Week 2 Summary Plant Morphology

BIOL 317 LECTURE NOTES – WEEK 2 SUMMARY

PLANT MORPHOLOGY; MAJOR LINEAGES & LIFE CYCLES OF LAND PLANTS

Plant Morphology - form or structure of a plant and its parts; vs. Plant Anatomy - cell and tissue structure of a plant.

Vegetative morphology - any portion of a plant that is involved in growth, development, photosynthesis, support, etc., but NOT involved with sexual reproduction. Example: roots, stems, leaves, seeds, etc.

Seed The seed is a young plant in which development is arrested and the plant is dormant. From the seed emerges a stem, or plumule, and a primary root, or radicle. Food is stored in the seed in the endosperm (3n) or in the cotyledons.

Roots If the primary root persists, it is called a “true root” and may take the following forms: taproot - single main root with small lateral roots; fibrous roots - many divided roots of +/- equal size. If the primary root dies (or even if it persists in some plants) new roots may arise from the stem. These are called adventitious roots. All roots on monocots are adventitious roots.

Stems At the tip of the growing shoot is a terminal bud or apical bud. A bud has bud scales surrounding it to protect the developing parts inside. Inside a bud are leaf primordia, lateral bud primordia, and the apical meristem. The apical meristem is responsible for new terminal growth. Below the terminal bud regions of the stem may be identified as follows: node - point of attachment of a leaf; lateral bud - always found in the axil of a leaf (between the leaf and stem, upward on the stem from the point of attachment of the leaf); internode - region of the stem between two nodes. Modified stems: rhizome - underground laterally growing stem (ginger); tuber - swollen elongate underground stem - modified rhizome (potato); bulb - underground stem with many swollen leaf bases surrounding it (onion); corm - swollen round underground stem (crocus); stolon – ‘runner’ or rhizome above the soil (strawberry plants).

Leaves At the point of attachment of the stem to the leaf, there sometimes is a small leaf- like structure attached to the stem called a stipule. Leaf structure: blade - the broad part of the leaf; petiole - the slender part of the leaf that attaches the blade to the stem; margin - the edge of the blade - may be smooth or variously shaped; ribs (or veins) - vascular bundles in the leaf - the central one is the midrib. Leaf shape: simple – undivided; blade compound - blade divided into leaflets, each resembling a leaf; a leaf may be compound more than once. You can tell a compound leaf from a simple leaf by looking for an axillary bud at the point of attachment of each blade. If you find one, the leaves are simple, if not, they are compound (there should be one at the base of the compound unit, however). Pinnate – veins/leaflets arranged like “pinnae” on a feather; palmate – veins/leaflets arranged like fingers radiating from the ‘palm’ of your hand. Leaves may be pinnately or palmately compound, however when there are three leaflets it may be impossible to tell which. In that case the leaves are called ternate. Dissected, or divided - blade lobed, but not comprising separate, individual leaflets. Venation (arrangement of veins): net veined (or reticulate) - a branching, network-like pattern - may be palmate or pinnate; parallel veined - primary veins all parallel (as in grass). Leaf arrangement (attachment of leaves to stem): alternate – one leaf per node; opposite – two leaves per node on opposite sides of stem; whorled – more than two leaves per node.

Reproductive morphology - any portion of a plant that is involved with or a direct product of sexual reproduction. Example: flowers, fruits, seeds, etc. Angiosperms (“clothed seed”) are characterized by the presence of flowers and, more specifically, the presence of a structure called a carpel which encloses the seed. Flowers consist of a series of parts that are modified leaves. Goethe (1749-1832) first suggested that flower parts are modified leaves.

Inflorescence - term for the flowering part of a plant; all of the flowers in aggregate. The inflorescence is attached to the stem by the peduncle. Infructescence - inflorescence after flowering is over and the fruits are maturing.

Pedicel (or peduncle if flower solitary) – modified stem leading to the flower. Receptacle - end of stem on which flower is borne.

Sepals - outer (lower) whorl of parts; often greenish - function to protect, photosynthesize, attract pollinators; calyx - collective term for sepals of one flower.

Petals - second whorl of parts; often colorful - function to attract pollinators, but often missing in wind-pollinated plants; corolla - collective term for petals of one flower.

