Perspective: leaf traces

Vegetative shoots consist of stems bearing leaves. In order to develop, and to synthesize various necessary compounds required by the plant, leaves must have access to a source of water and essential minerals which are transported into them from the stem through the primary xylem. Photosynthate and other compounds synthesized in the leaves are, in turn, transported through the primary phloem into the stem and root system for storage and/or use. This transport of substances takes place in primary vascular connections between the stem vascular system and the base of leaves called leaf traces. Traces may diverge from the stem vascular system some distance below, or very near, the nodes (sites of attachment of leaves to stems) at which they enter the leaves. Leaf traces are composed of protoxylem, metaxylem, protophloem, and metaphloem, and typically contain transfer cells in both primary xylem and primary phloem. In seed plants, leaf traces are often larger and contain more tracheary cells than the vascular bundles from which they diverge, and they may increase in size distally. A leaf may be vascularized by only one or by several to many leaf traces.

In order to understand the morphology of nodal regions of shoots we must observe both transverse and median longitudinal sections through these regions. Remember that stems bear many leaves in various spatial distributions.

Perspective

Most of the major taxa of vascular plants produce secondary xylem derived from the vascular cambium. Pteridophytes (except some extinct taxa), most monocotyledons, and a few species of largely aquatic dicotyledons, however, produce only primary vascular tissues. In woody plants secondary xylem comprises the bulk of the tissue in the stems and roots. It is the most important supporting tissue in arborescent dicotyledons and most gymnosperms, and the major tissue for the transport of water and essential minerals in woody plants. Secondary xylem is a complex tissue that consists not only of non-living supporting and conducting cells but also of important living components (rays and axial wood parenchyma) which, with those in the secondary phloem, comprise a three-dimensional symplastic pathway through which photosynthate and other essential molecular substances are transported thoughout the secondary tissues of the plant (Chaffey and Barlow, 2001; see pp. 206–207 for more detail). Additional increments of this tissue are added during each growing season (usually annually), but in older regions of most woody species only the outer increments are functional in transport although the number of increments that remain functional varies greatly among different species. Older increments gradually become plugged by the deposition in them of waste metabolites such as resins, tannins, and in some species by the formation of tyloses (balloon-like extensions of axial or ray parenchyma cells into adjacent conducting cells). The inner non-functional secondary xylem is called heartwood, the outer functional secondary xylem, sapwood.

Perspective

Land plants, plants that complete their life cycle entirely in a terrestrial environment, are represented largely by bryophytes and vascular plants. In all taxa except seed plants, however, at least a thin film of water is required for fertilization; and even in two primitive groups of seed plants, the cycads and Ginkgo, fertilization is by free-swimming spermatozoids released into a liquid medium in the archegonial chamber. A few angiosperms, although terrestrial in origin, have reverted to an aquatic existence.

Vascular plants are by far the dominant groups on the Earth comprising over 255 000 species in contrast to about 22 000 species of bryophytes and approximately 20 000 species of algae. The first vascular plants appear in the fossil record in the late Silurian, about 420 million years ago, but their green algal ancestors are thought to have appeared nearly 400 million years earlier! Shared features comprise the major evidence that vascular plants, possibly also bryophytes, evolved from green algae: both synthesize chlorophylls a and b, both store true starch in plastids; both have motile cells with whiplash flagella, and both (but only a few green algae) are characterized by phragmoplast and cell plate formation following mitosis. A green alga, with these and other significant characteristics, that may provide a model of an algal ancestor of vascular plants is Coleochaete, a member of the Charophyceae.

Perspective

It is difficult to overemphasize the importance of the vascular cambium which produces secondary xylem and secondary phloem. In the following two chapters we shall discuss in detail the structure, functions, and the importance to the plant of these tissues which also have great significance for mankind. Wood (i.e., secondary xylem) is a material of which the buildings in which we live and work are constructed. It is the source of the paper on which we write, on which newspapers, magazines, and books are printed, and of many synthetic fabrics such as rayon and nylon of which our clothes are made, to name only a few of its many uses. The phloem is of the utmost importance as the tissue through which photosynthate is transported from the leaves to sites of utilization or storage in the plant. It is the availability of photosynthate which makes possible the development of nutritious, edible parts of plants, such as fruits, nuts and grains, bulbs, tubers, other edible roots, and leaves, etc., the source of so much of the food supply of humans and other organisms. It is important, therefore, that we know more about the detailed structure and activity of the vascular cambium, a lateral meristem of such great significance.

