Esau's Plant Anatomy

CHAPTER ONE Structure and Development of the Plant Body—An Overview

The complex multicellular body of a vascular plant is a sisted of dichotomously branched axes without append- result of evolutionary specialization of long duration— ages, the leaf, the stem, and the root would be closely specialization that followed the transition of multicellu- interrelated through phylogenetic origin (Stewart and lar organisms from an aquatic habitat to a terrestrial one Rothwell, 1993; Taylor and Taylor, 1993; Raven, J. A. and (Niklas, 1997). The requirements of the new and harsher Edwards, 2001). The common origin of these three environments led to the establishment of morphological organs is even more obvious in their ontogeny (develop- and physiological differences among the parts of the ment of an individual entity), for they are initiated plant body so that they became more or less strongly together in the embryo as the latter develops from the specialized with reference to certain functions. The rec- unicellular zygote into a multicellular organism. At the ognition of these specializations by botanists became apex of the shoot the leaf and stem increments are embodied in the concept of plant organs (Troll, 1937; formed as a unit. At maturity, too, the leaf and stem Arber, 1950). At fi rst, botanists visualized the existence imperceptibly merge with one another both externally of many organs, but later as the interrelationships and internally. In addition, the root and the stem consti- among the plant parts came to be better understood, tute a continuum—a continuous structure—and have the number of vegetative organs was reduced to three: many common features in form, anatomy, function, and stem, leaf, and root (Eames, 1936). In this scheme, method of growth. stem and leaf are commonly treated together as a mor- As the embryo grows and becomes a seedling, stem phological and functional unit, the shoot. and root increasingly deviate from one another in their Researchers in evolution postulate that the organiza- organization (Fig. 1.1). The root grows as a more or less tion of the oldest vascular plants was extremely simple, branched cylindrical organ; the stem is composed perhaps resembling that of the leafl ess and rootless of nodes and internodes, with leaves and branches Devonian plant Rhynia (Gifford and Foster, 1989; attached at the nodes. Eventually the plant enters the Kenrick and Crane, 1997). If the seed plants have reproductive stage when the shoot forms infl orescences evolved from rhyniaceous types of plants, which con- and fl owers (Fig. 1.2). The fl ower is sometimes called

1 2 | Esau’s Plant Anatomy, Third Edition

cotyledons epicotyl

petals

sepals

C A

FIGURE 1.1 FIGURE 1.2 Some stages in development of the fl ax (Linum usitatis- Infl orescence and fl owers of fl ax (Linum usitatissimum). simum) seedling. A, germinating seed. The taproot A, infl orescence, a panicle, with intact fl owers showing (below interrupted line) is the fi rst structure to pene- sepals and petals. B, fl ower, from which the sepals and trate the seed coat. B, the elongating hypocotyl (above petals have been removed, to show the stamens and interrupted line) has formed a hook, which subsequently gynoecium. Flax fl owers usually have fi ve fertile stamens. will straighten out, pulling the cotyledons and shoot The gynoecium consists of fi ve united carpels, with fi ve apex above ground. C, after emergence above ground, distinct styles and stigmas. C, mature fruit (capsule) and the cotyledons, which in fl ax persist for about 30 days, persistent sepals. (Drawn by Alva D. Grant.) enlarge and thicken. The developing epicotyl—the stem-like axis or shoot above the cotyledons—is now apparent between the cotyledons. D, the developing epicotyl has given rise to several foliage leaves, and the taproot to several branch roots. (From Esau, 1977; drawn by Alva D. Grant.) Structure and Development of the Plant Body—An Overview | 3 an organ, but the classical concept treats the fl ower as Although the classifi cation of cells and tissues is a an assemblage of organs homologous with the shoot. somewhat arbitrary matter, for purposes of orderly This concept also implies that the fl oral parts—some of description of plant structure the establishment of cat- which are fertile (stamens and carpels) and others sterile egories is necessary. Moreover, if the classifi cations (sepals and petals)—are homologous with the leaves. issue from broad comparative studies, in which the vari- Both the leaves and the fl oral parts are thought to have ability and the intergrading of characters are clearly originated from the kind of branch systems that charac- revealed and properly interpreted, they not only are terized the early, leafl ess and rootless vascular plants descriptively useful but also refl ect the natural relation (Gifford and Foster, 1989). of the entities classifi ed. Despite the overlapping and intergrading of characters between plant parts, the division of the plant body The Body of a Vascular Plant Is Composed of Three into morphological categories of stem, leaf, root, and Tissue Systems fl ower (where present) is commonly resorted to because According to Sachs’s (1875) convenient classifi cation it brings into focus the structural and the functional based on topographic continuity of tissues, the body of specialization of parts, the stem for support and conduc- a vascular plant is composed of three tissue systems, the tion, the leaf for photosynthesis, and the root for anchor- dermal, the vascular, and the fundamental (or ground). age and absorption. Such division must not be emphasized The dermal tissue system comprises the epidermis, to the degree that it might obscure the essential unity that is, the primary outer protective covering of the of the plant body. This unity is clearly perceived if the plant body, and the periderm, the protective tissue that plant is studied developmentally, an approach that supplants the epidermis, mainly in plants that undergo reveals the gradual emergence of organs and tissues a secondary increase in thickness. The vascular tissue from a relatively undifferentiated body of the young system contains two kinds of conducting tissues, the embryo. phloem (food conduction) and the xylem (water conduction). The epidermis, periderm, phloem, and xylem are complex tissues.

