Lecture III.6. Plants.

Simplified plant phylogeny. The word “plants” is here taken to mean “embryophytes” (aka “land plants”), i.e., multicellu- lar photoautotrophs in which one of the life cycle stages is an embryo. This definition excludes green algae, which can be multicellular, but which lack such a stage.

Introduction.

 “Plants” = “embryophytes” = “land plants”. 1. Multicellular.

2. Chlorophyll a and b cap- ture energy from light. An even simpler phylogeny.

3. Store carbohydrates.

4. Develop from an embryo pro- tected by parental tissue.

 Charophyte algae the sister group.

1. Use CaCo3 for support.

2. Multicellular. Differentiated into rhizoids, thalli, branchlets.

3. Male (antheridia), female repro- ductive structures (oogonia).

4. Spores protected from desic- cation by sporopollenin, im- A stonewort. Note portant spore and pollen grain the orange male sex organs (antheridia). coat component.

2  Embryophytes can be divided up as follows:

1. All Embryophytes:

a. Non-tracheophytes. Vascular (water / nutrient conducting) tissue absent: Liverworts, hornworts, mosses.

b. Tracheophytes. Vascular tissue present.

2. Tracheophytes:

a. Seeds absent: Clubmosses, ferns and related.

b. Seeds present: Gymnosperms, angiosperms and related.

3. Angiosperms (flowering plants):

a. Monocots. Grasses, palm trees, etc. One seed leaf.

b. Dicots. Two seed leaves. The majority of plant species.

3 A more detailed plant phylogeny. Note the principal synapo- morphies: Ability to live on land; vascular tissue; seeds and flowers. If one defines “plants” as embryophytes, the presence of chloroplasts with chlorophyll a and b is a shared ancestral character.

4 Alternation of Generations – Review.

 Multicellular haploid and diploid individuals.

 Multicellular sporophyte (2n)

1. Develops mitotically from a diploid zygote.

2. Produces haploid spores by meiosis. Alternation of generations. Compare with figure on page 3. Spores contained in struc- 18 of Lecture 3.2. tures called sporangia.

 Multicellular gametophyte (n)

1. Develops mitotically from a haploid spore.

2. Produces haploid gametes – also by mitosis.

 Fusion of two gametes results in the formation of a diploid zygote from which the sporophyte develops, etc.

5 Alternation of Generations in Plants.

 In most plants, gametes produced by structures called gametangia. Two kinds:

1. Antheridia produce sperm.

2. Archegonia produce eggs (one per archegonium).

Gametangia (singular, gametangium) protect the gametes from desiccation and mechanical damage. The embryo develops in, and is nourished by, the archegonium (plural archegonia).

6  Archegonial Zygote Retention.

1. Archegonium the site of fer- tilization.

2. Functions as a womb, in which sporophyte develops.

3. Specialized transfer cells function as a placentae. Micrograph of a liverwort gametophyte showing two 4. In flowering plants (angio- developing sporophytes and sperms), these functions surrounding transfer cells. performed by other struc- tures.

 Charophyte algae (stone- worts) have archegonial re- tention of the zygote.

1. But these algae haplontic – only the zygote is diploid.

Charophyte life cycle. The 2. Therefore, if charophytes zygote is retained in the fe- embryophyte sister group, male gametangium, which is multicellular sporophyte called an oogonium. a derived trait.

7 Moss and fern life cycles. In both groups, sperm cells swim to the egg. In mosses, the gametophyte dominates; in ferns, the sporophyte.

8  In non-tracheophytes, such as mosses, the gameto- phyte is the “plant.” The sporophyte remains attached to the gametophyte on which it is nutritionally dependent.

 In tracheophytes, the sporophyte is the “plant”.

 Embryophyte evolution marked by gametophyte reduc- tion and sporophyte enlargement.

Progressive reduction of the gametophyte and enlargement of the sporophyte. a. In hornworts (non-tracheophytes), the sporophyte sits atop, and is nourished by, the photosynthetic gametophyte. b. In horsetails (non-seed tracheophytes), the gametophyte (left) is much smaller than the sporophyte (right).

9 Evolutionary Time Line.

 Cambrian. Molecular phylogenies suggest Cambrian / Precambrian origins.

 Ordovician. Non-tracheophytes.

 Silurian. Possible tracheophytes.

 Devonian.

1. Tracheophytes for sure.

2. Trees / forests.

3. Seed plants.

 Carboniferous. Conifers.

 Cretaceous.

1. Angiosperms.

2. Darwin’s “abominable mystery”.

 Cenozoic. Angiosperms dominate. Grasslands extensive.

