Lecture 23 - Gymnosperms

Selaginella

heterosporous; similar to conifer life cycle

microgametophyte

microsporangia contain microspore mother cells which undergo meiosis and divide into 4 spores clustered in a tetrad — each spore develops into a pollen grain

microgametophyte spores

one of the 4 cells is a generative cell — divides, producing 1 sterile and 1 spermatogenous cell

spermatogenous cell

divides, producing 2 sperm cells

microgametophyte GP

mature male GP only has 6 cells, no antheridia — just 2 sperm cells

megagametophyte

ovule eventually develops into seed

ovule

megaspore mother cell, megasporangium, and integument

megasporangium

nucellus

meiosis in megagametophyte

megaspore mother cell produces 4 megaspores, 3 disintegrate—1 functional megaspore left

megagametophyte gamete

1 functional megaspore forms mega-GP within megasporangium

mega-GP form

produces 2+ archegonia (each has egg cell)

pollination (megagametophyte)

pollen lands on ovule, finds micropyle to contact megasporangium; pollen tube grows into GP tissue toward archegonia

pollen grains

one sperm cell is delivered to the egg cell, the other sperm cell disintegrates (two pollen grains needed to fertilize two egg cells) → fertilization

mega-GP zygote

zygote develops into embryo which develops within mega-GP (1 of 2 embryo dies)

mega-GP purpose

food reserve for developing embryo

mature embryo

differentiation in root, shoot, and leaves (cotyledons); integuments dry out and harden to form the seed coat

gymnosperms

have archegonia, female GP has 7,000 cells

gymnosperm life cycle

pollen (male GP) is distributed by wind to female mega-GP — developed on base of cone scale (ovuliferous scale)

gymnosperm ovule

female GP develops within ovule, structure contains megasporangium surround by layers of cells (integument)

maturing ovule (Gymnosperm)

ovule matures as GP develops and fertilization takes place, finally becoming seed

pollen

male micro-GP

gymnosperm zygote

one cell of 6 cell micro-GP fertilizes egg cell within archegonium of female GP

seed

embryo + mega-GP + seed coat derived from ovule integuments

seed coat favorable conditions

water loss causes hardening of seed coat — seed enters dormancy state until favorable conditions.

seed germination — gymnosperms

root grows into the soil and anchors the young seedling, then hypocotyl elongates and pushes cotyledons into air

cotyledons become green, unfold, and produce energy (photosynthesis) → new sporophyte tree

difference between pollination and fertilization

pollen lands on micropyle of ovule, while nuclei of egg and sperm fuse to form zygote; separated in space and time

gymnosperm anatomy

leaves (needles in Conifers), and wood formation through secondary growth

needles

adaption to environment with shortage of water, epidermis with thick waxy cuticle, stomata below leaf surface, mesophyll cells are the main photosynthetic cells, vascular bundle in center

needs retain for 2-4 years in pine, functional needles can be as old as 45; some conifers are deciduous trees

conifer needles

wood formation through secondary growth

vascular cambium produces lignified xylem cells (secondary xylem) toward the inside of the stem and phloem toward the outside

xylem tissue in a stem

makes up the wood of a tree or shrub

secondary growth

increase in stem diameter

conifer xylem

consists of lignified dead cells that form a water-conducting structure that reaches from root to canopy

annual rings

Trees in cold and temperate climates go through periods of intense growth in the spring, slower growth in summer and fall, and a stop of growth in winter

tree ring growth in spring

most intense growth results in big xylem cells with relatively thin walls

tree ring growth in late summer

growth is ceasing and cells are small with thick walls; as a consequence, the wood is compacting towards the end of the growing season producing a thin dark ring, the year ring

tropical trees

don’t generally produce year rings

evolution of gymnosperms

arose in late Devonian, dominant during age of dinos

extant groups of gymnosperms

coniferophyta, cycadophyta, ginkgophyta, gnetophyta (all trees except gnetophyta)

cycadophyta, cycads

palm-like plants, abundant during dinosaur era, cycads are living fossils (reached peak in Mesozoic) — often very toxic, 140 species mostly in tropics

cycad life cycle

male and female cones on different plants, insects important for pollination

cycad reproduction

have flagellated sperm and fertilization depends on presence of water; cycads may represent a transition between SLVPs and gymnosperms

male GP produces pollen tube that doesn’t reach archegonium

cycad male GP pollen tube

pollen tubes grow and anchor themselves in the remaining megasporangium tissue (helps absorb nutrients for developing male gametophyte sperm cells)

pollen tube ruptures and 2 multi-flagellated sperm are released which actively swim towards archegonium to fertilize egg cell (combo of flagellated sperm and pollen tube)

cycadophyta and conifers

gingkophyta (Ginkgo biloba)

unchanged over last 150 million years, no wild representatives but cultivated (originally in Japan and China), deciduous

deciduous

plants that seasonally shed their leaves, usually in response to environmental changes

ginkgophyta reproduction

ovule and microsporangia on different trees, ovules produce fleshy coated seeds (most cultivated trees are male); fertilization by flagellated sperm

ginkgophyta seeds

seeds inside a fleshy seed coat — derived from integuments (gymnosperms)

male gametophyte in Ginkgo biloba

development of seed from ovule

only one megaspore mother cell is formed per megasporangium

megaspore within megasporangium

development of microgametophyte

seed development in conifers