❙ INTERNAL ORGANIZATION OF THE PLANT BODY The fundamental tissue system (or ground tissue system) includes the simple tissues that, in a sense, The plant body consists of many different types of cell, form the ground substance of the plant but at the same each enclosed in its own cell wall and united with time show various degrees of specialization. Paren- other cells by means of a cementing intercellular sub- chyma is the most common of ground tissues. Paren- stance. Within this united mass certain groupings of chyma cells are characteristically living cells, capable of cells are distinct from others structurally or function- growth and division. Modifi cations of parenchyma cells ally or both. These groupings are referred to as tissues. are found in the various secretory structures, which The structural variations of tissues are based on differ- may occur in the ground tissue as individual cells or as ences in the component cells and their type of attach- smaller or larger cell complexes. Collenchyma is a ment to each other. Some tissues are structurally living thick-walled tissue closely related to parenchyma; relatively simple in that they consist of one cell type; in fact, it is commonly regarded as a form of paren- others, containing more than one cell type, are chyma specialized as supporting tissue of young organs. complex. The fundamental tissue system often contains highly The arrangement of tissues in the plant as a whole specialized mechanical elements—with thick, hard, and in its major organs reveals a defi nite structural and often lignifi ed walls—combined into coherent masses functional organization. Tissues concerned with con- as sclerenchyma tissue or dispersed as individual or as duction of food and water—the vascular tissues— small groups of sclerenchyma cells. form a coherent system extending continuously through each organ and the entire plant. These tissues connect places of water intake and food synthesis Structurally Stem, Leaf, and Root Differ Primarily in with regions of growth, development, and storage. the Relative Distribution of the Vascular and The nonvascular tissues are similarly continuous, Ground Tissues and their arrangements are indicative of specifi c inter- Within the plant body the various tissues are distributed relations (e.g., between storage and vascular tissues) in characteristic patterns depending on plant part or and of specialized functions (e.g., support or storage). plant taxon or both. Basically the patterns are alike in To emphasize the organization of tissues into large that the vascular tissue is embedded in ground tissue entities showing topographic continuity, and reveal- and the dermal tissue forms the outer covering. The ing the basic unity of the plant body, the expres- principal differences in the structure of stem, leaf, and sion tissue system has been adopted (Sachs, 1875; root lie in the relative distribution of the vascular and Haberlandt, 1914; Foster, 1949). ground tissues (Fig. 1.3). In the stems of eudicotyledons 4 | Esau’s Plant Anatomy, Third Edition

primary phloem shoot apex young leaves primary xylem leaf bases

epidermis

cortex pith stem in primary growth B vascular bundles leaf trace gap primary phloem F primary xylem cortex pith procambium epidermis primary phloem fibers secondary phloem cortex vascular cambium pith secondary xylem primary xylem leaf blade

epidermis

stem in mesophyll secondary growth vascular bundles C

leaf subtending the axillary shoot midvein vascular rays

axillary shoot

phellem (cork) lateral vein pericycle rootroot inin vascular cambium secondarysecondary growthgrowth G

