26 the Plant Kingdom: Seedless Plants

26 The Plant Kingdom: Seedless Plants

Outline

I. Plants have adapted to life on land A. A cuticle prevents desiccation of plant tissues B. Stomata allow diffusion of carbon dioxide into the intercellular spaces of leaf tissue C. Multicellular gametangia have sterile (non-reproductive cells) as well as gametes The fertilized egg develops within the female gametangium

1 II. There are four major groups of plants A. The bryophytes are seedless and disperse via haploid spores

B. The ferns, gymnosperms, and flowering plants have vascular tissue 1. Xylem conducts water and minerals 2. Phloem conducts dissolved sugars and other materials 3. Lignin strengthens cell walls C. The seedless vascular plants disperse primarily via spores and diversified during the Silurian and Devonian periods D. The gymnosperms are vascular plants that produce seeds, typically in a cone E. The angiosperms are vascular plants which produce seeds after flowering

2 III. The plant life cycle alternates between haploid and diploid generations A. Plants exhibit alternation of generations 1. The haploid portion is the gametophyte generation a. It produces haploid gametes by mitosis in the antheridia or the archegonia b. The gametes fuse to form the diploid zygote, the first stage of the sporophyte generation 2. The diploid portion is the sporophyte generation a. The zygote develops within the archegonium b. It produces haploid spores by meiosis c. The spores divide by mitosis and develop into the gametophyte generation

3 IV. Mosses and other bryophytes are nonvascular plants A. There are over 15,000 species of mosses, liverworts, and hornworts 1. The lack of vascular tissue generally limits them to moist environments and restricts them to small size 2. This may be a polyphyletic group B. Moss gametophytes are small leafy shoots 1. Members of phylum Bryophyta are colonial plants 2. They have rhizoids that anchor the plant to the soil 3. Mosses lack true leaves, roots and stems Some have conducting cells which function similar to the vascular tissue of higher plants

4 4.Alternation of generation is apparent a. The gametophyte dominates and bears the gametangia VI. at the apex of the plant b. Some mosses have separate sexes, others bear archegonia and antheridia on the same plant c. Flagellated sperm are transported to the archegonium by splashing raindrops and swim to the archegonium where they fertilize the egg 1). Sperm may also be transported by insects 2). The archegonium secretes sucrose which attracts the sperm

d. The diploid zygote grows into a multi-cellular sporophyte by mitosis e. The sporophyte grows on, and is nutritionally dependent on the gametophyte The gametophyte has a foot, a seta (stalk) and a capsule, which contains sporogenous or spore mother cells f. The sporogenous cells divide meiotically to produce haploid spores g. The spore germinates into a protonema h. The protonema develops into the gametophyte 5. Sphagnum is the most commercially important moss a. Cells of the gametophyte hold much water and Sphagnum is used as a soil conditioner

5 C. Liverwort gametophytes are either thalloid or leafy 1.Members of phylum Hepaticophyta have a dominant gametophyte phylum Hepaticophyta have a dominant gametophyte generation like mosses, but their body form is a flattened, lobe- like thallus 2.Rhizoids anchor the plant to the soil 3.Sexual reproduction involves archegonia and antheridia on gametophytes as is seen in mosses 4. Asexual reproduction involves production of gemmae which disperse via raindrops or small animals a. Hornworts may reproduce asexually by thallus branching

D. Hornworts are inconspicuous thalloid plants 1. Members of phylum Antherocerophyta resemble liverworts but may not be closely related to them 2. Hornworts have a single disc-shaped chloroplast in each cell similar to many algae algae 3. Antheridia and archegonia are embedded in the thallus, and the sporophyte which develops after fertilization forms a “horn” 4. Meiosis occurs within sporangia, and spores are formed

E. Bryophytes are used for experimental studies Members of this group are small and easy to grow 6 V. The evolution of bryophytes is obscure A. Bryophytes may represent a side line in evolution B. Bryophytes may even have descended from vascular plants by becoming simpler and losing vascular tissue