Structure of the vascular cambium

It is generally agreed that the vascular cambium is composed of a layer of cells only one cell thick, and that all of these cells are meristematic cambial initials from which cells of the secondary xylem and phloem are derived.

Perspective

Except in the very youngest regions, the stems and roots of woody plants (specifically, gymnosperms and dicotyledons) are covered by bark consisting of the functional secondary phloem and rhytidome, a complex tissue comprised of successively formed periderms, often of overlapping shell-like morphology, between which are enclosed dead cortical and/or phloem tissues. The outer covering of stems of large monocots differs from that of woody dicotyledons and will be discussed later. The outer bark, consisting primarily of rhytidome, is a protective layer which restricts entrance of both insects and microorganisms and also protects the inner living tissues from temperature extremes. It also inhibits water loss through evaporation, but at the same time allows gaseous exchange through specialized regions in the periderm called lenticels. In addition it supplements the secondary xylem in stiffening young stems (Niklas, 1999), thus, contributing to their ability to withstand the bending forces exerted by excessive wind and/or the weight of ice.

Periderm: structure and development

Periderm consists of phellem and phelloderm, both derived from a single-layered secondary meristem, the phellogen (Fig. 13.1a, b). Cells of the phellogen are tabular, radially thin, somewhat elongate, and polygonal as viewed tangentially. In many plants the phellogen forms at about the same level in the stem and at about the same time as the vascular cambium. The site of its initiation is highly variable but often is an outer layer of cortical parenchyma one or two layers beneath the epidermis (Fig. 13.1b).

Perspective

Nearly all plant cells are characterized by an enclosing, cellulosic wall. Those that are not, such as gametes, are either very short-lived or are protected by enclosure within a sheath or tissue of walled cells. In addition to its vital role in communication between cells, the wall serves both supporting and protective functions. Cell walls were first observed, in cork, by Robert Hooke in 1663 and considered to be “dead” structures. Furthermore, the cell wall, produced by and to the exterior of the protoplast, has been considered by some biologists to be an extracellular structure. Most botanists, however, have persisted in considering the wall to be the outer part of the cell, a view based largely on the integration of cytokinesis and cell wall formation. Strong justification for this viewpoint has been provided by research during the past several decades which has shown that the wall is a dynamic structure that receives biochemical information from the protoplast and sends information to it. Recent studies suggest that the wall is an integral component of a cell wall–plasma membrane–cytoskeleton continuum which provides a pathway for molecular and mechanical signals between cells in a tissue, or between cells and the external environment (Wyatt and Carpita, 1993; Reuzeau and Pont-Lezica, 1995; see also Wojtaszek, 2000). Major components of this continuum are plasmodesmata, highly specialized regions of endoplasmic reticulum which traverse the walls and connect the protoplasts of adjacent cells, microtubules, thought to play important roles in determining the orientation of cellulose microfibrils in the cell wall (Baskin, 2001), and actin microfilaments which have been implicated in cytoplasmic streaming and in the transport of vesicles containing precursor compounds to the sites of wall synthesis (Chaffey et al., 2000).

Perspective

Among the unusually interesting and unique aspects of plants is their indeterminate mode of growth. This results from the presence of apical meristems by which new cells and tissues are added to the plant body during every period of growth. As a consequence plants have the potential to increase in size at regular intervals throughout their lives. This accounts for the large size of some plants such as the redwoods of California as well as many hardwood tree species of temperate and tropical forests.

A meristem is a localized region of tissue which, by cell division, adds new cells to a plant or plant part. In the shoots of vascular plants the activity of meristems results in an increase in length and/or diameter, and following cell growth and differentiation, formation of the various mature tissue regions of the axes as well as the formation of organs such as leaves, cone scales, sporophylls, stipules, flower parts, etc. Some meristems are self-perpetuating and thus, can be considered to be “permanent” meristems. Most apical meristems and the vascular cambium are meristems of this type and, as a result of their activity, provide vascular plants with their mode of indeterminate growth. Others, such as the meristems that contribute to the formation of the petiole and blade of leaves, flower parts, and the various other lateral appendages of non-seed plants, cease functioning when these organs, characterized by determinate growth, reach their genetically predetermined size and form.