vascular cylinder epidermis cortex D primary phloem secondary phloem secondary xylem primary xylem epidermis vascular cylinder cortex primary phloem E endodermis pericycle rootcap A root apex H root in primary growth FIGURE 1.3 Organization of a vascular plant. A, habit sketch of ﬂ ax (Linum usitatissimum) in vegetative state. Transverse sections of stem at B, C, and of root at D, E. F, longitudinal section of terminal part of shoot with shoot apex and developing leaves. G, transverse section of leaf blade. H, longitudinal section of terminal part of root with root apex (covered by rootcap) and subjacent root regions. (A, ×2/5; B, E, F, H, ×50; C, ×32; D, ×7; G, ×19. A, drawn by R. H. Miller.) Structure and Development of the Plant Body—An Overview | 5 1 mm 1 mm

A B

FIGURE 1.4 Types of stem anatomy in angiosperms. A, transverse section of stem of Helianthus, a eudicot, with discrete vascular bundles forming a single ring around a pith. B, transverse section of stem of Zea, a monocot, with the vascular bundles scattered throughout the ground tissue. The bundles are more numerous near the periphery. (From Esau, 1977.)

(eudicots), for example, the vascular tissue forms a of the leaf (Fig. 1.5). The extensions from the vascular “hollow” cylinder, with some ground tissue enclosed system in the stem toward the leaves are called leaf by the cylinder (pith, or medulla) and some located traces, and the wide gaps or regions of ground tissue between the vascular and dermal tissues (cortex) (Figs. in the vascular cylinder located above the level where 1.3B, C and 1.4A). The primary vascular tissues may leaf traces diverge toward the leaves are called leaf appear as a more or less continuous cylinder within the trace gaps (Raven et al., 2005) or interfascicular ground tissue or as a cylinder of discrete strands, or regions (Beck et al., 1982). A leaf trace extends from bundles, separated from one another by ground tissue. its connection with a bundle in the stem (called a stem In the stems of most monocotyledons (monocots) the bundle, or an axial bundle), or with another leaf trace, vascular bundles occur in more than one ring or appear to the level at which it enters the leaf (Beck et al., scattered throughout the ground tissue (Fig. 1.4B). In 1982). the latter instance the ground tissue often cannot be Compared with the stem, the internal structure of distinguished as cortex and pith. In the leaf the vascular the root is usually relatively simple and closer to that of tissue forms an anastomosing system of veins, which the ancestral axis (Raven and Edwards, 2001). Its rela- thoroughly permeate the mesophyll, the ground tively simple structure is due in large part to the absence tissue of the leaf that is specialized for photosynthesis of leaves and the corresponding absence of nodes and (Fig. 1.3G). internodes. The three tissue systems in the primary The pattern formed by the vascular bundles in the stage of root growth can be readily distinguished from stem reﬂ ects the close structural and developmental one another. In most roots, the vascular tissues form a relationship between the stem and its leaves. The term solid cylinder (Fig. 1.3E), but in some they form a hollow “shoot” serves not only as a collective term for these cylinder around a pith. The vascular cylinder comprises two vegetative organs but also as an expression of their the vascular tissues and one or more layers of nonvas- intimate physical and developmental association. At cular cells, the pericycle, which in seed plants arises each node one or more vascular bundles diverge from from the same part of the root apex as the vascular the strands in the stem and enter the leaf or leaves tissues. In most seed plants branch, or lateral, roots attached at that node in continuity with the vasculature arise in the pericycle. A morphologically differentiated 6 | Esau’s Plant Anatomy, Third Edition

5 7 5 6 48646846575 collenchyma periderm

leaf trace

3 333 4 6 8 5 6 7 5 4 7 5 6 6 8 4 222

leaf trace gap A sympodium B 11 11

median trace lateral trace FIGURE 1.5 Diagrams illustrating primary vascular system in the stem of elm (Ulmus), a eudicot. A, transverse section of stem showing the discrete vascular bundles encircling the pith. B, longitudinal view showing the vascular cylinder as though cut through median leaf trace 5 and spread out in one plane. The transverse section (A) corresponds to the topmost view in B. The numbers in both views indicate leaf traces. Three leaf traces—a median and two lateral traces—connect the vascular system of the stem with that of the leaf. A stem bundle and its associated leaf traces are called a sympodium. (From Esau, 1977; after Smithson, 1954, with permission of the Council of the Leeds Philo- sophical and Literary Society.)