10 Above Left. Cooksonia (Silurian). Some species may have had xylem and phloem. Note the terminal sporangia (spore-bearing structures. Living plant was about 1½ inches tall. Above Right. Thursophyton (Devonian) with scale-like leaflets reminiscent of living clubmosses (primitive tracheophytes). Next Page. A Devo- nian forest. Acanthostega, a labyrinthodont amphibian, is shown in the foreground. The proposed (G. J. Retallack, 2011. J. Geol. 119: 235-258) origin of tetrapods in flooded woodlands supports an “eat here” hypothesis as opposed Alfred Romer’s earlier sug- gestion that land dwelling was a response to increasing aridity. Still another scenario (J. A. Clack. 2007. Int. Comp, Biol. 47: 510–523) links tetrapod evolution to increasing global anoxia, which would have been more of a problem in stagnant waters than on land.

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12 Coevolution with Other Organisms.

 Fungi – mycorrhizal associations provide nutrients.

 Herbivores. 1. Defense, e.g., phytolyths.

2. Chemical defenses – “secondary compounds.”

 Pollinators. 1. Flowers attract pollinators. Root tip, root hairs (arrows) 2. Examples: and mycorrhizae. a. Bee flowers – yellow, strong scent, concentrat- ed nectar. b. Hummingbird flowers – red, tubular c. Hawkmoth flowers – long, white tubes.

 Seed dispersers (animals) – e.g., ingestion / elimination. Datura and hawkmoth.  Mixed interactions – Hawk- moth adults pollinate Datura; larvae eat the leaves.

13 Yucca–Yucca Moth. An Obligate Mutualism.

 Fertilization requires services of the moth. 1. Pollen agglutinated1 into masses called pollinia. 2. Ovary superior, i.e., above the pollinia.

 Female moth. 1. Collects pollinia with ten- Yucca moth gathering pollen. tacles that only she has. 2. Chews them up to free the pollen grains 3. Pushes masticated pollinia down stigmatic canal. 4. Injects eggs into the plant’s ovary with special- ized ovipositor. 5. Go here to watch

1. Developing larvae eat the immature seeds; exit thru fruit wall and pupate in the soil near base of plant. Top. Yucca moth on flower. 2. Yucca plants selectively Bottom. a. seed pod; b. seed abort fruits with too many pod with larval exit holes; c. developing larvae. larva inside seed row.

1 Stuck together.

14 Transition to Land.

 Embryophyte ancestors lived in fresh water – facilitated fertilization by flagellated sperm.

 Terrestrial existence offered abundant resources:

1. Light.

2. CO2.

 And it presented problems:

1. Desiccation / gas exchange.

2. Support / transport.

3. Acquiring water / nutrients from the soil.

4. Reproduction – getting the sperm to the egg.

15 Desiccation / Gas Exchange.

 Waxy cuticle – prevents evaporative water loss.

 Stomata are pores that permit gas exchange. 1. Each pore surrounded by a pair of guard cells.

2. When the guard cells swell, the pore opens. a. Normally, pores open during the day; close at night.

b. When the plant is water-stressed, stomata remain closed during the day.

Transverse section of a leaf. Cuticle (upper and lower surface) prevents water loss. Stomata (lower surface) permit gas ex- change via guard cells that swell when exposed to light if the leaf is adequately hydrated.

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Stomatal opening and closing. In response to light and water avail- ability, potassium ions, 푲+, are pumped into the guard cells. Water follows. This causes the guard cells to bow outward, thereby open- ing the pore. The structural basis of this response is two-fold: 1. Radially oriented cellulose microfibrils prevent guard cell di- ameter from increasing. As a result, the response to water entry is stretching. 2. The thinner outer guard cell walls stretch more easily than the thicker inner walls. This causes the cells bow away from each as shown in the figure.

17 Transport and Support.

 Vascular plants have specialized structures to transport water, minerals and sugars throughout the plant.

1. Xylem conducts water / minerals from roots to leaves.

a. Series of continuous “pipes” formed from dead cells.

b. Developed gradually in the course of plant evolution.

2. Xylem also provides support via secondary cell wall.

Evolution of xylem in embryophytes. Secondary cell wall formed of a complex biopolymer called lignin, provides strength. Tracheids (cells with pores in the secondary cell wall) and vessel elements (cells with larger openings penetrating both primary and secondary cell walls) enhance transport. Phylogenetic note: Vessel elements evolved inde- pendently in gymnosperms and angiosperms.

18  Water Transport / Support in Non-Tracheophytes.

1. Some, e.g., mosses, have simple conducting cells that lack the perforated secondary cell walls of the tra- cheids found in vascular plants.

2. Many

a. Grow in dense moist masses through which water moves by capillary action.

b. Have leaf-like structures that catch and hold water.

c. Small enough for nutrients to circulate by diffusion.

3. No support, they just sprawl.

19  Water Transport in Tracheophytes.

1. In aquatic environment, plant surrounded by water. a. Cytosol ~ isotonic with seawater. b. Cell wall prevents membrane rupture in freshwater where the water flow is into the cell.

2. On land, water must be ac- quired from the soil and transported to the leaves. a. Root hairs absorb water from the soil. b. Xylem transports the wa- ter upwards.