VI. Seedless vascular plants include ferns and their allies A. There are about 11,000 species of ferns 1. Extant ferns are representatives of a once larger group (in size) 2. The fern allies are the whisk ferns, club mosses and horsetails a. These plants have vascular tissue (xylem and phloem) for conduction and support b. Most have true roots and leaves 3. Megaphylls and microphylls are the two types of leaves a. Microphylls are small and have a single vascular strand 1). Microphylls probably evolved as extensions of stems 2). The club mosses are the only extant group which possess microphylls b. Megaphylls are larger and have more than one vascular strand 1). Megaphylls probably evolved from stem branchings 2). Ferns, horsetails, and seed plants have megaphylls

B. Ferns have a dominant sporophyte generation 1. Members of phylum Pterophyta are primarily found in moist, tropical habitats; a few are aquatic 2. Alternation of generation includes a dominant sporophyte generation 3. Fern bodies consist of a rhizome (an underground stem), roots, and leaves called rhizomes a. Newly emerging fronds are fiddleheads b. All sporophyte structures contain vascular tissues 4. Spore production occurs on the undersides of the fronds in sporangia, which are typically found in clusters called son

7 5. Spores germinate and grow by mitosis into a gametophyte called the prothallus a. The prothallus is typically heart shaped and lacks vascular tissues b. The prothallus is anchored by rhizoids c. The prothallus bears antheridia and archegonia on the ventral surface d. Water is still required for transportation of the flagellated sperm to the archegonium e. The zygote grows into the free-living sporophyte plant

8 C. Whisk ferns are the simplest vascular plants 1. Members of phylum Psilotophyta lack true roots and leaves but have vascular stems 2. Whisk ferns show dichotomous branching of the stem (a primitive condition) 3. The sporangia are born on the stems and produce spores a. The spores germinate to form underground, haploid prothalli b. The prothalli have a symbiotic relationship with mycorrhizal fungi 4. Most species are extinct; the extant species are limited mostly to warmer climates

9 D. Horsetails have hollow, jointed stems 1. Members of phylum Sphenophyta were once among the dominant plants and grew as tall as modern trees a. Horsetails contributed to the coal deposits 2. Genus Equisetum is the only extant genus and grows in wet habitats 3. Horsetails have true roots, stems, and leaves 4. The stems of the horsetail have silica which makes them abrasive. Pioneers used them to clean pots and pans. “scouring rushes” 5. Strobili are the reproductive structures which bear sporangia

E. Club mosses are small plants with rhizomes and short, erect branches 1. Members of phylum Lycophyta were also dominant in the i 10 Carboniferous period 2. Extant species such as Lycopodium are small plants with true roots and stems and microphylls 3. Strobili are borne on the stems and produce the spores

F. More advanced plants are less dependent on water as a transport medium DC for reproductive cells 1. Algae typically have motile spores and sperm 2. Primitive plants have nonmotile spores but motile sperm wil 3. Advanced plants have nonmotile spores and sperm

11 G. Some ferns and club mosses are heterosporous To 1.Homosporous plants produce only one type of spore a. Bryophytes, whisk ferns, horsetails, and most ferns and club foil mosses are homosporous The 2.Heterosporous plants produce megaspores and microspores a. Some mosses and ferns are heterosporous Is t~ b. Seed plants are heterosporous The c. Microsporangia produce microsporocytes (also known as microspore mother cells) which produce microspores by The meiosis The- Microspores can develop into a male gametophyte which produces sperm in the antheridia d. Megasporangia produce megasporocytes (also known as This megaspore mother cells) which produce megaspores by Is thj meiosis The Microspores can develop into a female gametophyte the which produces eggs in archegonia 3. Heterospory was the forerunner of the evolution of seeds was the forerunner of the evolution of seeds H. Seedless vascular plants are used for experimental studies Mezn( Megasporangium

Microsporangium

VIII. Seedless vascular plants arose more than 420 million years ago The oldest megafossils of vascular plants are from the mid-Silurian period