Perianth – collective term for calyx and corolla. Tepals – term used for sepals & petals when they look alike.

Stamens - pollen producing structures - provide ‘male’ function in reproduction (pollen = male, or microgametophyte; sperm are produced here) - consist of filament supporting the anther, where pollen is produced; androecium - collective term for ‘male’ portion of flower.

Carpels – ovule-producing structures - provide ‘female’ function in reproduction (ovules contain the female, or megagametophyte; eggs are produced here) - consist of swollen ovary at base, elongate style supporting the stigma at the tip, where pollen is deposited; ovule - the egg-producing gametophyte is contained in this structure; placenta - tissue lining the ovary that bears the ovules.

Pistil - generalized term for each individual female structure in a flower. Flowers may have a single carpel or many carpels; the carpels may be separate or fused together. When single or separate, each carpel equals one pistil – this is a simple pistil. There may be one or more than one simple pistil per flower. When carpels are fused together, one pistil may consist of many carpels – this is a compound pistil. In nature, there are plants with one or more simple pistils and plants with one compound pistil, but no plants with more than one compound pistil per flower. Gynoecium - collective term for ‘female’ portion of flower (may be one or many pistils).

How can you tell if a pistil is simple or compound? There are two ways to tell that will work in almost all cases: (1) look for more than one stigma lobe at the top of the pistil; compound pistils often have separate styles or lobes on the stigma. (2) Make a cross-section of the ovary and look for compartments (locules) or more than one point of attachment of the ovules (placentae), that indicate fusion of two or more carpels.

Inside the ovary the ovules attach to the placenta in two basic ways: parietal placentation - ovules attached along the ovary wall; axile placentation - ovules attached to a central septum in the center of the ovary. Variations on axile placentation include: free central - axile with the septum missing; basal - derived from either axile or parietal, but ovules only attaching at the base of the ovary. Locule – The chamber or opening inside the ovary where the ovules are found. There is not really empty space here (although sometimes a space develops as the seeds mature in the fruit), but the ovules are only attached to the ovary wall at the placenta, thus the locule is a chamber filled with ovules.

Flowers may be: complete - all four whorls present; incomplete - one or more whorls missing; perfect - both reproductive whorls present (bisexual); imperfect - one reproductive whorl missing (unisexual).

Plants may be: hermaphroditic (or bisexual) – plants with perfect flowers; monoecious - plants with imperfect flowers, in which both sexes are on the same plant; dioecious - plants with imperfect flowers, in which each sex is on a separate plant.

In many flowers the parts in each whorl or the whorls themselves may be fused together: connate - parts of one whorl fused to each other (gamopetaly or sympetaly in the case of connate petals; syncarpy in the case of connate carpels); adnate - parts on one whorl fused to parts in another whorl (eg. epipetalous stamens - stamens adnate to petals).

The position of the ovary relative to other flower parts is often diagnostic: superior ovary – hypogynous (‘hypo-‘ means ‘under’ and refers to location of other parts relative to the ovary; inferior ovary – epigynous (‘epi’ means ‘over’). Sometimes there is a fusion of parts at the base of the flower that makes it unclear where the ovary is relative to the attachment of the other parts, and it may appear to be anywhere between fully superior and fully inferior. This situation is called a half-inferior ovary, or perigynous (‘peri-‘ means ‘around’).

Flower symmetry: radial (or regular, or actinomorphic) symmetry - many planes of symmetry; bilateral (or irregular, or zygomorphic) symmetry - one plane of symmetry.

Fruit – mature ovary. Once the eggs within the ovules have been fertilized and the ovary starts to expand, it is usually called a fruit. Functions (one or more of these may exist for same fruit): dispersal of seed, nutrition for developing embryos, protection of seed.

Land plant relationships Major groups of organisms belonging to the group Viridiplantae (= green plants). Note that we are learning one (most recent) view of land plant relationships, among several alternative theories. Some evidence provides contradictory views; eg. that ‘bryophytes’ might be related differently than shown here; or that the extant (those alive today) ‘gymnosperms’ are paraphyletic, and are related differently than shown here.

Green plants (viridiplantae) “Green Algae” - paraphyletic basal grade. Either Coleochaetales or Charales are the closest living relatives to land plants.

