I. Evolution of vascular plants (seedless & seed producing):
A. To prevent desiccation:
1. spores developed thick, protective walls, & plants developed a cuticle
(waterproof, consists of cutin + waxes)
2. stomata for gas exchange.
B. Other early adaptations:
1. conducting systems (xylem & phloem) -- transport water & food
2. incorporation of lignin in cell walls of supporting & water-conducting cells
3. true roots, stems, & leaves
4. reduction of gametophyte stage; sporophyte dominates
5. seeds (= embryonic sporophyte) -- seed plants only
C. Direct ancestor -- perhaps a multicellular green alga = Coleochaete
Oldest known fossil = Cooksonia (~ 420 mya)
D. Became numerous, diverse by Devonian period (408-360 mya)
E. 9 phyla (~ 250,000 species)
II.
General chs. of seedless vascular plants:
A. All of above, except seedless
B. Flagellated sperm -- must have water to reach egg
C. Sporophyte is dominant, often long lived
D. Sporophyte & gametophyte are nutritionally independent of each other
Gametophyte is often short lived --photosynthetic in ferns & horsetails,
saprophytic in club mosses & whisk ferns
E. archegonium ---> 1 egg; antheridium ----> several sperm
Life cycle resembles that of nonvascular plants
III.
Plant body organization (all vascular plants):
A. Early sporophyte was dichotomously branched (evenly forked) -- lacked leaves & roots; had underground rhizomes
B. Organ systems later evolved:
1. root system --anchorage, water & mineral absorption
2. shoot system --stems & leaves (photosynthesis, conduction of water & minerals)
C. 3 tissue systems (continuous in all organs):
1. dermal = outer, protective covering
2. vascular = conductive tissue (xylem & phloem); embedded in ground tissue
3. ground tissue (mesophyll, cortex, etc.)
D. Primary & secondary growth
1. primary -- occurs near tips of roots & stems; initiated by apical meristems; involved in extension of plant body; primary plant body; occurs in all vascular plants!!!
2. secondary -- thickens stems & roots; produced by lateral meristems (such as vascular cambium);
secondary plant body; occurs in woody plants, such as gymnosperms, woody angiosperms, & ancient seedless vascular plants
Allows plants to be of enormous size
Many trees are almost all secondary plant body
E. Conducting cells:
1. tracheid (xylem) = elongate tracheary element with tapering ends; = the first water-conducting cells; in all vascular plants; lignified walls; dead at maturity
What are the vascular plants? Seedless vascular plants, gymnosperms, and angiosperms
2. vessel element (xylem) = more specialized water-conducting cell of angiosperms; not present in other vascular plants except the gnetophytes (type of gymnosperm); dead at maturity
3. sieve element (phloem) -- soft walled; live at maturity
F. Stele = arrangement of vascular tissues (plus pith, if present) in central cylinder of stem & root of the primary body; pith = ground tissue
1. protostele -- most primitive type: solid strand of vascular tissue (no pith); found in roots of most plants (except some monocots)
Examples? Found in stems of Psilotum & Lycopodium
2. siphonostele -- has a pith in the center & surrounded by cylinders (rings) of xylem & phloem
Stem may have leaf gaps -- places where vascular strand is missing from the siphonostele; associated with leaf traces -- vascular tissue that enters leaves
Examples? typically found in ferns & woody dicot stems
3. eustele -- has discrete bundles of vascular tissue; pith
Examples? Typically found in stems of: horsetails, ferns, some angiosperms (herbaceous dicots & monocots)
An atactostele is a special type of eustele
IV.
Origin of roots & leaves
Stem evolved first as axis of the plant
A. Roots are fairly simple & primitive in structure;evolved from underground stems (rhizomes); retained a protostele
B. Leaves = primary lateral appendages of stem
1. arise as protuberances (or primordia) from apical meristem
2. 2 major kinds of leaves:
a. microphylls -- relatively small; contain a single strand of vascular tissue
Associated with stems having protosteles; found in lycophytes
b. megaphylls -- larger; have a complex system of veins
Associated with stems having siphonosteles or eusteles; found in most plants
V.
