Exam 2 Mean = 67.8 Median = 69.0 38. In bryophytes meiosis produces the. a. sporophyte b. gametophyte c. gametes d. sporangium e. spores 1
49. Which of the following is least closely related to bryophytes? a. Seed plants b. Ferns c. Horsetails d. Green algae e. Common ancestor of all land plants Bryophytes Seedless Vascular Plants Gymno sperms Angiosperms Hornworts Liverworts Mosses Rhyniophytes Lycophytes Monilophytes Gametophytes not free living Seed and pollen Flowering Plants Land Plants Vascular Plants Seed Plants 2
From bryophytes to gymnosperm: The sporophyte generation in size moss club moss fern The gametophyte generation in size pine tree Sporophyte Not free-living Sporophyte free-living moss club moss Gametophyte free-living fern pine tree Gametophyte not free-living 3
The evolution of secondary growth in seed plants is important because it provided strength for very tall and long-lived trees that could out-compete other plants. Secondary growth in gymnosperms All gymnosperms have vascular cambium, secondary xylem and phloem, rays, and bark. The phloem contains sieve cells (instead of sieve elements and companion cells). Many gymnosperms produce resin that accumulates in resin ducts and inhibits herbivory 4
rays resin duct The wood is regular because there is no vessels (except in Gnetophytes). summerwood springwood tracheids Progymnosperms Existed around 360 million years ago. Now extinct Had secondary stem growth similar to today s conifers. However, they do not belong to gymnosperms because they produced and released spores from leaves like ferns and did not have seeds Leaves in progymnosperms 5
Leaves in progymnosperms were a. microphylls b. megaphylls c. neither microphylls nor megaphylls First seed plants: seed ferns Appeared >325 million years ago. Now all extinct Small trees with fern-like leaves that bore seeds instead of sori (clusters of sporangia) Despite the name, they belong to gymnosperms 6
Gymnosperms: gymnos naked sperma seed. They are seed plants with exposed seeds (no fruit). The seeds are single or borne on the scales of cones. Only flowering plants produce fruit What is a seed? Baby plant in a box with its lunch : seed coat, food supply, and new sporophyte Advantages of seeds: The young sporophyte no longer needs to grow and photosynthesize immediately and can go dormant Can be dispersed by air or animal (origin of nutrient-rich food for animals) The young sporophyte is protected Disadvantage: costly to make A seed is a fertilized, mature ovule 7
Evolution of ovule protecting reproduction Ovule: Spore in sporangium Sporangium in integument leaf with female sporangium and spores leaf wraps around sporangium protection = integument Fossil evidence for origin of the integument 8
Ovule consists of 3 parts: Integument: the outer most protective layer(s) of an ovule (2n). The integument has an opening at one end called the micropyle that allows the pollen tube entry Megasporangium (called a nucellus in seed plants): it is 2n sporophyte tissue and produces the megaspore mother cell (= megasporocyte) that undergoes meiosis Integument (2n) Megaspore (n) (2n) Megaspore: product of meiosis and it gives rise to the megagametophyte In the gymnosperms, the gametophytes (n) are not free living The female gametophyte is maintained in the megasporangia. The male gametophyte is the pollen and is released and travels to the female gametophyte for fertilization. 9
Ovule to seed: Integument à seed coat (2n, old sporophyte) Megasporangium à much reduced Zygote à embryo (2n, new sporophyte) Female gametophyte à food supply for the embryo (n) Evolution of seeds: 1. Heterospory - this arose independently several times. It allowed for one mobile spore - microspore and one larger nutrient laden megaspore. 2. Retention of the megaspore within the sporophyte as opposed to releasing them. 3. Reduction of megaspore number from 4 to 1. 4. Development of the 2n protective layer called the integument. 10
Just before fertilization, a gymnosperm ovule contains all of the following except: a. female gametophyte b. egg c. integument d. megaspore After fertilization, the integuments develop into the: a. seed coat. b. ovule. c. nucellus. d. micropyle. 11
The evolution of pollen is important because previously a film of water was required for sperm to swim to the archegonia. With pollen the sperm only need to swim (or move) down the pollen tube. Release of pine pollen Pollen=immature microgametophyte Mature pollen leaves the microsporangium consisting of 2-5 haploid nuclei. Pine pollen 12
When pollen reaches the ovule, it matures and produces sperm nuclei in a pollen tube. Egg is fertilized by a sperm nucleus Four groups of living gymnosperms: Cycads Ginkgos Gnetophytes Conifers 13
During the Permian (~250-300 million years ago), as environments became warmer and dryer, the conifers and cycads flourished Gymnosperms dominated the Mesozoic era (~65-250 million years ago), during which the continents drifted apart and large dinosaurs lived. Conifers ( cone bearer ) ~700 living species, the most abundant of the gymnosperms. All are trees All have seed cones (some lack woody cones - yews, junipers the fleshy covering of their seeds is not a fruit, it is integument or modified scales) 14
Conifer leaves are simple needles or scales Pine needles vascular bundle phloem xylem mesophyll endodermis resin duct Cross section of a pine needle. Showing thick cuticle, sunken stomata, no air space, epidermis. All modifications to reduce desiccation. guard cells mesophyll stomatal pore resin duct schlerenchyma epidermis 15
Structure of the male cone of pine. 16
Structure of the female cone of pine 17
The megasporangium (nucellus) produces a megasporocyte that undergoes meiosis. One megaspore survives to become the female gametophyte. The sperm nucleus is delivered by the pollen tube instead of swimming to the egg 18
