Chapter 27: Seedless Plants

Heterospory

certain ferns and club mosses (known as spike mosses) exhibit this condition, in which they produce two types of spores: microspores and megaspores.

alternation of generations

clearly defined in plants; the process during which plants spend part of their lives in a multicellular haploid stage and part in a multicellular diploid stage.

bryophytes

consist of about 16,000 species of mosses, liverworts, and hornworts; the only living nonvascular plants.

rhizoids

each individual gametophyte plant has these tiny, hairlike absorptive structures.

Strobilus

each reproductive branch of a horsetail bears a terminal conelike _______.

Polyploids

ferns are interesting research plants for studies in genetics because they are these, having multiple sets of chromosomes. (Many ferns have hundreds of chromosomes).

Microspores

formed when microsporocytes (microspore mother cells) undergo meiosis; each one of these microscopic, haploid structures develops into a male gametophyte that produces sperm cells within antheridia.

Plant megafossils

fossilized roots, stems, leaves, and reproductive structures; currently the oldest known ________ of early vascular plants are from Silurian (420 mya) deposits in Europe.

phloem

one of two vascular tissues; it conducts dissolved organic molecules such as sugar.

xylem

one of two vascular tissues; it conducts water and dissolved minerals

Photoperiodism

plant responses to varying periods of night and day length is referred to as this.

Monophyletic group

plants are described as this; that is, all plants probably evolved from a common ancestral green alga.

Megasporangia

produce megasporocytes (also called megaspore mother cells).

sporophyte generation

produced when two gametes fuse; the diploid portion of the life cycle that produces haploid spores (the first stage in the other of the two generations) by meiosis.

archaeplastids

red algae, green algae, and land plants are collectively classified as this.

Microsporangia

sporangia that produce microsporocytes, also called microspore mother cells, which undergo meiosis to form microscopic, haploid microspores.

Sporangium

spores formed within each of these structures when meiosis occurs.

archegonia

Gametophytes also produce these female gametangia, each bearing a single egg.

wyrt

the Old English word meaning plant from which the suffix "wort" is derived.

Apical meristem

the area at the tip (apex) of a root or shoot where growth - cell division, elongation, and differentiation - occurs.

Anthoceros natans

the common hornwort, within which archegonia and antheridia are embedded in the gametophyte thallus rather than on Archegoniophores and Antheridiophores.

Marchantia polymorpha

the common liverwort; it is thalloid.

Homospory

the condition in which plants only produce one type of spore as a result of meiosis; it is characteristic of bryophytes, horsetails, whisk ferns, and most ferns and club mosses.

Prothallus

the fern gametophyte that lacks vascular tissues and has tiny, hairlike, absorptive rhizoids to anchor it.

Rhizome

the fern sporophyte consists of this horizontal underground stem that bears leaves (fronds) and true roots.

antheridia

the haploid gametophytes produce these male gametangia, in which sperm cells form.

gametophyte generation

the haploid portion of the life cycle is given this name because it gives rise to haploid gametes by mitosis.

Frond

the leaf or leaflike part of a palm, fern, or similar plant.

spores

the mature sporophyte has special cells called sporogenous cells (spore-producing cells, also called spore mother cells) that divide by meiosis to form these haploid structures.

Wattieza

the name given to the treetops of the oldest known megafossils of fernlike trees before they were realized to be part of an entire tree (including the trunks, Eospermatopteris).

Aglaophyton

the new genus into which R. major was reclassified.

gametangia

the sex organs; on most plants, they're multicellular, whereas those of algae are unicellular.

Sphagnum

Commercially, the peat mosses in this genus are the most important mosses; one of the distinctive features of this genus' "leaves" is the presence of many large, empty cells that absorb and hold water. This feature makes peak moss a useful packing material for shipping live plants as well as a good soil conditioner. Added to sandy soils, for example, peat moss helps absorb and retain moisture.

Apical cell

Ferns and other seedless vascular plants have a single large one of these located at the center tip of the apical meristem.

embryophytes

Many botanists refer to land plants as this.

peat

Over time, the organic material compresses to form this.

thallus

The body form of liverworts is often this flattened, lobed structure.

calyptra

The caplike structure that covers the capsule of some species; it is derived from the archegonium.

plant

a complex multicellular eukaryote that has cellulose walls, chlorophylls a and b in plastids, and starch as a storage product that may have cells with two anterior flagella.

Physcomitrella patens

a particularly important research organism for studying plant evolution because its features and genome can be compared with those of algae and flowering plants.

Rhynia major

a plant that grew about 50 cm (20 in.) tall and probably lived in marshes was considered by botanists for many years a classic example of a rhyniophyte. Fossils indicate that this plant had rhizoids, dichotomously branching rhizomes, and upright stems that terminated in sporangia. However, recent microscopic studies of fossil rhizomes indicate that the central core of tissue lacked the xylem cells characteristic of vascular plants, so ___________ was reclassified into a new genus, Aglaophyton, and is no longer considered a rhyniophyte.

