Lecture #27 Plant Kingdom Tracheophytes Vascular Plants (Ferns, Pine and Apple Trees And

Lecture #27—Plant Kingdom—Tracheophytes—vascular plants (ferns, pine and apple trees and friends)

This lecture continues following some of the major steps of plant evolution. It is primarily a story of how plants progressively got better at surviving on land and invading more and drier environments.

Tracheophytes are plants with vascular tissue (xylem & phloem) to move water and nutrients around the body. Even the earliest Tracheophytes (ferns, etc.) had evolutionary advancements over the Bryophytes which allowed them to invade much drier environments and grow very large.

 xylem and phloem  lignin = wood  thick waxy cuticle  stomates  profuse branching

o increased young tissues for photosynthesis

o increased areas for sporangia (as opposed to only one sporangium per sporophyte in the Bryophytes)  highly differentiated plant tissues and organs

However the early Tracheophytes were still dependent on living in moist environments; they had swimming sperm. Only later did the Tracheophytes (e.g. Gymnosperms and Angiosperms) develop efficient reproductive methods that were better adapted to land (pollination, seeds, and fruit).

The life cycle of Tracheophytes shows another major innovation: the haploid gametophyte generation is longer dominant. In Tracheophytes the dominant phase of the life cycle is the diploid sporophyte generation. This is seen in the life cycle of the fern. The obvious large leafy plant is the sporophyte,2n stage; the gametophyte (n) is tiny and lives inconspicuously on the moist forest floor. Seed Plants (Gymnosperms and Angiosperms) also have the sporophyte as the dominant generation. In fact, the gametophyte generation is so tiny it consists of a few cells crammed together inside of a pine cone; it almost appears as if the stage has disappeared altogether. So the pine cone is a part of the sporophyte generation and produces in turn (by meiosis): a spore, gametophyte cells, and a gamete.

Here is another wrinkle: the advanced Tracheophytes like pines produce two kinds of spore in two different pine cones: small male cones that make spores and a few tiny gametophyte cells that produce pollen (=sperm); large female cones that make spores and few tiny gametophyte cells that produce eggs.

We call this two spore arrangement heterosporous.

In the pine life cycle pollen is released from the male pine cone and floats by air and gets caught by sticky woody leaf-like bracts that make up the female cone. The pollen germinates and grows a pollen tube to the egg where fertilization occurs. The zygote grows into an embryo that is covered with a tough covering along with a food supply (called an endosperm) becoming a seed. So the advanced Tracheophytes have specialized by developing a two spore solution to reproduction. And they have developed non-swimming sperm and a tough seed both adaptations that allow them to invade dry environments.

Plant Adaptations to Land.

Challenges: Support, water loss from plant body, reproduction without water.

Support: First plants lived in water and were supported.

On land, the turgor pressor in their cells gave support inside cell walls.

Tracheophytes evolved lignin (=wood) a strong complex carbohydrate used in cell walls giving support.

Water Availability:

First plants lived in water—no problem.

On land danger of drying out. Bryophytes live in moist habitats.

Tracheophytes evolve tough waxy cuticle.

Tracheophytes evolve stomata to control water loss.

Evolve roots, root hairs, and develop mutualistic relationships with mycorrhizae to obtain water and minerals from soil.

In desert conditions—Tracheophytes reduce leaf size.

Drop leaves when photosynthesis is dormant.

Reproduction without water.

First plants lived in water and had swimming sperm.

First land plants (Bryophytes and early Tracheophytes like fern had swimming sperm and were confined to moist environments.

Seed plants evolved mechanisms to deliver sperm via the wind or by animals (bees, etc.) and did away with swimming sperm Seed plants evolved seeds—embryos packaged together with food supply (endosperm) and covered with a tough seed coat to withstand drying.

Seed adaptations evolved to spread the seeds throughout the environment by wind or animal distribution ( e.g. encasing the seeds in tasty nutritious fruit which will be eaten).

Terms/Concepts to Define: Tracheophytes Gymnosperm Angiosperm Stomate (stomata) Guard cells Alternation of Generations Sporophyte Gametophyte Zygote Embryo Spore Sorus (sori) Homosporous Heterosporous Lignin Mycorrhizae Endosperm Seed Anachronistic Fruit

Can you answer these questions?

1. Name three evolutionary advances that Tracheophytes over the Bryophytes which makes them better adapted to the terrestrial environment.

2. Discuss the changes in reproduction that have occurred as plants adapted to the land environment. 3. Diagram the alternation of generation model.

4. Describe the fern life cycle.

5. Discuss the shift that occurs in the importance of the gametophyte stage of the life cycle as plant evolution progressed through time.

6. Describe the shift in spore formation that occurred in plant evolution. Over time.

7. Describe and discuss the events that occur in pine cones.

8. List the major problems that plants faced when they invaded land and give one major adaptation that the Tracheophytes evolved to solve that problem.

9. Explain the role that mycorrhizae play in plant adaptation to land.

10. Describe 3 methods of seed dispersal and explain why the Osage orange is an enigma.