Premed · Premed · General Biology 2
Lecture 9: Plant Structure, Growth, and Reproduction
General Biology II — Organismal, Evolution & Ecology
Learning Objectives
By the end of this lecture, students will be able to:
- Describe the three main plant tissue systems and their functions
- Distinguish between the structure and function of roots, stems, and leaves
- Compare primary and secondary growth in plants
- Explain the mechanisms of asexual and sexual reproduction in angiosperms
- Describe fruit and seed development, dispersal, and germination
- Explain the roles of meristems in plant growth
Lecture Content
I. Plant Tissue Systems
The plant body is organized into three tissue systems that are continuous throughout all organs.
A. Dermal Tissue System
The dermal tissue system forms the outermost protective layer of the plant. In primary growth, this layer is the epidermis, a single layer of cells covered by a waxy cuticle that reduces water loss. The epidermis contains several specialized cell types: guard cells that regulate stomatal opening and closing to control gas exchange and transpiration, trichomes (hair-like outgrowths) that reduce water loss, reflect light, and deter herbivores, and root hairs -- extensions of epidermal cells in the root zone -- that dramatically increase the surface area available for water and mineral absorption. In woody plants undergoing secondary growth, the epidermis is replaced by the periderm, which includes cork cells that are dead at maturity and filled with suberin, a waterproof wax.
B. Vascular Tissue System
The vascular tissue system provides the plant's internal transport network. Xylem conducts water and dissolved minerals from roots to shoots through two types of conducting cells, both of which are dead at functional maturity and possess thick, lignified cell walls. Tracheids are elongated cells with tapered ends found in all vascular plants, while vessel elements -- wider, shorter, and open-ended -- are primarily an angiosperm innovation that allows more efficient water transport. Phloem conducts sugars (mainly sucrose) from photosynthetic source organs to consuming or storing sink organs. Sieve-tube elements, the main conducting cells of phloem, are living but lack nuclei and most organelles at maturity; they depend on adjacent companion cells for metabolic support. Xylem and phloem are packaged together into vascular bundles, which are arranged in a ring in eudicot stems and scattered throughout the ground tissue in monocot stems.
C. Ground Tissue System
The ground tissue system comprises all tissue that is neither dermal nor vascular. It consists of three cell types. Parenchyma cells are the most common plant cells -- thin-walled, metabolically active, and responsible for photosynthesis, storage, and secretion. They are totipotent, retaining the ability to divide and differentiate, which is why plant cuttings can regenerate. Collenchyma cells have unevenly thickened walls that provide flexible support in growing tissues; the strings in celery are bundles of collenchyma. Sclerenchyma cells have thick, lignified walls and are dead at maturity, providing rigid structural support. They come in two forms: fibers (long and slender, as in hemp and flax) and sclereids (short and irregular, responsible for the gritty texture of pears and the hardness of nut shells).
II. Plant Organs
A. Roots
Roots anchor the plant, absorb water and minerals, store nutrients, and engage in symbiotic relationships such as mycorrhizae. The internal structure of a root, from outside to inside, consists of the epidermis with root hairs, the cortex (composed of parenchyma cells that store starch), the endodermis (a single cell layer bearing the Casparian strip, a waxy barrier of suberin that forces water and dissolved minerals to pass through endodermal cell membranes rather than slipping between cells, thereby enabling selective filtration), the pericycle (just inside the endodermis, which gives rise to lateral roots), and the vascular cylinder or stele containing the xylem and phloem. In eudicots, the xylem forms a star-shaped core with phloem nestled between its arms; in monocots, vascular bundles are arranged in a ring around a central pith.
The root tip is organized into distinct zones. The root cap protects the delicate apical meristem, secretes mucilage to ease passage through soil, and senses gravity through the settling of dense starch-containing statoliths. Behind the cap lies the zone of cell division (the apical meristem itself), followed by the zone of elongation where cells lengthen dramatically, and finally the zone of differentiation where cells assume their mature identities. Root systems take two basic forms: the taproot system of eudicots, with one dominant root, and the fibrous root system of monocots, with many thin, branching roots. Adventitious roots arise from stems or leaves.
B. Stems
Stems provide structural support, serve as conduits for transport between roots and leaves, and can function in storage and photosynthesis. Nodes are the points where leaves attach, and internodes are the stem segments between them. At each node, an axillary bud in the leaf axil has the potential to develop into a lateral branch, while the terminal (apical) bud at the shoot tip drives primary growth in length.
C. Leaves
Leaves are the primary site of photosynthesis. A typical leaf consists of a blade (the flat, expanded portion that captures light) and a petiole (the stalk connecting it to the stem). Internally, the upper and lower epidermis, each coated with cuticle, sandwich the mesophyll tissue. The palisade mesophyll, composed of columnar cells packed with chloroplasts just below the upper epidermis, is the primary site of photosynthesis. The spongy mesophyll below it consists of loosely arranged cells with abundant air spaces that facilitate gas exchange. Vascular bundles form the branching network of veins, and stomata -- located primarily on the lower epidermis in most species -- regulate the exchange of CO2, O2, and water vapor. Leaves have been modified by evolution for many specialized functions: tendrils for climbing, spines for defense (as in cacti), fleshy storage leaves in succulents, and insect-trapping leaves in carnivorous plants.