Psilotum nudum

a representative whisk fern that has both a horizontal underground rhizome and vertical areal stems.

lignin

a strengthening polymer in the walls of cells that function for support and conduction; production of this was a key step in the evolution of vascular plants.

Indeterminate growth

a unique feature of hornworts is that the sporophytes, unlike those of mosses and liverworts, continue to grow from their bases for the remainder of the gametophyte's life. What is this characteristic called?

Hornwort

after fertilization and development, the needlelike sporophyte projects out of the gametophyte thallus, forming a spike or "horn," hence the name.

charophytes

also known as stoneworts; structural and molecular data indicate that land plants probably descended from this group of green algae.

Drosophila

an important model organism for studies of animal inheritance, development, and evolution; in some ways equivalent to Physcomitrella.

Fiddlehead

as each young frond first emerges from the ground, it is tightly coiled and resembles the top of a violin, hence this name.

Diploid

gene expression in ferns is exactly what one would expect of this sort of plant.

protonema

if a moss spore lands in a suitable spot, it germinates and grows into this filament of cells.

embryo

in plants the fertilized egg develops into this multicellular structure (young plant) within the female gametangium; this last character is one that distinguishes plants from green algae.

Antheridiophores

in some liverworts the gametangia are borne on these stalked structures, which bear antheridia.

Archegoniophores

in some liverworts the gametangia are borne on these stalked structures, which bear archegonia.

Hornworts

located within phylum Anthocerophyta, these are a small group of about 100 species of bryophytes whose gametophytes superficially resemble those of the thalloid liverworts. They live in disturbed habitats such as fallow fields and roadsides.

liverworts

located within phylum Hepatophyta, these consist of about 6000 species of nonvascular plants with a dominant gametophyte generation, but the gametophytes of some liverworts are quite different from those of mosses.

Horsetails

located within phylum Pteridophyta and about 300 mya, these were among the dominant plants and grew as large as modern trees. Because they contributed to Earth's vast coal deposits, these ancient horsetails, like ancient club mosses, are still significant today.

Whisk ferns

located within phylum Pteridophyta, only about 12 species of these exist today, and the fossil record contains several extinct species.

Sori

many species bear the sporangia (located on the fronds) in these clusters.

Club mosses

members of phylum Lycopodiophyta; these were important plants millions of years ago, when species that are now extinct often reached great size.

Ferns

members of phylum Pteridophyta; most of the 11,000 species of these are terrestrial, although a few have adapted to aquatic habitats.

Lycopodium

one of about 1200 or so species of club mosses that are living today; small (less than 25 cm, or 10 in., tall), attractive plants common in temperate woodlands.

capsule

one of the three main parts of the sporophyte; it contains sporogenous cells (spore mother cells).

seta

one of the three main parts of the sporophyte; known as the stalk.

foot

one of the three main parts of the sporophyte; this structure anchors the sporophyte to the gametophyte and absorbs minerals and nutrients from it.

Eospermatopteris

the trunks of the oldest known megafossils of fernlike trees were given this name when they were discovered in the late 1800s; not until 2007 was a fossil of an entire tree pieced together. The treetops had previously been found separate from the trunks and named Wattieza.

Psilotum

the upright stems of these are green and are the main organs of photosynthesis.

Megaspores

these haploid structures are formed when megasporocytes undergo meiosis; each of these develops into a female gametophyte that produces eggs in archegonia.

Megaphylls

these structures probably evolved from stem branches that gradually filled in with additional tissue (webbing) to form most leaves as we know today; these have more than one vascular strand, as we would expect if they evolved from branch systems.

leafy liverworts

these superficially resemble mosses, with leaflike blades, "stems," and rhizoids rather than a lobed thallus.

zygote

this is formed when sperm cells reach the female gametangium from one of many ways, and one sperm cell fertilizes the egg.

cuticle

this waxy structure covers the aerial portion of a plant; an important difference between plants and algae.

Gemmae

tiny balls of tissue formed by some liverworts that allow them to reproduce asexually; borne in the saucer-shaped structure, the gemmae cup, directly on the liverwort thallus.

stomata

tiny pores which dot the surfaces of leaves and stems of almost all plants that facilitate gas exchange.

Microphyll

usually small and has a single vascular strand; probably evolved from small, projecting extensions of stem tissue (enations); only one group of living plants, the club mosses, have these.

Rhynia gwynne

vaughanii - an example of an early vascular plant that superficially resembled whisk ferns in that it consisted of leafless upright stems that branched dichotomously from an underground rhizome; it lacked roots, although it had absorptive rhizoids.

Dichotomous branching

whenever the stem forks, or branches, it always divides into two equal halves through the process of _________ _________.