Reproduction in vascular plants
A. Early vascular plants are homosporous - produce only one kind of spore -> typically produce bisexual gametophytes
Examples? psilophytes, horsetails, some lycophytes, & most ferns
Gametophytes develop outside spore wall
B. More advanced vascular plants are heterosporous
– produce 2 types of spores (in 2 kinds of sporangia):
microspores (from microsporangia) -- produce microgametophytes
megaspores (from megasporangia) -- produce megagametophytes
Gametophytes develop within spore wall
Examples: Selaginella, some ferns, all gymnosperms and angiosperms
C. Sporangia may be single or aggregate into a cone (= strobilus)
VI. 4 phyla of seedless vascular plants:
A. Phylum Psilotophyta (only 2 genera)
1. Psilotum (whisk fern) -- occurs in FL, LA, TX, AZ, Hawaii,& Puerto Ric
Only living vascular plant that lacks both leaves and roots!
a. dichotomously branched stem with small outgrowths; underground rhizome with rhizoids; sporangia borne on the ends of short, lateral branches; stem has a protostele
b. Has bisexual, subterranean gametophytes
2. Tmesipteris -- epiphyte; occurs in the South Pacific; has leaflike appendages
B. Phylum Lycopodophyta (club mosses)
1. 3 orders (3 families) of living species; all are herbaceous, & sporophyte has microphylls, stems, roots
2. Lycopodiaceae -- club mosses (e.g., Lycopodium); evergreen
a. sporophyte has branching rhizome from which aerial branches & adventitious roots emerge; stem & roots have protosteles
b. microphylls are spirally arranged
c. homosporous, & sporangia are borne on fertile microphylls called sporophylls. In many species, sporophylls are grouped into strobili
3. Selaginellaceae -- Selaginella (only extant genus)
a. leaves occur in 2 ranks
b. heterosporous; both kinds ofspores occur in same strobilus
c. has a ligule (scalelike growth) near base of upper surface of each microphyll & each sporophyll
d. stem & root have protosteles
4. Isoetaceae -- Isoetes (only extant genus)
C. Phylum Equisetophyta -- horsetails or scouring rushes
(Equisetum = only living genus
1. appeared during Devonian; during late Devonian & Carboniferous periods, represented by trees called calamites 18 m tall, 45 cm diameter
2. sporophyte is herbaceous; occurs in moist places, along streams,edges of woods:
a. conspicuously jointed stems, small scalelike leaves, rough texture
b. leaves are whorled at nodes, & branches (if present) occur laterally at nodes, alternating with leaves
c. internodes are ribbed & contain siliceous deposits; adventitious roots arise at nodes of rhizome
d. perennial; homosporous
spores ---> sporangia ---> sporangiophore ---> strobilus
e. gametophyte is the size of a pinhead, is green & freeliving
D. Phylum Pteridophyta -- ferns (~ 12,000 species; largest group of vascular plants next to flowering plants)
1. abundant since Carboniferous period (360 mya)
2. tropical & temperate in distribution; greatest diversity in tropics (~ 1/3 of these are epiphytes)
3. sporophyte has megaphylls, stems, adventitious roots, rhizomes; sporangia are borne on fertile leaves or a fertile segment
4. most are homosporous; 2 groups of water ferns are heterosporous
5. Order Ophioglossales (eusporangiate)
grape ferns & adder's tongues. (e.g., Botrychium & Ophioglossum)
6. Order Pteridales -- most ferns in this order; leptosporangiate; homosporous; rhizome bears new sets of fronds each year; often these are compound & have leaflets called pinnae; sporangia -- on lower surface of fronds, on specially modified leaves, or on separate stalks; often in clusters called sori; sorus may or may not be covered by flap of tissue called indusium;
prothallus = gametophyte; bisexual, flat, membranous, green, heart shaped
7. Order Marsileales & Salviniales -- water ferns
• Marsilea -- rhizome grows in mud, leaves often float
• Salvinia (duckweed) -- entire plant floats & is tiny
• Azolla -- also tiny and entire plant floats