Perspective: the plant life cycle

Reproduction in higher plants is relatively complex, involving a life cycle consisting of two phases, a diploid sporophyte phase and a haploid gametophyte phase, comprising what is called an alternation of generations. The prominent bodies of angiosperm trees, shrubs, perennials, and annuals as well as those of gymnosperms, ferns, sphenophytes, and lycophytes are sporophytes, having developed from fertilized egg cells (zygotes). The gametes which fused to form the zygotes, however, were produced by gametophytes, very small plant bodies, parasitic on the sporophytes in seed plants, but somewhat larger and free-living in pteridophytes (except in heterosporous species in which gametophytes when mature remain, at least in part, within the the walls of the spores from which they develop).

The sporophyte in pteridophytes is dominant, and although dependent initially for its nutrition on the gametophyte, soon becomes independent. The gametophyte is much reduced in size but is free-living and either autotrophic or saprophytic. In seed plants, the sporophyte is also dominant and initially dependent on the gametophyte, but soon becomes independent. The gametophyte is greatly reduced, however, and parasitic on the sporophyte. In angiosperms it is exceptionally small, consisting in many taxa of only seven cells and eight nuclei, and can be observed only with a microscope.

The life cycle of a vascular plant can be summarized as follows. The sporophyte produces specialized cells called sporocytes that undergo meiosis producing haploid spores. The spores germinate to form the gametophytes in which gametes are produced.

Perspective: evolution of the phloem

With increase in the size of plants over geologic time, efficient systems for the transport of water and minerals (primary and secondary xylem) as well as for photosynthates, hormones and other substances (primary and secondary phloem) evolved (see Chapter 1). The protoplasts of differentiating conducting cells of the xylem (tracheids and vessel members) were eliminated through autolysis, thus providing at functional maturity open, but non-living, passageways through which water could be pulled upward and out through the leaves by the force of transpiration (see Chapter 11). Evolution in the phloem took a different course. An open, but living, system of interconnected tubes, formed by overlapping sieve cells in gymnosperms (and more primitive vascular plants), and superposed sieve tube members forming sieve tubes in angiosperms evolved. The protoplasts of sieve elements became degraded, losing the nucleus, tonoplast (vacuolar membrane) and all other organelles except some mitochondria and endoplasmic reticulum. In conifers and dicotyledons, distinctive plastids and P-proteins (phloem proteins) evolved and, with the mitochondria and ER, became located peripherally in the cells. Concurrently, plasmodesmata which connected contiguous sieve tube members evolved into open pores, thus forming a symplastic system of essentially unimpeded passageways (Ehlers et al., 2000) through which photosynthate and other molecular materials are transported throughout the plant. Although living, but because of the loss of the nucleus, the sieve elements were no longer able to control their genetic and metabolic activities.

Perspective: role of the vascular cambium

As the vascular cambium becomes active and secondary tissues are formed, the consequent increase in diameter of the stem may have profound effects on the primary body. This is especially true in woody, arborescent taxa among conifers and dicotyledons. The vascular cambium is an extensive, permanent secondary meristem, one cell thick, conical in form, often described as cylindrical, that begins its development between primary xylem and primary phloem. In most gymnosperms and dicotyledons it is present in all main stems and roots and their branches, extending from near their tips to the bases of stems and roots. In some woody plants it even extends into leaf petioles. In most woody taxa it differentiates first in developing vascular bundles at about the same time as metaxylem begins its development (Figs. 9.1a, b, 9.2), that is, after elongation in the provascular strands has ceased. This fascicular cambium may become active, producing some secondary xylem and phloem before cambial differentiation occurs between the bundles (Fig. 9.1b), that is, in the interfascicular regions. In many woody, arborescent taxa, additional provascular strands differentiate between the initial vascular bundles, often so close together that they may contact each other laterally (Fig. 9.1b, c). The vascular cambium then becomes continuous across the vascular bundles (Fig. 9.1c, d).

