatomy

By Hansjoerg Kraehmer and Peter Baur

Weeds affect everyone in the world by reducing crop yield and crop quality, delaying or interfering with harvesting, interfering with animal feeding (including poisoning), reducing animal health, preventing water flow, as plant parasites, etc. Weeds are common everywhere and cause many $ billions worth of crop losses annually, with the global cost of controlling weeds running into $ billions.

The anatomy of plants is generally well understood, but the examples used for explanations in most books are often restricted to non-weed species. Weeds have many features that make them more competitive, for example enabling them to more quickly recover after herbicide treatment. Some of these adaptations include rhizomes, adapted roots, tubers and other special structures. Until now,
no single book has concentrated on weeds’ anatomical features. A comprehensive understanding of these features is, however, often imperative to the successful implementation of many weed control measures.

Beautifully and comprehensively illustrated, in full colour throughout, Weed Anatomy provides a comprehensive insight into the anatomy of the globally-important weeds of commercial significance. Commencing with a general overview of anatomy, the major part of the book then includes sections covering monocotyledons, dicotyledons, brackens and horsetails, with special reference to their anatomy. Ecological and evolutionary aspects of weeds are also covered and a number of less common weeds such as Adonis vernalis, Caucalis platycarpos and Scandix pecten-veneris are also included.

The authors of this book, who have between them many years of experience studying weeds, have put together a true landmark publication, providing a huge wealth of commercially-important information. Weed scientists, plant anatomists and agricultural scientists, including personnel within the agrochemical and crop protection industry, will find a great deal of useful information within
the book’s covers. All libraries in universities and research establishments where agricultural and biological sciences are studied and taught should have copies of this exceptional book on their shelves.

• Language: English
• Publisher: Wiley
• Release date: Jan 31, 2013
• ISBN: 9781118503348

Book preview

Chapter 1

Tissues

No structure without substructure

The reproductive development of higher plants starts with a fertilised egg cell. This cell and its descendants divide and form cell clusters. The growing embryo differentiates into a seedling and characteristic tissues or functional plant parts (Figure 1.1), which later on result in an organism consisting of stem, leaf and root. In between, a number of distinct tissues develop which can be classified according to their appearance and function. The following terms will be used in the next chapters.

Epidermis with interfacial cuticle: the plants’ outer protective layer of cells; it allows the exchange of water, some ions, CO2 and O2 with the environment; many agrochemicals can enter the plant via this layer.

Parenchyma: ground tissue of cells with thin walls; this kind of tissue often fills spaces between other tissues or between organs.

Collenchyma and sclerenchyma: tissues that stabilise the form of stem, leaf and root.

Vascular tissues called phloem and xylem: they transport water and assimilates from one plant part into another.

Meristem: these consist of embryonic cells which divide, form new tissues and are already apparent in seedlings, as shown in Figure 1.2.

Secretory tissues: many plants excrete all kinds of secondary products, for example oils, resins, gums, mucilages and others; these products are stored, transported or excreted by secretory tissues.

Figure 1.1 Germinating seed of Abutilon theophrasti Medik.

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Figure 1.2 Embryo of Galium aparine L., longitudinal section.

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Explanations of our figures often describe how tissues were dissected. Figure 1.3 depicts three major ways of generating sections as you may find them in most of our examples.

Figure 1.3 (A) longitudinal, radial; (B) longitudinal, tangential; (C) transverse. Staining and preparation techniques will be described in Section 9 of this book.

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Chapter 2

Parenchyma

In animal histology, parenchyma is the tissue in which organs are embedded. In plants, parenchyma cells make up the bulk of the organs and are often found as a sort of filling material, for example in the pith. They are characterised as isodiametric in some textbooks. This is true for many cases but not for all. It is rather difficult to measure a diameter within the spongy mesophyll of leaves. One characteristic of parenchymatic cells is that their cell walls are usually thin (∼0.2 to 2 µm in diameter; this figure results from different published data, for example from Schopfer and Brennicke 2006 and Taiz and Zeiger 2007). Parenchyma is living tissue, it contains complete protoplasts. These cells keep their ability to divide and most look undifferentiated. Typically, they are surrounded by intercellular spaces or larger cavities for effective gas exchange (Figure 2.1).

Figure 2.1 Parenchymatic cells in the cortex of an Echinochloa crus-galli (L.) P. Beauv. root.