endodermis (the innermost, and compactly arranged, properties in relation to their positions in the plant body. layer of cells of the cortex in seed plants) typically sur- Some cells undergo more profound changes than others. rounds the pericycle. In the absorbing region of the root That is, cells become specialized to varied degrees. Cells the endodermis is characterized by the presence of Cas- that are relatively little specialized retain living proto- parian strips in its anticlinal walls (the radial and trans- plasts and have the capacity to change in form and func- verse walls, which are perpendicular to the surface of tion during their lifetimes (various kinds of parenchyma the root) (Fig. 1.6). In many roots the outermost layer cells). More highly specialized cells may develop thick, of cortical cells is differentiated as an exodermis, rigid cell walls, become devoid of living protoplasts, and which also exhibits Casparian strips. The Casparian cease to be capable of structural and functional changes strip is not merely a wall thickening but an integral (tracheary elements and various kinds of sclerenchyma band-like portion of the wall and intercellular substance cells). Between these two extremes are cells at varying that is impregnated with suberin and sometimes lignin. levels of metabolic activity and degrees of structural The presence of this hydrophobic region precludes the and functional specialization. Classiﬁ cations of cells and passage of water and solutes across the endodermis and tissues serve to deal with the phenomena of differentia- exodermis via the anticlinal walls (Lehmann et al., tion—and the resultant diversiﬁ cation of plant parts— 2000). in a manner that allows making generalizations about common and divergent features among related and unre- lated taxa. They make possible treating the phenomena of ontogenetic and phylogenetic specialization in a com- ❙ SUMMARY OF TYPES OF CELLS AND TISSUES parative and systematic way. As implied earlier in this chapter, separation of cells and Table 1.1 summarizes information on the generally tissues into categories is, in a sense, contrary to the fact recognized categories of cells and tissues of seed plants that structural features vary and intergrade with each without special regard to the problem of structural and other. Cells and tissues do, however, acquire differential functional intergrading of characteristics. The various Structure and Development of the Plant Body—An Overview | 7

Casparian strip endodermis less defi nitely organized formations on the surface of the plant. The principal secretory structures on plant surfaces are glandular epidermal cells, hairs, and various glands, such as fl oral and extrafl oral nectaries, certain hydathodes, and digestive glands. The glands are usually differentiated into secretory cells on the surfaces and nonsecretory cells support the secretory. Internal secretory structures are secretory cells, intercellular cavities or canals lined with secretory cells (resin ducts, oil ducts), and secretory cavities resulting from disintegra- tion of secretory cells (oil cavities). Laticifers may be placed among the internal secretory structures. They pericycle are either single cells (nonarticulated laticifers) usually much branched, or series of cells united through partial A 10 µm dissolution of common walls (articulated laticifers). Laticifers contain a fl uid called latex, which may be rich in rubber. Laticifer cells are commonly multinucleate. primary phloem primary xylem

❙ DEVELOPMENT OF THE PLANT BODY The Body Plan of the Plant Is Established during Embryogenesis The highly organized body of a seed plant represents the sporophyte phase of the life cycle. It begins its existence with the product of gametic union, the unicellular zygote, which develops into an embryo by a process known as embryogenesis (Fig. 1.7). Embryogenesis establishes the body plan of the plant, consisting of two superimposed patterns: an apical-basal pattern along B the main axis and a radial pattern of concentrically Casparian strip arranged tissue systems. Thus patterns are established in the distribution of cells, and the embryo as a whole FIGURE 1.6 assumes a speciﬁ c, albeit relatively simple, form as con- Structure of endodermis. A, transverse section of part trasted with the adult sporophyte. of a morning glory (Convolvulus arvensis) root showing The initial stages of embryogenesis are essentially position of the endodermis in relation to vascular cylin- the same in eudicots and monocots. Formation of the der consisting of pericycle, primary xylem, and primary embryo begins with division of the zygote within the phloem. The endodermis is shown with transverse walls embryo sac of the ovule. Typically the ﬁ rst division of bearing Casparian strips in focus. B, diagram of three the zygote is transverse and asymmetrical, with regard connected endodermal cells oriented as they are in A; to the long axis of the cell, the division plane coinciding Casparian strip occurs in transverse and radial walls with the minimum dimension of the cell (Kaplan and (i.e., in all anticlinal walls) but is absent in tangential Cooke, 1997). With this division the polarity of the walls. (From Esau, 1977.) embryo is established. The upper pole, consisting of a small apical cell (Fig. 1.7A), gives rise to most of the mature embryo. The lower pole, consisting of a larger basal cell (Fig. 1.7A), produces a stalk-like suspensor (Fig. 1.7B) that anchors the embryo at the micropyle, types of cells and tissues summarized in the table are the opening in the ovule through which the pollen tube considered in detail in Chapters 7 through 15. Secretory enters. Through a progression of divisions—in some cells—cells that produce a variety of secretions—do not species (e.g., Arabidopsis; West and Harada, 1993) quite form clearly delimited tissues and therefore are not orderly, in others (e.g., cotton and maize; Pollock and included in the table. They are the topics of Chapters Jensen, 1964; Poethig et al., 1986) not obviously so—the 16 and 17. embryo differentiates into a nearly spherical structure, Secretory cells occur within other tissues as single the embryo proper and the suspensor. In some angio- cells or as groups or series of cells, and also in more or sperms polarity is already established in the egg cell and ■ TABLE 1.1 Tissues and Cell Types