Development of seeds from fertilized ovules Pollen (male gametophyte) delivers the male gametes to the ovule In the seed, the embryo is surrounded by female gamotophyte (1n) tissue. 19
20
Bristlecone Pines World's oldest living thing: 4,950 years 21
Larch and tamaracks (genus Larix) Unusual because they are deciduous (they lose all of their leaves in the Fall) Older branches have clusters of multiple needles, oneyear old branches have single or spiral needles. Cedars (genus Cedrus) needles are single Seed cones 22
Dawn redwood (Metasequoia glyptostroboides) Deciduous, was thought to be extinct but rediscovered in 1944 in China California coast redwood (Sequoia sempervirens) 23
Sierra redwood (Sequoiadendron giganteum) bald cypress (Taxodium distichum) 24
Taxaceae: Yews shrubs or trees dark, linear, pointed leaves single seeds with a freshy covering (aril) these are the only conifers to lack cones Examples English Yew (Taxus baccata) Cycads There are 125 extant species, mostly tropical They have compound, palm-like leaves. They grow slowly and live a long time (1000 years). The tissues of many species are highly toxic to humans if ingested. 25
Cycads produce seed cones and pollen cones, each on separate plants (dioecious) The seeds are eaten by birds, and dispersed in that way. The seeds are poisonous to humans causing a slow degenerative brain disease. Pollination can take up to 10 years. Some are pollinated by insects. 26
Ginkgo - one species left, only found in the wild in China, planted at monasteries in Japan and China for centuries. Now it is planted as an ornamental tree throughout the world, and all over the St. Paul campus. The fossil (left) is from the Eocene of Washington, US Ginkgo has fan-shaped leaves with dichotomous venation 27
Gingko is dioecious (male and female plants) with X and Y sex chromosomes like human Large flagellated sperm. Ovules are borne on stalks rather than cones. Gingko male cones Gingko ovules Gingko seeds (not fruit). Stinky from the fleshy seed coat Gingko embryo. 28
Gnetophytes Three genera: Ephedra, Gnetum, and Welwitschia vessel elements in xylem, unusual in being gymnosperms -ovules surrounded by fleshy layer -similarities to flowering plants represent convergent evolution -convergent evolution: independent evolution of a trait such as wings in reptiles and mammals. Ephedra viridis (Mormon tea) It has some stimulant properties. Ephedrine and pseudoephedrine are the main active ingredients. Female plant Male plant 29
Gnetum: small tropical trees Welwitchia: Native to the Namib desert of Africa. It has two leaves. It grows slowly, this plant is about 1000 years old. Female cone Male cone 30
In a gymnosperm seed, the three different generations are represented by the: a. integument, nucellus, and egg. b. seed coat, embryo, and food supply. c. seed coat, archegonium, and food supply. d. integument, ovule, and embryo. Lecture 20 study guide The word gymnosperm means naked seed. The seeds are borne on scales and fruit is not present. In gymnosperms the sporophyte is dominant, the gametophyte is not free living so we say the gametophyte is dependent. Pollen, seed, and dependent gametophyte stage first appeared in early seed plants Know that the following are living gymnosperms: Conifers, Cycads, Gingkos, and Gnetophytes and all are vascular plants and all have secondary growth (wood). Seed ferns are extinct gymnosperms. What is pollen? The microgametophyte. When did the earliest seed plants appear? >325 million years ago. 31
What is a seed? It is a plant embryo, with some stored food, enclosed in a seed coat. The stored food in gymnosperms is primarily female gametophyte tissue. In angiosperms it is endosperm. In some angiosperms the endosperm is abundant (as in maize) in others the endosperm is almost completely used up by the time the seed matures (as in beans) and the stored food is present in the cotyledons. What are the advantages to seeds? Think about the differences between young sporophytes of ferns vs. pines. The young sporophytes in gymnosperms can be dormant, can be dispersed by wind or animals, and are protected. Those are advantages of producing seeds. Know the definitions of ovule, integument, nucellus, micropyle, and megaspore as related to the gymnosperm lifecycle. Gymnosperms are heterosporous and the spores are called the microspore and the megaspore. Pollen is the immature male gametophyte in gymnosperms. When the pollen lands on a female cone it grows and matures before it produces sperm cells. In gymnosperms the megaspore develops into a multicellular female gametophyte. Some gymnosperms: Gingko - Dioecious (male and female plants), Large trees with fan shaped leaves. The male gametophytes produce flagellated sperm. Ovules are borne on stalks rather than cones. Cycads - Dioecious, long-lived, compound leaves, flagellate sperm, pollination can take up to 10 years, some pollinated by insects. Conifers - Most have woody seed cones (do not be confused about conifers with fleshy seeds such as yews they are not fruit), wind pollination, it takes about two years for conifers to produce seeds, so a long life cycle compared to angiosperms (or some earlier plants such as mosses and ferns). I gave you the lifecycle of pine as an example of gymnosperm lifecycles and you need to know the main features: dominant sporophyte generation, dependent gametophytes, the megasporangium (nucellus) produces a megasporocyte that undergoes meiosis, one megaspore survives to become the female gametophyte, development of seeds from fertilized ovules, pollen delivers the male gametophyte to the ovule, in the seed, the embryo is surrounded by female gamotophyte (1n) tissue, etc. You need to know that some gymnosperms are deciduous Ephedra, Gnetum, and Welwitschia are gnetophytes and they have vessel in xylem 32