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A special form of parenchymatic tissue is the chlorenchyma (Figures 2.2 and 2.3). It contains chloroplasts and is often located close to the surface of assimilating stems. The leaf mesophyll consists mainly of chlorenchyma cells. Intercellular cavities facilitate gas exchange. This is particularly pronounced in the aerenchyma of plants living in water where it consists of stellate cells, as shown in Figure 2.4.

Figure 2.2 Chlorenchyma cells in the stem of Galium aparine L. Intercellular cavities (arrow) facilitate gas exchange.

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Figure 2.3 Chlorenchyma cells from the stem of G. aparine L. with chloroplasts.

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Figure 2.4 Six-rayed stellate parenchyma cell in the pith of Juncus effusus L.

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Chapter 3

Collenchyma

Cell walls of collenchyma cells are thicker than those of parenchyma cells. Thickening is often restricted to specific areas. Such a partial thickening results in angular collenchyma when corners are thicker or in lamellar collenchyma when longitudinal walls are thicker (Figure 3.1). Collenchyma is stabilising tissues with living cells. It stays, therefore, elastic when hydrated. Cell walls consist primarily of cellulose; they are never lignified. Collenchyma is often found directly under a stem surface, that is directly under the epidermis, such as in Figure 3.2.

Figure 3.1 Mixed form of angular and lamellar collenchyma (transverse) in Cirsium arvense (L.) Scop. Cellulose is stained blue.

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Figure 3.2 Angular collenchyma of Kochia scoparia (L.) Roth. Cellulose is stained blue in a transverse stem section. The epidermis is covered by an orange-stained cuticle (arrow).

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Epidermal cells, and especially their periclinal bases, are thickened from time to time (Figures 3.3 and 3.4).

Figure 3.3 Mixed form of angular and lamellar collenchyma (transverse) in Adonis annua L. Cellulose is stained purple. The epidermis (as shown in Figure 3.4) is linked to the collenchyma by thick, periclinal cell walls forming a layer made up by cellulose.

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Figure 3.4 Lamellar thickening in transverse section of Consolida regalis Gray. Cellulose is stained blue.

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Chapter 4

Sclerenchyma, a Typical Contributor to Weediness

Sclerenchyma cells have a support function just like collenchyma cells. Their walls are thick as well but more rigid as a result of lignification. This limits solute transport and inevitably causes cell death. The lumina of sclerenchyma cells disappear, often completely. Vascular bundles are protected by sclerenchyma tissue in most monocot and dicot species (Figure 4.1).

Figure 4.1 Fibres enclosing and stabilising (protecting) the phloem of Convolvulus arvensis L. Lignin is stained red in this transverse stem section.

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In dicots, bundle caps consist mostly of sclerenchyma (Figures 4.2 and 4.3). In monocots, leaf bundles are often surrounded by a sheath, called the mesotome sheath. The cells of this layer are sclerified to a great extent. Phloem and xylem can both contain sclerenchymatic cells. There are two types of sclerenchyma cells: fibres and sclereids. Fibres are long cells usually occurring in strands. They are frequently associated with vascular bundles. They occur in bundle caps within phloem and in xylem. Sclereids can occur in many tissues: within the epidermis, within the pith, in leaves or in fruits as stone cells. They are variable in shape, that is they can be branched, isodiametric or can have lobes. Many types of sclereids are isolated from other cells. Sometimes they occur in clusters like in the pericarp of fruits (e.g. nutlike fruits contain sclereids). The stability of grass culms is based on fibres (Figure 4.4).

Figure 4.2 Fibres in stem of Conyza canadensis (L.) Cronquist, transverse section.

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Figure 4.3 Fibres (greenish) in a bundle cap above orange phloem of C. canadensis (L.) Cronquist; transverse section.

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Figure 4.4 Transverse section of Avena fatua L. Culm stability is caused by fibres (red). Vascular bundles are surrounded by sclerenchyma (red).

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Stolons or rhizomes of some perennial weeds (e.g. Cynodon dactylon and Cyperus rotundus; Figures 4.5 and 4.6) contain sclerified tissue surrounding vascular bundles. These tissues protect the plant against mechanical damage and water loss. Some Amaranthus species can survive frost due to lignified parenchyma cells and fibres (Figure 4.7). They stay erect while others collapse completely in late autumn after the first frost because they are less lignified (Costea and DeMason 2001). Some species are characterised by perivascular fibre rings in the stem (e.g. Fallopia convolvulus and Plantago species; Figures 4.8 and 4.9).

Figure 4.5 Transverse section through a stolon of Cynodon dactylon (L.) Pers.