Tissues Cell Type Characteristics Location Function

Dermal Epidermis Unspecialized cells; guard cells Outermost layer Mechanical and cells forming trichomes; of cells of the protection; minimizes sclerenchyma cells primary plant water loss (cuticle); body aeration of internal tissue via stomata Periderm Comprises cork tissue (phellem), Initial periderm Replaces epidermis as cork cambium (phellogen), and generally protective tissue in phelloderm beneath roots and stems; epidermis; aeration of internal subsequently tissue via lenticels formed periderms deeper in bark Ground Parenchyma Parenchyma Shape: commonly polyhedral Throughout the Such metabolic (many-sided); variable plant body, as processes as Cell wall: primary, or primary and parenchyma respiration, digestion, secondary; may be lignifi ed, tissue in cortex, and photosynthesis; suberized, or cutinized pith, pith rays, storage and Living at maturity and mesophyll; conduction; wound in xylem and healing and phloem regeneration Collenchyma Collenchyma Shape: elongated On the periphery Support in primary Cell wall: unevenly thickened, (beneath the plant body primary only—nonlignifi ed epidermis) in Living at maturity young elongating stems; often as a cylinder of tissue or only in patches; in ribs along veins in some leaves Sclerenchyma Fiber Shape: generally very long Sometimes in Support; storage Cell wall: primary and thick cortex of stems, secondary—often lignifi ed most often Often (not always) dead at associated with maturity xylem and phloem; in leaves of monocots Sclereid Shape: variable; generally Throughout the Mechanical; protective shorter than fi bers plant body Cell wall: primary and thick secondary—generally lignifi ed May be living or dead at maturity Vascular Xylem Tracheid Shape: elongated and tapering Xylem Chief water-conducting Cell wall: primary and secondary; element in lignifi ed; contains pits but not gymnosperms and perforations seedless vascular Dead at maturity plants; also found in angiosperms Vessel Shape: elongated, generally not as Xylem Chief water-conducting element long as tracheids; several vessel element in elements end-on-end constitute a angiosperms vessel Cell wall: primary and secondary; lignifi ed; contains pits and perforations Dead at maturity Structure and Development of the Plant Body—An Overview | 9

■ TABLE 1.1 Continued

Tissues Cell Type Characteristics Location Function

Phloem Sieve cell Shape: elongated and tapering Phloem Food-conducting Cell wall: primary in most species; element in with sieve areas; callose often gymnosperms associated with wall and sieve pores Living at maturity; either lacks or contains remnants of a nucleus at maturity; lacks distinction between vacuole and cytosol; contains large amounts of tubular endoplasmic reticulum; lacks proteinaceous substance known as P-protein Strasburger Shape: generally elongated Phloem Plays a role in the cell Cell wall: primary delivery of substances Living at maturity; associated with to the sieve cell, sieve cell, but generally not including derived from same mother cell as informational sieve cell; has numerous molecules and ATP plasmodesmatal connections with sieve cell Sieve-tube Shape: elongated Phloem Food-conducting element Cell wall: primary, with sieve element in areas; sieve areas on end wall with angiosperms much larger pores than those on side walls—this wall part is termed a sieve plate; callose often associated with walls and sieve pores Living at maturity; either lacks a nucleus at maturity or contains only remnants of nucleus; lacks distinction between vacuole and cytosol; except for those of some monocots, contains a proteinaceous substance known as P-protein; several sieve-tube elements in a vertical series constitute a sieve tube Companion Shape: variable, generally Phloem Plays a role in the cell elongated delivery of substances Cell wall: primary to the sieve-tube Living at maturity; closely element, including associated with sieve-tube informational elements; derived from same molecules and ATP mother cell as sieve-tube element; has numerous plasmodesmatal connections with sieve-tube element