<image>A cross-sectional diagram of a eudicot leaf. The upper epidermis with cuticle is shown at the top, followed by a layer of columnar palisade mesophyll cells (densely packed with chloroplasts), then a layer of loosely arranged spongy mesophyll cells with labeled air spaces. A vascular bundle (vein) is shown in cross-section within the mesophyll, with xylem on top and phloem on the bottom, surrounded by a bundle sheath. The lower epidermis is shown at the bottom with two guard cells flanking an open stoma; arrows indicate CO2 entering and O2 and H2O exiting through the stoma. Chloroplasts are visible as green dots within mesophyll cells.</image>
III. Primary and Secondary Growth
A. Primary Growth
Primary growth is produced by apical meristems at the tips of roots and shoots and is responsible for increasing the plant's length. The apical meristem gives rise to three primary meristems: the protoderm, which differentiates into the epidermis; the ground meristem, which produces ground tissue; and the procambium, which generates primary xylem and phloem. All plants undergo primary growth.
B. Secondary Growth
Secondary growth increases a plant's girth and is produced by lateral meristems. It occurs in woody eudicots and gymnosperms but not in monocots. The vascular cambium is a cylinder of meristematic cells that produces secondary xylem (wood) to the inside and secondary phloem to the outside. In temperate climates, the alternation between large-celled spring wood and small-celled summer wood creates visible annual growth rings. As a tree ages, older xylem in the center ceases to conduct water and becomes heartwood, darker and denser, filled with resins and tannins. The outer, lighter-colored sapwood remains functional. The cork cambium produces cork cells to the outside, which die and fill with waterproof suberin to form bark. Bark, technically, is everything outside the vascular cambium -- secondary phloem plus cork. Lenticels are pores in the bark that allow gas exchange with the living tissues beneath.
<image>A cross-section of a three-year-old woody eudicot stem showing secondary growth. The center shows pith surrounded by three visible annual growth rings of secondary xylem (wood), each ring labeled with alternating lighter (spring wood) and darker (summer wood) bands. The vascular cambium is shown as a thin ring just outside the xylem. Outside the vascular cambium is a layer of secondary phloem. Outside that, the cork cambium and cork (bark) layers are labeled. An inset magnifies the vascular cambium region showing cells dividing inward (producing secondary xylem) and outward (producing secondary phloem). Arrows indicate the direction of cell addition.</image>
IV. Asexual Reproduction in Plants
Asexual reproduction, also called vegetative reproduction, produces genetically identical offspring (clones) without the involvement of gametes or fertilization. Plants accomplish this through several mechanisms: fragmentation, where a piece of the plant breaks off and develops into a new individual; stolons (runners), horizontal above-ground stems that produce new plants at their nodes (as in strawberries); rhizomes, horizontal underground stems (as in ginger and iris); tubers, enlarged underground stems that store energy (as in potatoes, where each "eye" is an axillary bud); bulbs, underground buds with fleshy leaf bases (as in onions and tulips); and apomixis, the production of seeds without fertilization.
Asexual reproduction offers the advantages of rapid colonization, guaranteed reproduction without the need for a mate or pollinator, and faithful propagation of favorable genotypes. Its major disadvantage is the absence of genetic diversity, which leaves clonal populations vulnerable to environmental change and disease.
V. Sexual Reproduction in Angiosperms
Sexual reproduction in angiosperms begins with the development of male and female gametophytes within the flower. In the anthers, microspore mother cells undergo meiosis to produce microspores, which develop into pollen grains -- the male gametophyte. A mature pollen grain contains two cells: a tube cell and a generative cell that will divide to produce two sperm. In the ovule, a megaspore mother cell undergoes meiosis to produce one surviving megaspore (the other three degenerate), which develops into the embryo sac -- the female gametophyte. The mature embryo sac consists of seven cells and eight nuclei, including the egg cell, two synergids, three antipodal cells, and a central cell containing two polar nuclei.
Pollination -- the transfer of pollen from anther to stigma -- may occur within the same flower (self-pollination) or between different individuals (cross-pollination). Many species possess self-incompatibility mechanisms that biochemically prevent self-fertilization, promoting outcrossing and genetic diversity.
Double fertilization, unique to angiosperms, proceeds as follows. The pollen grain germinates on the stigma and grows a pollen tube down through the style to reach the ovule. Upon arrival, one sperm fuses with the egg to produce a diploid zygote (2n), while the second sperm fuses with the two polar nuclei in the central cell to produce the triploid endosperm (3n), which will nourish the developing embryo. The zygote develops into an embryo with identifiable structures: the radicle (embryonic root), hypocotyl, epicotyl, and one or two cotyledons. The ovule as a whole becomes the seed, consisting of the embryo, endosperm, and a protective seed coat.
As the seed develops, the ovary wall matures into the fruit (pericarp). Fruits come in diverse forms: simple fruits develop from a single ovary (berries, drupes, pods), aggregate fruits from the many carpels of a single flower (raspberries), and multiple fruits from many flowers clustered together (pineapples). Fruit adaptations for dispersal are equally varied -- feathery structures for wind dispersal (dandelion), buoyant shells for water dispersal (coconut), fleshy, nutritious tissues that attract animals (berries), hooks and barbs for attachment to fur (burrs), and explosive dehiscence that flings seeds mechanically (touch-me-not).
Seed germination requires water (for imbibition, the initial swelling of the seed), oxygen, and an appropriate temperature. Some seeds require additional cues such as scarification (physical abrasion of the seed coat), stratification (a period of cold treatment), or exposure to light. The radicle is the first structure to emerge, anchoring the seedling and beginning water uptake, followed by the emerging shoot.
<image>A diagram showing seed structure and germination of a eudicot (bean). Panel A: Labeled cross-section of a bean seed showing the seed coat, two cotyledons (food storage), embryonic axis with radicle (embryonic root), hypocotyl, epicotyl, and plumule (embryonic shoot). Panel B: Sequential stages of germination — (1) seed imbibition and swelling, (2) radicle emergence through the seed coat, (3) hypocotyl arch pushing through the soil, (4) cotyledons and epicotyl emerging above soil surface, (5) young seedling with first true leaves expanding and cotyledons shrinking. Arrows label each stage and direction of growth.</image>