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Figure 4.6 Transverse section through a wiry rhizome of Cyperus rotundus L. Sclereids are stained orange.

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Figure 4.7 Longitudinal section through a strand of fibres in Amaranthus retroflexus L.

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Figure 4.8 Perivascular fibre ring in Plantago lanceolata L., transverse section.

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Figure 4.9 Perivascular fibre ring in stem of Fallopia convolvulus (L.) A. Löve, transverse section.

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SECTION II

Meristematic, Secretory, Storage and Boundary Structures

5 | Meristems

6 | Secretory Structures

7 | External Secretory Structures

8 | Internal Secretory Structures

9 | Stored Compounds

10 | Epidermis

11 | Stomata

12 | Non-glandular Trichomes and Papillae

Chapter 5

Meristems

No form without matter

Plants are open systems in contrast to animals, according to Troll (1973). The organs of an animal are usually developed during the embryonic stage. A plant, however, mostly adapts its growth and development to environmental conditions. The plant body can vary in size, number of leaves, roots or in many cases of flowers and shoots. Plants have tissues with the ability to stay undifferentiated and whose only task is to produce new cells. These tissues are called meristems. Apical meristems are found either in stem or root tips (Figures 5.1, 5.2 and 5.3); lateral meristems or cambia are found in peripheral circles of stems or roots (Figures 5.4 and 5.5).

Figure 5.1 Apical meristem of Bidens pilosa Gray.

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Figure 5.2 Shoot apex of Cirsium arvense (L.) Scop.

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Figure 5.3 Tip of Galium aparine L. radicle.

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Figure 5.4 Lateral meristem of C. arvense (L.) Scop. (tangential section). Cambium cells are elongated and narrow.

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Figure 5.5 Lateral meristem of Euphorbia heterophylla L.(tangential section); notice the steeply angled cross-walls.

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Buds are derived from apical meristems (Figures 5.6 and 5.7). Some tissue-forming areas become active only later in plant development, and these are called secondary meristems. Secondary cambium within vascular bundles is called fascicular cambium, the meristem between bundles interfascicular cambium (Figures 5.8, 5.9). The vascular cambium becomes a cylinder visible as rings in transverse sections (Figures 5.10 and 5.11). Cork-producing cambia are also secondary; their actively dividing cell layer is called phellogen (Figure 5.12).

Figure 5.6 Longitudinal section through a shoot apex of C. arvense (L.) Scop. with buds.

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Figure 5.7 Flower bud of Mercurialis annua L.

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Figure 5.8 Beginning of interfascicular cambial activity (arrows) in C. arvense (L.) Scop.

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Figure 5.9 Interfascicular cambium (arrow) in the shoot of Ambrosia artemisiifolia L.

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Figure 5.10 Closed shoot cambium ring (arrow) of Abutilon theophrasti Medik. producing xylem towards the centre and phloem towards the periphery.

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Figure 5.11 Closed cambium ring of Sesbania exaltata (Raf.) Rydb. ex A.W. Hill surrounding the central xylem cylinder of the shoot.

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Figure 5.12 Cork cambium of Mimosa pudica L.

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Chapter 6

Secretory Structures

Special structures for special substances

Vacuoles often store products that are no longer involved in primary metabolic events. They may serve as an inner reservoir for the excretion of such substances later on. Many plants have developed additional structures that allow them to store or transport secondary metabolites outside cells. The evolutionary advantage of the excretion of substances (e.g. gum, mucilage, resins, crystals) is not always obvious. Some phenols and terpenoids may have antifungal or bactericidal properties, others may repel animals. Several authors are tempted to define excreted or secreted substances as those no longer needed for primary purposes. This view is only valid as long as we do not recognise the function of excreted substances. Some substances are indeed lost, such as volatile terpenoids or such as ions which are often washed off from the plant surface by rain. Mauseth (1988) defines all plant cells as ‘secretory’ as, for example, cell walls are a special form of secretion. We go beyond this definition and describe in this chapter and in Chapters 7 and 8 only structures that do not occur in every plant.