Source: Raven et al., 2005. 10 | Esau’s Plant Anatomy, Third Edition

embryo sac endosperm

20 µm two-celled proembryo20 µm suspensor with basal cell AB

protoderm emerging cotyledons

endosperm procambium

endosperm

root tip

nucellar tissue

suspensor with 50 µm 50 µm basal cell CD FIGURE 1.7 Some stages of embryogenesis in shepherd’s purse (Capsella bursa-pastoris, Brassicaceae), a eudicot, in longitudinal sections. A, two-celled stage, resulting from unequal transverse division of the zygote into an upper apical cell and a lower basal cell; B, six-celled proembryo, consisting of a stalk-like suspensor as distinct from the two terminal cells, which develop into the embryo proper. C, the embryo proper is globular and has a protoderm, the primary meristem that gives rise to the epidermis. D, the embryo at the so-called heart stage, when the cotyledons are emerging. (Note: The basal cell of the suspensor is not the basal cell of the two-celled proembryo.) Structure and Development of the Plant Body—An Overview | 11 zygote, where the nucleus and most of the cytoplasmic shoot organelles are located in the upper (chalazal) portion of apical meristem the cell, and the lower (micropylar) portion is domi- nated by a large vacuole. hypocotyl Initially the embryo proper consists of a mass of seed coat relatively undifferentiated cells. Soon, however, cell divisions in the embryo proper and the concomitant differential growth and vacuolation of the resulting cells initiate the organization of the tissue systems (Fig. 1.7C, D). The component tissues are still meristematic, but their position and cytologic characteristics indicate a relation to mature tissues appearing in the subsequently developing seedling. The future epidermis is represented by a meristematic surface layer, the protoderm. Beneath it the ground meristem of the future cortex is distinguishable by cell vacuolation, which is more pronounced here than it is in contigu- ous tissues. The centrally located, less vacuolate tissue extending through the apical-basal axis is the precur- sor of the future primary vascular system. This meristematic tissue is the procambium. Longitudinal divisions and elongation of cells impart a narrow, elongated form to the procambial cells. The protoderm, ground meristem, and procambium—the so-called primary meristems, or primary meristematic radicle tissues—extend into other regions of the embryo as embryogenesis continues. During the early stages of embryogenesis, cell divi- 100µm sion takes place throughout the young sporophyte. As root the embryo develops, however, the addition of new apical meristem cotyledons cells gradually becomes restricted to opposite ends of basal cell the axis, the apical meristems of future root and FIGURE 1.8 shoot (Aida and Tasaka, 2002). Meristems are embry- Mature shepherd’s purse (Capsella bursa-pastoris) onic tissue regions in which the addition of new embryo in longitudinal section. The part of the embryo cells continues while other plant parts reach maturity below the cotyledons is the hypocotyl. At the lower end (Chapters 5, 6). of the hypocotyl is the embryonic root, or radicle. The mature embryo has a limited number of parts— frequently only a stem-like axis bearing one or more leaf-like appendages, the cotyledons (Fig. 1.8). Because of its location below the cotyledon(s), the stem-like axis is called hypocotyl. At its lower end (the root pole), the axils of leaves produce axillary shoots (exogenous the hypocotyl bears the incipient root, at its upper end origin), which in turn have other axillary shoots. As a (the shoot pole) the incipient shoot. The root may be result of such activity, the plant bears a system of represented by its meristem (apical meristem of the branches on the main stem. If the axillary meristems root) or by a primordial root, the radicle. Similarly the remain inactive, the shoot fails to branch as, for example, apical meristem of the shoot located at the shoot pole in many palms. The apical meristem of the root located may or may not have initiated the development of a at the tip of the hypocotyl—or of the radicle, as the case shoot. If a primordial shoot is present, it is called may be—forms the primary root (ﬁ rst root; Groff and plumule. Kaplan, 1988). In many plants the primary root produces branch roots (secondary roots) (Figs. 1.1D and 1.3A) from new apical meristems originating from the With Germination of the Seed, the Embryo Resumes pericycle deep in the primary root (endogenous origin). Growth and Gradually Develops into an Adult Plant The branch roots produce further branches in turn. After the seed germinates, the apical meristem of the Thus a much branched root system results. In some shoot forms, in regular sequence, leaves and nodes and plants, notably monocots, the root systems of the adult internodes (Figs. 1.1D and 1.3A, F). Apical meristems in plant develop from roots arising from the stem. 12 | Esau’s Plant Anatomy, Third Edition

