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Last day… got started on Kingdom Plantae, general characteristics, & looked at ‘non-vascular’ groups (Hepatophyta, Anthocerophyta, & Bryophyta)
Then, started looking at characteristics of the vascular plants
Vascular plants evolved specialized roots to absorb water & minerals from the soil
- and specialized leaves, microphylls (small, single vein) in lycophytes & megaphylls (large, branched veins) in most vascular plants
Another innovation was sporophylls, modified leaves bearing sporangia
Some seedless vascular plants (and all seed plants) became heterosporous: megaspores for female gametophytes, males from microspores
Seedless vascular plants placed in 2 phyla:Phylum Lycophyta – Club mosses, spike mosses, quillworts – 1200 spp.- club mosses & spike mosses produce clusters of sporophylls called strobili- some are epiphytes (on other plants), quillworts are aquatic
Phylum Pterophyta – ferns, horsetails & whisk ferns - 12,000 species (15 horsetails, 7 ‘whisk ferns’)
Ferns widespread, but primarily in moist areas, may reach tree size- horsetails may be abundant in temperate areas, hollow stems with silica (‘scouring rush’)- whisk ferns may be ferns that lost leaves, roots?
Success of seedless vascular plants may have caused an ice age… and global warming
Ferns & other seedless vascular plants not of huge economic importance but…
Only 2 main groups of plants left : Gymnosperms & Angiosperms…
But, those 2 groups contain most living plants, the Gymnosperms & Angiosperms - seed plants seem to be monophyletic
Success of seed plants related to advantages of producing seeds: fertilized embryo & its food supply, surrounded by a protective coat
Along with seeds, 4 associated evolutionary changes:
1) reduction of gametophyte stage
2) consistently heterosporous: microspores male gametophytes, megaspores female gametophytes
- separate gametophytes will live different lives
3) Ovules & production of eggs: sporophyte produces protective layer of tissue (integument) enclosing megasporangum & its megaspore
Ovule = integument + megasporangium + megaspore
Megaspore germinates within ovule, develops into tiny
female gametophyte - produces egg which will eventually be fertilized
Seed contains tissue from three generations of plants: 1) seed coat from mature sporophyte 2) food supply from female gametophyte 3) embryo is new sporophyte generation
Seed is protected, capable of dispersal, can remain dormant for very long periods, & has food supply to begin growth
4) Pollen grains: male gametophyte surrounded by pollen wall (partly secreted by sporophyte)- a separate, multicellular generation, even if reduced to an extreme (2 cells, or 3 after germination)
Pollen grain capable of long dispersal, tough & resistant, & sperm nuclei do not require external H2O for fertilization
Seed Plants divided into 2 main groups:Gymnosperms (‘naked seeds’) & Angiosperms (‘container seeds’)
Some ancestors of seed plants found in fossil record ~ 380 MYA, & ‘seed ferns’ by 360 MYA
Carboniferous forests dominated by lycophytes, horsetails & ferns but early gymnosperms by 305 MYA
Drier conditions of Permian period favored gymnosperms over seedless vascular plants, & dominated through whole Mesozoic era
Living gymnosperms in 4 phyla, some not very diverse…
Phylum Cycadophyta – Cycads (Sago palms)- only about 130 spp. now but abundant in Mesozoic
Male and female cones on separate plants
Phylum Ginkgophyta – Ginkgos- well known as fossils (80 MYA), but only one surviving sp.- fan-like leaves, fruit with interesting odor
- turns yellow, & drops leaves abruptly
Phylum Gnetophyta – Gnetophytes- only 3 genera…
Welwitschia – one sp. in SW Africa
Ephedra – one genus w. about 40 spp. in arid areas
…and Gnetum, one genus of trees, vines or shrubs, about35 spp. in warm areas of Old World
Final phylum of gymnosperms: Phylum Coniferophyta - the conifers (including pines, spruces, cedars…) with
about 600 spp. (largest group of gymnosperms)
- often large & long-lived, dominate large areas of northern forest, & grow elsewhere in rocky or sandy soils
Next big group: Phylum Anthophyta - Angiosperms or ‘flowering plants’
- extremely diverse group with about 170,000 spp.
Flowering plants are diploid sporophytes, produce microspores and/or megaspores
- asexual reproduction, no gametes or fertilization…
- microspores develop into male gametophytes (inside pollen grain),
- megaspores develop into female gametophyte = embryo sac
Pollen transferred to stigma, normally cross-pollination
Female gametophyte now has 8 nuclei (7 cells)
- 2 sperm cells carry out double fertilization: one fertilizes egg, other fuses w. 2 nuclei in central cell
- central cell becomes triploid endosperm (food supply for seed)
Embryo, w. endosperm & integuments, develops into seed
- surrounding ovary tissue forms fruit
Angiosperms probably split from gymnosperms about 305 MYA, but subsequent history not well known- living lineages shared a common ancestor ~150 MYA
Earliest branch in phylogeny is Amborella, shrub found only on New Caledonia- lacks vessels in xylem
Other early branches include water lilies & the star anise & relatives- star anise family has female gametophyte w. only 4 nuclei, some spp. used as spices and medicines
Magnoliids are more speciose (~8,000 spp.), including magnolias, laurels, & black pepper plant
The 2 big groups of angiosperms are the Monocots (~70,000 spp.) & the Eudicots ( ~ 170,000? spp.)
Monocots include orchids, palms, lilies, grasses…
Eudicots include… lots! (oaks, peas, roses, potatoes, etc.)
A number of distinctive characteristics generally make Monocots & Eudicots fairly easy to distinguish
The value of seed plants to humans is… priceless
- six species (maize, rice, wheat, potatoes, cassava & sweet potatoes) provide 80% of calories consumed
- source of building material, fuel, pulp, etc.
- coffee, tea, spices & sugar also impt.
Many drugs from seed plants, currently or originally discovered in plants- most plants not investigated yet for potential uses
No less important to organisms around the world, at least in terrestrial habitats
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