Many specialised plant hairs have the function of glands. Salt glands allow Atriplex species growing on salt-containing soils to remove the salt from tissues. This property enables Atriplex sagittata Borkh. and Atriplex micrantha C.A. Meyer in Ledeb. to grow, for example, along motorways in Europe where salt is used to defrost the asphalt (Wittig 2008). Nectaries attract insects by producing sugars; they are close to the flower, and are called floral nectaries, or separated from flowers, and called extrafloral. Some plants have developed special secretory structures such as cavities or canals. Euphorbia species are known for their latex-containing laticifers (Figures 6.1 and 6.2). Many fruits of species in the family Apiaceae have typical secretory organs. Evert (2006) differentiates between external and internal secretory structures and we have followed this distinction in Chapters 7 and 8. Glandular trichomes play a major role in external secretion. The classification of trichomes is described in detail by several authors, for example Uphof and Hummel (1962) or Callow (2000). It could be that some weeds have gained a natural advantage through the storage of poisonous compounds that protect them from enemies.

Figure 6.1 Branched laticifer of Euphorbia heterophylla L., wild poinsettia, longitudinal section.

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Figure 6.2 Old laticifer of E. heterophylla L., transverse section.

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Chapter 7

External Secretory Structures

Plants have developed all kinds of surface structures for the secretion of compounds. One tool for targeted secretion are trichomes. End cells of glandular trichomes are usually filled with secondary metabolites. Many of these compounds have pharmacological activity. Today, several groups are trying to understand the genetic basis for the production of alkaloids, terpenoids and other drug precursors. In 2006, the Solanum Trichome Project was started. It is a collaborative functional genomics project funded by a National Science Foundation grant to Michigan State University, the University of Michigan and the University of Arizona (http://www.trichome.msu.edu/). One part of the project is the morphological characterisation of Solanum trichomes (Figures 7.1, 7.2, 7.3 and 7.4). Lamiaceae (=Labiatae) such as Lavandula, Mentha or Salvia have long been known as drug-producing plants. Glandular hairs of Lamium (Figures 7.5 and 7.6) were described by Metcalfe and Chalk (1965) in the last century. Bisio et al. (1999) published similar findings for Salvia blepharophylla Brandegee ex Epling. Cutter (1978) shows an electron microscope image of a Lamium trichome. According to the interpretation of this image, secretion from the head cells passes through the cell wall and accumulates under the cuticle.

Figure 7.1 Capitate glandular trichomes of Solanum nigrum L. (blue); SEM, computer-enhanced colours.

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Figure 7.2 Capitate, uniseriate, glandular trichome of S. nigrum L.; SEM.

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Figure 7.3 Capitate, uniseriate, glandular trichomes of S. nigrum L.; light microscopy.

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Figure 7.4 Capitate, uniseriate, glandular trichome of S. nigrum L., light microscopy.

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Figure 7.5 Glandular trichome of Lamium amplexicaule L.

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Figure 7.6 Capitate glandular trichome of L. amplexicaule L. Casparian-like wall thickening.

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Some hairs are constructed as stilettos or as stinging devices (Figure 7.7).

Figure 7.7 Stinging trichome of Pharbitis purpurea (L.) Voigt.

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Nectaries are external secretary structures producing sugars to attract animals. Floral nectaries are associated with pollination. Extrafloral nectaries may have different functions. They often attract ants that prey on plant herbivores (Evert 2006). Typical floral nectaries can be found in the spurge family (Euphorbiaceae) (Figure 7.8).

Figure 7.8 Floral nectaries of Euphorbia heterophylla L., wild poinsettia.

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Inflorescences of spurges are rather complicated. They are described in Chapter 43 (Secondary Reproduction Characteristics). Figure 7.9 shows the connection of anthers and inflorescence in a longitudinal section. Sugars are secreted by epithelial cells at the apex of nectaries (Figures 7.10, 7.11 and 7.12). Schnepf and Deichgräber (1984) have described the morphology and function of nectaries of several ‘non-weedy’ spurges with detailed electron microscope views.

Figure 7.9 Floral nectary of E. heterophylla L., wild poinsettia. The nectary is complete.

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Figure 7.10 Section through a floral nectary of E. heterophylla L. showing the epithelium.

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Figure 7.11 Epithelial cells in the floral nectary of E. heterophylla L., wild poinsettia.

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Figure 7.12 Surface view of E. heterophylla L. nectary with a sugar-containing droplet.