The growth outlined above constitutes the vegetative activity of lateral meristems. Growth and differentiation stage in the life of a seed plant. At an appropriate time, require synthesis and degradation of protoplasmic and determined in part by an endogenous rhythm of growth cell wall materials and involve an exchange of organic and in part by environmental factors, especially light and inorganic substances circulating by way of the con- and temperature, the vegetative apical meristem of the ducting tissues and diffusing from cell to cell to their shoot is changed into a reproductive apical meristem, ultimate destinations. A variety of processes take place that is, in angiosperms, into a fl oral apical meristem, in specialized organs and tissue systems in providing which produces a fl ower or an infl orescence. The vege- organic substances for metabolic activities. An outstand- tative stage in the life cycle of the plant is thus suc- ing feature of the living state of a plant is that its per- ceeded by the reproductive stage. petual changes are highly coordinated and occur in The plant organs originating from the apical meri- orderly sequences (Steeves and Sussex, 1989; Berleth stems pass a period of expansion in length and width. and Sachs, 2001). Moreover, as do other living organ- The initial growth of the successively formed roots and isms, plants exhibit rhythmic phenomena, some of shoots is commonly termed primary growth. The which clearly match environmental periodicities and plant body resulting from this growth is the primary indicate an ability to measure time (Simpson et al., 1999; plant body, which consists of primary tissues. In Neff et al., 2000; Alabadi et al., 2001; Levy et al., 2002; most seedless vascular plants and monocots, the entire Srivastava, 2002). life of the sporophyte is completed in a primary plant body. The gymnosperms and most angiosperms, including some monocots, show an increase in thickness of stem and root by means of secondary growth. REFERENCES The secondary growth may be a cambial secondary AIDA, M., and M. TASAKA. 2002. Shoot apical meristem formation growth resulting from production of cells by a meri- in higher plant embryogenesis. In: Meristematic Tissues in Plant stem called cambium. The principal cambium is the Growth and Development, pp. 58–88, M. T. McManus and B. E. vascular cambium, which forms the secondary vas- Veit, eds. Sheffi eld Academic Press, Sheffi eld. cular tissues (secondary xylem and secondary phloem) ALABADI, D., T. OYAMA, M. J. YANOVSKY, F. G. HARMON, P. MÁS, and causes thereby an increase in thickness of the axis and S. A. KAY. 2001. Reciprocal regulation between TOC1 and (Fig. 1.3C, D). This growth is usually accompanied by LHY/CCA1 within the Arabidopsis circadian clock. Science 293, the activity of a cork cambium, or phellogen, which 880–883. develops in the peripheral region of the expanding axis and gives rise to the periderm, a secondary protective ARBER, A. 1950. The Natural Philosophy of Plant Form. Cambridge tissue system replacing the epidermis. University Press, Cambridge. The secondary growth of the axis may be diffuse in BECK, C. B., R. SCHMID, and G. W. ROTHWELL. 1982. Stelar that it occurs by overall cell division and cell enlarge- morphology and the primary vascular system of seed plants. ment in ground parenchyma tissue without involving a Bot. Rev. 48, 692–815. special meristem restricted to a certain region of the BERLETH, T., and T. SACHS. 2001. Plant morphogenesis: Long- axis. This kind of secondary growth has been desig- distance coordination and local patterning. Curr. Opin. Plant nated diffuse secondary growth (Tomlinson, 1961). Biol. 4, 57–62. It is characteristic of some monocots, notably the palms, EAMES, A. J. 1936. 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