Perspective

Waste products in animals are excreted to the exterior through the digestive system, the urinary system, and, to a lesser extent, through sweat glands. By contrast, in the plant, waste products of metabolism as well as substances that will be further utilized are stored within individual cells or transferred to regions of living or non-living tissues or into cavities and ducts within the organism. A good example is the transfer of waste metabolites into the secondary wood (with the consequent formation of heartwood) where they are isolated from the functional regions of the plant body. The transfer of metabolites from one site to another is referred to as secretion rather than excretion although some substances are transferred to the plant surface, such as precursor compounds of cutin and waxes and a variety of substances that exit the plant through glands and glandular hairs. This concept of secretion also includes the transfer of substances within single cells such as, e.g., the movement of enzymes to chloroplasts, sites of photosynthesis, and the transport in vesicles of precursors of cellulose to sites of wall synthesis. We can, thus, define secretion in plants as the transfer of certain intermediate or end products of metabolism from one region to another within the cell or out of the protoplast to another part of the plant body.

Perspective

The primary vascular system extends throughout the root system, the stem and its lateral branches, and appendages of the stem such as leaves, flowers, and fruits. The basic pattern of the primary vascular system is established initially by the arrangement of provascular tissue in the embryo. As development of the young plant proceeds, the provascular tissue becomes restricted to the shoot apex and to the root tip proximal to the root cap. Differentiation in the provascular tissue leads to the development of mature, functional primary xylem and primary phloem (Fig. 6.1). In primitive plants with central columns of primary vascular tissue (protosteles) (many pteridophytes as well as the roots of most plants), phloem surrounds the xylem (Fig. 6.1a). In those with tubular vascular systems (siphonosteles) this is usually also true, but in some taxa phloem may bound the xylem on the interior as well as on the exterior (Fig. 6.1b). In seed plants in which the primary vascular systems consist of discrete, or relatively discrete, vascular bundles (eusteles) (Fig. 6.1c, d), the spatial relationship of primary xylem and primary phloem varies according to the bundle type, i.e., whether collateral, bicollateral, amphicribral, or amphivasal. In collateral bundles, the primary xylem comprises the part of the bundle toward the inside of the stem and the primary phloem comprises the outer part (Figs. 6.1, 6.2, 6.4) whereas in bicollateral bundles phloem occurs both to the inside and to the outside of the primary xylem.

Perspective: origin of multicellularity

Since early in the study of plants botanists have been interested in the structure, function, development, and evolution of cells, tissues, and organs. Because some green plants are very small and unicellular, but others are large and multicellular, the origin of multicellularity in plants also has been of great interest to botanists. Among the green algae from which higher plants are thought to have evolved, some colonial taxa such as Pandorina, Volvox, and relatives consist of aggregations of motile cells that individually appear identical to apparently related unicellular forms (Fig. 2.1). Consequently, it was concluded early in the history of botany, and widely accepted, that multicellular plants evolved by the aggregation of unicellular organisms. This viewpoint led to the establishment of the cell theory of multicellularity in plants which proposes that cells are the building blocks of multicellular plants (Fig. 2.2). As early as 1867, however, Hoffmeister proposed that cells are simply subdivisions within an organism. This viewpoint, supported and expanded upon in 1906 by Lester Sharp at Cornell University, has been elucidated and clarified more recently by Hagemann (1982), Kaplan (1992), and Wojtaszek (2000) among others.

Perspective: evolution of the leaf

All vascular plants except their most primitive ancestors are characterized by leaves (see Chapter 1). As the primary photosynthetic organs, leaves are of great significance not only to the plant but also to many other organisms, including humans, that rely on plants as a source of food. Botanists interested in the evolution of plant structures believe that leaves evolved in at least two ways, and in possibly five independent lines in vascular plants (see Niklas, 1997). The leaves of lycophytes are considered enations because they are thought to have evolved as simple outgrowths from stems. These leaves, often referred to as microphylls, are commonly small although those of some extinct taxa attained great lengths (up to 1 meter in some lepidodendrids). Like all microphylls, however, they were vascularized by only a single midvein. In seed plants and ferns (possibly also in sphenophytes) leaves are thought to represent evolutionarily modified lateral branch systems. This hypothesis (the telome hypothesis) is based on the fact that the earliest seed plant ancestors were leafless, but bore small lateral branch systems. The fossil evidence indicates that over time, three-dimensional branch systems became flattened and subsequently laminate. Seed plant leaves which, on average, are much larger, and much more complex than those of lycophytes in both gross morphology and internal structure, are often referred to as megaphylls. For more detailed discussions of the evolution of leaves see Steward and Rothwell (1993) and Taylor and Taylor (1993).