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Chapter 8

Internal Secretory Structures

Not everything stored is for consumption

Internal secretory elements can be unicellular. These are special forms of idioblasts and may be sacs or tubes. Cells with crystals are also often called idioblasts. Such cells are described in Chapter 9 (Stored Compounds). Other structures for the storage of secreted substances are extracellular cavities, for example resin ducts or ducts formed by special cells such as laticifers. Many species of the family Apiaceae have developed resin ducts in their fruit and stems. Some species of this family are used as culinary herbs, for example fennel or caraway. Young fruit of Caucalis (Figures 8.1 and 8.2) produce a very interesting scent when cut. Some representatives of the Apiaceae family are deadly poisonous, for example poison hemlock, Conium maculatum L., or Western water hemlock, Cicuta douglasii (DC.) Coult. & Rose. It appears that all secondary metabolites stored in internal secretory structures can contribute to the weediness of these plants. They are either protected against fungal attacks or avoided by animals, and benefit from a competitive advantage for this reason.

Figure 8.1 Resin duct in fruit of Caucalis platycarpos L. (longitudinal section).

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Figure 8.2 Resin duct in fruit of C. platycarpos L.(transverse section).

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Resin ducts are sometimes found in the pericarp of fruits as in Caucalis platycarpos L. (Figure 8.3), or in the cortex between collenchymatic tissues and vascular bundles. This is shown for Scandix pecten-veneris L. in Figure 8.4.

Figure 8.3 Resin duct in fruit of C. platycarpos L., transverse section.

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Figure 8.4 Resin duct in stem of Scandix pecten-veneris L., transverse section.

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In the Asteraceae family (=Compositae) similar structures can be found, for example in hairy beggarticks, Bidens pilosa L. (Figure 8.5).

Figure 8.5 Resin duct in stem of Bidens pilosa L., transverse section.

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Another kind of secretory structures in Asteraceae are laticifers. These are either single cells or a series of connected cells containing latex. Plant latex is a stable dispersion or emulsion of polymer microparticles in water. Latex seems to be important for the closure of plant wounds. Some forms of latex can cause allergies, for example latex of spurge plants (Euphorbiaceae). Laticifers are typical of Sonchus species (Figures 8.6 and 8.7).

Figure 8.6 Laticifers of Sonchus oleraceus L. at the periphery of a vascular bundle, transverse section.

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Figure 8.7 Laticifers of S. oleraceus L. at the periphery of a vascular bundle, longitudinal section.

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Another representative of the Asteraceae family is dandelion, Taraxacum officinale agg. Weber. It shows articulated laticifers in all plant parts (Figure 8.8).

Figure 8.8 Articulated laticifers of Taraxacum officinale F.H. Wigg.

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The production of plant latex in the form of natural rubber is of commercial interest. Recent molecular biology projects have analysed genes involved in the production of Taraxacum latex (Foucou 2006).

The spurge family (Euphorbiaceae) has evolved various secretory structures. One of these is non-articulated laticifers (Figures 8.9 and 8.10). These have been described by Mahlberg and Pleszczynska (1984) from a chemotaxonomical point of view. Older laticifers can be rather wide and have thick walls.

Figure 8.9 Laticifer of Euphorbia heterophylla L., wild poinsettia, transverse section.

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Figure 8.10 Laticifer cell of E. heterophylla L., wild poinsettia, longitudinal section.

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Chapter 9

Stored Compounds

Plants store carbohydrates, oils, proteins and secondary metabolites in different forms. Esau (1977) classifies these storage products as ergastic substances. Some herbicides (e.g. photosynthesis inhibitors or amino acid biosynthesis inhibitors) block pathways leading to the synthesis of such substances. The fact that plants can store them explains why they do not die immediately after the application of such herbicides. Starch is stored in large amounts in tubers and roots of various plants (Figure 9.1), allowing them to survive periods with reduced photosynthesis, dry or cold periods. Examples of storage organs are shown in Chapters 46 to 51.

Figure 9.1 Starch grains in roots of Convolvulus arvensis L. (longitudinal section).

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Salts of organic or inorganic acids are often stored in the vacuole in high concentrations. Crystals of calcium oxalate are found in many plant tissues, for example in fruits, in the cortex or in the phloem of stems. These crystals may occur in various modifications, for example as prisms (Figures 9.2 and 9.3), amorphous (Figure 9.4), druses or as ‘crystal sand’ (see Figure 33.15).

Figure 9.2 Crystals in the pericarp of Adonis annua L. (transverse section).

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Figure 9.3 Crystals in the most inner cortex layer of Mimosa pudica L. (longitudinal section).

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Figure 9.4 Crystals in the outer phloem layer of Fallopia convolvulus (L.) A. Löve L. (longitudinal section).

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Chapter 10

Epidermis

The epidermis is the outermost layer of the primary plant body (Mauseth 1988). Its surface towards the environment is covered by a cuticle that primarily consists of waxes and wax-like substances