3. Transport driven by evapo- transpiration. a. Like sucking soda through a straw. b. Evaporation provides the “suck” – tension. c. Water molecules “stick” together (cohesion). d. Result is a column of wa- ter traveling up the xylem. Top. Water transport from e. When the column gets too soil to leaves. Bottom. Red- long, it breaks. This limits wood leaves get smaller as tree height. one goes up the tree.

20  A Really Cool Experiment.

1. Observation: Leaf size decreases as one goes up a redwood tree.

2. Alternative Hypotheses:

a. Lack of water.

b. Increased sunlight (“sun” vs. “shade” leaves).

3. Suggestive observation: At 95 m., leaves of epi- phytic2 redwoods larger than those of host.

4. Experiment: Branches re- Top. Epiphyte and host tree leaves. Bottom. Host tree moved from the host tree at branches grown in pots. 90 m. above the ground and grown in pots expand. [Photos from Koch et al. 2004. Nature. 428: 851-854.]

5. Conclusion: leaf size depends on water availability and the degree to which leaves are expanded.

2 Obtain water from soil accumulated on host tree, i.e., in crooks & crannies.

21  Sucrose Transported by Phloem.

1. Sucrose actively transported from source cells into the phloem sieve tube cells. 2. Water taken up from nearby xylem by osmosis. 3. Water pressure drives sap down the sieve tube to companion cells in roots. 4. Water moves back to the xylem.

 Water transport in xylem does not require energy expenditure (water flows down concentra- tion gradient). Sucrose trans- port, does.

22 Plant phylogeny with support / transport synapomorphies. Note the in- dependent evolution (circled in red) of vessel elements in angiosperms and gnetophytes (gymnosperms).

23 Reproduction.

 Retention of embryo in archegonia reduces need for water.

1. Embryo develops in moist, internal environment. But,

2. In non-seed plants motile sperm must swim to the egg.

3. Complete independence of water not achieved until the evolution of seeds and pollen.

 In non-seed plants, bisexual gametophytes produce both antheridia and archegonia (homospory).

 In seed plants, two types of spores, and gametophytes are male or female.

1. This condition called heterospory.

2. Microsporangia produce microspores that develop into ♂♂ gametophytes that produce sperm.

3. Megasporangia produce megaspores that develop into ♀♀ gametophytes that produce eggs.

24 4. Micro- or megasporangia can be on the same or dif- ferent individuals – i.e., sporophytes can be male, female, hermaphrodite.

 Pollen Grains

1. Male gametophytes, each surrounded by a protective outer coat of sporopol- lenin (exine) and an inner coat of cellulose (intine). Pollen grain schematic.

2. Dispersed by wind or animal pollinators.

3. Produce sperm that is delivered to the female gameto- phyte.

4. Result is fertilization, a diploid embryo and a seed.

25 5. Non-angiosperm seeds contain tissue from three generations. a. Megasporangium (2n) – be- comes the seed coat.

b. Female gametophyte (n) – nu- tritive tissue.

c. Embryo (2n) – the new sporo- phyte. Development arrested un- til seed germinates.

6. In angiosperms (flowering plants), a. ♀ gametophyte reduced to 7 cells; no longer nourishes em- Above right. Simpli- bryo – i.e., no archegonium. fied schematic of an angiosperm seed. a. b. Nutritive tissue develops from seed coat; b. triploid triploid (3n) endosperm via a pro- endosperm; c. seed cess called double fertilization. leaf; d. embryonic stem. c. Seeds protected by ovary / fruit tissue.

7. In gymnosperms (conifers), a. ♀ gametophyte reduced but still nourishes embryo.

b. No endosperm. Naked seeds (no ovary).

26 Flowers.

 Key (along with seeds) to angiosperm success – 250,000+ species.

Flower structure. Pistil and stamens are respectively the female and male parts. Ovaries contain ovules; ovules, megasporangia; and megasporangia, megagametophytes. Likewise, anthers contain microsporangia, etc. Petals / sepals are often brightly colored to attract pollinators. Stamens, pistils, sepals and petals all evolved from leaves.

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Plant phylogeny with reproductive synapomorphies.

28 Questions.

1. Plants transitioned from freshwater to land in the Ordovi- cian; vertebrates, in the Devonian. List and compare the adaptations with regard to gas exchange and support that enabled each group to make the switch.

2. List and compare the adaptations regarding reproduction that enabled vertebrates and plants to transition from freshwater to land.

3. Compare dispersal in plants and animals.

4. Physical constraints – inability to transport water to a height of greater than about 120 m – limit the maximum height of trees. Why should trees be tall in the first place? Why don’t they allocate some of the energy that goes into trunk tissue to making more leaves?

5. What are phytoliths? In what kinds of plants are they found? How do they help the plant defend against herbi- vores? Requires outside reading.

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