Perspective

Most terrestrial plants live in a highly evaporative environment and one in which they are constantly exposed to toxic substances, to attack and invasion by various small insects and pathogens, to the potentially damaging effects of solar radiation, and to potential damage from high winds. Consequently, several protective tissues have evolved that reduce water loss from the plant, restrict the entry of organisms and toxic substances into the plant body, mitigate the effects of radiation, and strengthen and support the plant thereby reducing its susceptibility to damage from rapid air movement. These include the epidermis of shoot and root systems (sometimes called rhizodermis in the root), the periderm and the rhytidome. These tissues, while providing these functions, must also under certain conditions allow oxygen used in respiration to enter the plant and carbon dioxide utilized in photosynthesis to exit the plant. Consequently, the epidermis and other surficial, protective tissues represent both structural and functional compromises. As the bounding tissue of all young parts of a plant, and of the aerial parts of plants that are comprised solely or largely of primary tissues, the epidermis also provides an important supporting function. In the stem of Tulipa (tulip), for example, the epidermis plus a layer of subepidermal collenchyma can contribute as much as 50% to overall stem stiffness (Niklas and Paolillo, 1997). We shall consider the epidermis in some detail in this chapter, and periderm and rhytidome in Chapter 13.

Perspective: evolution of the root

The anatomy of the root reflects its origin, its subterranean environment, and its function. The first vascular plants (Rhyniophyta) lacked roots, and absorption of water and nutrients was facilitated by rhizoids. Roots evolved in the seed plant clade (rhyniophytes, trimerophytes, progymnosperms, seed plants) as well as in lycophytes, sphenophytes, and ferns in response to the pressures of a land environment, enhanced by increasing plant size. During their evolution important functions such as anchorage, absorption and transport of minerals and water, and storage of photosynthate were established. In some ways, however, roots changed relatively little through time. This is the result of the subterranean environment in which they evolved, and the fact that roots were, thus, not exposed to the same intense selection pressures as stems.

The seed plant root (Fig. 16.1a, b) is considered by most researchers to be an evolutionarily modified stem although it has also been suggested that it might be an entirely new organ that evolved independently of the stem. The predominant view is supported by the fact that the structure of the root of extant plants is remarkably similar to the anatomy of the stem of their ancestors. Even in many plants with stems that feature specialized siphonostelic or eustelic structure, the roots are protostelic (Fig. 16.1b), also a feature of the stems of very primitive plants. Roots with central piths have an alternate arrangement of xylem and phloem that may reflect a protostelic origin.

Introduction

Tropical rainforests are renowned for their ecological complexity and the seeming ubiquity of coevolved relationships among species (Janzen 1969; Gilbert 1980). Unfortunately, these forests are being destroyed and fragmented at alarming rates, to the extent that many tropical protected areas are becoming virtual islands in a sea of heavily degraded land (Laurance & Bierregaard 1997; DeFries et al. 2002). The future of tropical biodiversity will be largely determined by the extent to which natural ecological processes and communities can be maintained in isolated fragments of forest.

Here I summarize available information on the alteration of biotic interactions, such as predation and key symbioses like pollination and seed dispersal, in fragmented tropical forests. This review is necessarily preliminary, given the great diversity of species and ecological interactions in the tropics and the fact that the alteration of biotic linkages is among the most poorly understood consequences of habitat fragmentation.

Initial impacts of fragmentation

Forest fragmentation leads to the reduction and isolation of remnant forest patches. Although each fragmented landscape is unique, a common pattern is that most forest fragments in human-dominated landscapes are small, ranging in size from a hectare or less to a few hundred hectares (Gascon et al. 2000; Cochrane & Laurance 2002). The biota of such small forest patches are expected to be vulnerable to edge effects and many other negative consequences of fragmentation (e.g. Laurance et al. 1998, 2002).