Land plants (embryophyta) “Bryophytes” paraphyletic group consisting of 3 monophyletic groups: liverworts, mosses, and hornworts.

Vascular plants (tracheophyta) (note that some mosses have vasculature, but not in the form of special cells called ‘tracheids’) Lycophytes - Lycopodium (club mosses), Selaginella (resurrection plant) Ferns & fern allies (sphenophytes - Equisetum (horse tails), psilophytes - Psilotum)

Seed Plants (spermatophyta) Gymnosperms – cycads, Ginkgo, gnetophytes (Gnetum, Ephedra, Welwitschia), and conifers. Extant gymnosperms are a monophyletic group according to molecular evidence, but when extinct taxa are considered, the consensus is that gymnosperms are paraphyletic. Recent evidence suggests that cycads and Ginkgo are each other’s closest living relatives, and that gnetophytes are derived within conifers.

Flowering plants, or angiosperms (magnoliophyta) Most land plants are flowering plants, and most terrestrial ecosystems are dominated by flowering plants. Class for the rest of the quarter will be focused on angiosperms.

Within angiosperms two features may each be used to define two groups: (1) number of cotyledons (seed leaves) - one or two (monocots and “dicots”); (2) number of apertures on the pollen grain - one or three. However, neither character splits the flowering plants into two monophyletic groups, and there are some families that are left out: “basal angiosperms” have two seed leaves and pollen with one aperture, so “dicots” are paraphyletic. Dicots with tricolpate pollen (3 apertures) form a large monophyletic group, sister to monocots, and are recognized by the name eudicots (sometimes also called tricolpates). Most flowering plants are eudicots.

Synapomorphies for angiosperms: (1) carpel (ovary) enclosing the ovules; (2) ovules with two integuments (bitegmic ovules - integuments surround the ovule and become the seed coat); (3) reduced female gametophyte (embryo sac typically comprising 8 nuclei); (4) triploid endosperm, the product of double fertilization (sperm + 2 polar nuclei from the embryo sac); (5) sieve tubes in the phloem (special vessel elements conducting fluids through the plant - have open ends, and stack like straws inside the plant).

Plant life cycles: alternation of generations Multicellular diploid generations alternating with multicellular haploid generations. Most animals have only a single multicellular stage in their life cycle (usually diploid).

Each plant generation (sporophyte or gametophyte) may be: free-living – depending only on itself for nutrients; dependent – depending on its parent (previous generation in the life cycle) for nutrition. Usually one generation is dominant, but in some algae both are +/- equivalent. Mosses - gametophyte dominant and free-living; sporophyte dependent. Ferns - sporophyte dominant and free-living; gametophyte free-living, but small and short-lived. Seed plants - sporophyte dominant and free-living; gametophyte dependent and reduced (gametophyte VERY reduced in angiosperms - pollen 2-3 cells; embryo sac 8 cells).

Sporophyte – the diploid (2n) generation in plants. It begins with fertilization, which produces the zygote, the first cell of the diploid generation. Cell division (mitosis) produces the embryo and, ultimately, the mature sporophyte, which makes special structures called sporangia, where meiosis takes place.

Spore – the product of meiosis; the first cell in the haploid (1n) or gametophyte generation in plants. In most animals the product of meiosis functions as a gamete.

Gametophyte – The haploid (1n) generation in plants. Flowering plants are heterosporous, meaning that they produce two different kinds of spores: (1) the microspore becomes the pollen grain (male, or microgametophyte) and (2) the megaspore becomes the embryo sac (female, or megagametophyte).

Gamete – the haploid cells that participate in fertilization, egg and sperm. These are produced by specialized structures in the gametophyte generation of plants called archegonia and antheridia, respectively (collectively called gametangia). In flowering plants the gametophytes are so reduced (pollen grain and embryo sac) that distinct gametangia are not found.

Fertilization – the act of fusion between sperm and egg to produce a zygote – must follow pollination in seed plants; fertilization occurs in all sexually reproducing organisms.

Pollination – the act of transferring pollen from anther (or male cone) to stigma (or female cone). Must take place before fertilization can occur. Only happens in seed plants.