Gymnosperm lecture

I. Kingdom Plantae

A. Gymnosperms - first seed plants (higher plants)

• next major plant group after the SVP’s

• first to evolve seed and pollen

  • freed plants from needing water to carry spores, sperm and egg = internal fertilization

Gymnosperm characteristics

1. make pollen

   • has wings to carry them on the wind

2. make naked seeds

   • seeds on a cone, not on a fruit so they’re considered naked

3. internal fertilization

   • sperm is being carried by a pollen grain and pollen tube

B. How did gymnosperms arise

Theory: SVP’s evolved into progymnosperms which then evolved into gymnosperms upon the evolution of the seed

Ex: archaeopteris - progymnosperm

• spores only - no seeds (wet environment)

• heterospory as a synapomorphy

• 2 xylem and 2 phloem

  • 1st bifacial cambium

• softwood - tracheids only

C. Very first gymnosperm - now extinct

Ex: elksinia - oldest known seed plant

• integuments are incomplete

Ex: seed fern medulossa

• integuments are complete/fused

First gymnosperms had:

• 2nd xylem and phloem

• megaphylls

• seeds!

Debate: which extinct gymnosperm gave rise to living gymnosperms today

Gymnosperm phyla living today

1. phylum: coniferophyta

   ex: redwoods, pines, firs, spruces, cedars

• shrubs/trees

• softwoods

  • tracheid’s only for water conduction

• scale/needle like leaves

• most are monoecious

  • male and female on the same plant

Juniper berries = fleshy cone scales

• used for flavoring gin

Yew tree = makes the drug Taxol

• anti cancer drug

• now synthesized

2. phylum: cycadophyta

   ex: cycads

   • largest cone in the world

• pinnate leaves

• dioecious (male and female plants are separate)

• native to the tropics

• resemble palms

  • but palms are flowering plants that produce fruit

3. phylum: ginkophyta

   ex: ginkgo biloba

• native to china

• dioecious

• no cone - seed coat is fleshy

• female seeds have butyric acid (stinky)

  • that’s why you plant the male trees

• cleft leaves (open line through the leaf)

• go through sex conversion

  • some branches of male trees become female

  • this is due to the fact that they evolved from a monoecious ancestor

4. phylum: gnetophyta

   ex: ephedra (mormon tea)

• lives in arid environments and the desert

• shrub - leaves reduced

• xylem has vessels and fibers

• double fertilization

• dioecious

ex: welwitschia

• large woody stem, close to the ground

• has 2 large meristematic leaves that split into segments

• dioecious

• weevil pollinated

• native to namibia/angola-southern africa region

Pine life cycle

• have a sporic life cycle (alteration of generation)

• pines are heterosporous

• micro = male, mega = female

1. pollen (microgametophyte) formation

• pollen is made in male cones called staminate strobilus

• male cones develop by MITOSIS out of the branches

• the scales of the male cone = modified leaves called microsporophylls

• on each microsporophyll, there are two microsporangia (pollen sacs) attached to each microsporophyll

• inside microsporangia - microgametophytes (pollen grains) form

  • all cone tissue is 2N because they form by mitosis from the tree (a 2N sporophyte)

• inside microsporangia are 2N microsporocytes

  • microsporocytes go through meiosis to make 1N microspores

• 1N microspores go through mitosis to make a 4 celled microgametophyte (pollen grain)

microsporocytes (2N) → meiosis → microspores (1N) → mitosis → 4 celled microgametophyte (pollen grain)

• generative cell is inside of the tube cell, why?

  • prothallial cells degenerate

  • wings form

  • pollen grain is an immature microgametophyte because no sperm has formed yet

• once pollen grain is made, they blow from the male cone to the female cone

• pollen grain then germinates on the female cone

• the tube cell becomes a pollen tube

• generative cell stays near the pollen tube tip as it grows (inside the tube cell) because it becomes sperm, and the tube cell delivers the sperm

• generative cell will divide to make 2 unflagellated sperm

  • pollen tube eliminates the need for swimming sperm

Huge evolutionary advances over SVP’s and Progymnosperms

• allowed them to live away from water in arid environments

new derived traits that come with internal fertilization:

1. pollen grains

2. tube cells/pollen tube

3. generative cell

4. unflagellated sperm

pollen tube delivers the sperm to the female cone

Seed formation - female

• young new cones are tiny and fleshy

• takes 1-2 years to mature into woody cones

1 integument + 1 megasporangium = 1 ovule (immature seed, doesn’t have embryo)

• ovuliferous scales

January every year:

• soft fleshy cones form on tree by mitosis (2N)

• cone scales are open to receive pollen

  • integuments open, opening is called micropyle

• pollination begins = pollen grain lands on micropyle

• when pollination is complete, ovule secretes a sugar droplet

  • integuments closed after pollination

• pollen germinates out pollen tubes

• pollen tubes release enzymes that start the formation of gametophyte

• after, pollen tube growth stops and waits for the female gametophyte to develop

Megasporocyte (2N) → meiosis → 4 megaspores (1N)

• 3 megaspores degenerate due to internal fertilization

  • 1 big spore to make a large gametophyte

remaining 1 megaspore → mitosis → female gametophyte with archegonia and eggs

• stage is set for fertilization

• eggs mature to release enzymes to cause the pollen tube to re-initiate growth

• pollen tube digests its way through the megasporangium to the egg

• fertilized egg develops into the zygote and embryo

zygote (2n) → mitosis → embryos (2n)

• embryo grows into the female gametophyte

• female gametophyte becomes food for the embryo

• megasporangium dries and becomes papery

• integuments harden into the seed coat, embryo matures

• cones open and drop seeds

• mature cones have woody scales

coat and megasporangium = 2N

female gametophyte = 1N

embryo = 2N

• female cones are modified branches because they have modified leaves below them = bracts