Gymnosperms
naked
seed
Four major living groups.
Cycads (Cycadophyta),
Welwitschia group (Gnetophyta),
Ginkgo, the Maiden Hair Tree (Ginkgophyta),
and the conifers (Pinophyta) .
Cycas bougainvilleana
Cycads
Cycads resemble palms or tree-ferns in
overall appearance but differ greatly in
detailed structure and reproductive
behaviour. Cycads are dioecious (i.e.
male and female reproductive structures
are borne on separate plants), and
reproduction is by seeds produced on
open carpophylls or seed-bearing leaves.
Although technically woody plants,
unlike other woody plants, cycads have a
thick, soft stem or trunk made up of
mostly storage tissue with very little true
wood.
The living cycads include about 250
species, with 11 genera in 3 families.
Microcycas calocoma Photo Dennis Stevenson
Female cone Cuban species
Gnetophyta
Welwitschia mirabilis
Lives in coastal desert regions of Namibia
and Angola. Morning fogs provide
moisture.
Female cones
Male cones
Large, straplike leaves that grow continuously along the ground.
During its entire life, each plant produces only two leaves, which
often split into many segments as a result of the leaves being
whipped by the wind. Carbon-14 datings of the largest plants have
shown that some individuals are over 1500 years old.
Ginko biloba
Illustration in Pen Tsao
Kang Mu of Ginkgo with
seeds (1578)
There are no native ginkgoes living in the wild.
Several ginkgoes were the only living survivors
of the atomic bomb blast at Hiroshima
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Why is Washington called the
Evergreen State?
The Evergreen State
Most conifers are evergreen
Evergreens have potential for growth all-year round
What is the advantage of the potential for all -year
round growth?
Why aren’t all plants evergreen?
What is the advantage of the deciduous habit
over the evergreen habit?
Why is so much of Washington covered by
evergreen conifers?
What is the advantage of the potential for all –
year round growth?
Opportunistic growth when conditions may be
favorable and lack of dependence upon a particular
time when growth must be made.
Evergreenness
Why aren’t all plants evergreen?
Evergreen plants are frequently slow growing. The
features that enable them to survive harsh conditions
do not always enable them to grow quickly.
What is the advantage of the deciduous habit
over the evergreen habit?
Deciduous plants are frequently faster growing than
evergreen plants and can rapidly exploit favorable and
reliable habitats. Leaves do not require to withstand
severe cold and leaf abscission enables nutrients and
carbohydrates to be withdrawn into the plant for re-use
Why is so much of Washington covered by
evergreen conifers?
It regularly has periods of summer drought but mild wet
winters. This can favor evergreens over deciduous trees.
In what other type of environment are
evergreen conifers frequently found?
Relative numbers of species of major
groups of plant across geological time
Angiosperms
Ferns
Mosses
Time scale
Gnetales
Cycads
Ginkgos
Conifers
Origins of conifers in the
Permian
The coal forming period
Millions of years
Pangea
Gymnosperms show adaptations
to drier conditions than mosses
and ferns both in their
reproduction and vegetative
growth
Continental type climates
typically have dry periods or
seasonal rains. These can be
seasonal and may be called
monsoons
In the Permian the land masses
came together and formed a large
continent call Pangea
360 to 286 mya
Carboniferous
Adaptations for the drier environment
Reproductive
1. Airborne male gametophyte (pollen) carried by wind to
the female gametophyte where sperm fertilizes the egg located
in the female gametophyte in the ovule which is retained,
sustained and protected by the sporophyte.
2.
Production of a durable seed that at maturity consists
of a protective seed coat, a source of nutrition and an
embryo sporophyte all in one package.
Vegetative
3.
Seed plants have wood producing tissue well developed
in some species for water conduction and support. This
enables plants to grow tall and out-compete neighbors
Reproduction in the conifers
The cone is a modified branch. Unlike the flower it does
not have sepals or petals. Usually separate male and
female cones are borne on the same plant, i.e., monoecious.
Each of the numerous scales, or sporophylls, of the male
cone bears pollen and each female cone scale bears ovules
in which egg cells are produced. .
Life cycle
Fig. 17.8
From ovule to seed
Cedrus deodora
The diminutive female cone
Male cones – all lined
up and ready to go!
Developing seed cones
Continuing
vegetative shoot
Pollen grains
Male cone
Pictures of male cone and pollen
Microsporangium
Female gametophyte
Female cone
Archegonia
Pictures of female cone
Egg
Megagametophyte
Ovule
Single scale (sporophyll
Longitudinal section through ovulate cone
Bristlecone pine
Pinus longaeva
Male
cone
Current year
Last year’s
empty sporangia
Female
cone
The oldest aged specimen is 5,600 y
Sections you need to have read
17.8 and Introduction to Chapter 31
Courses that deal with this topic
ESC 200 Trees in Our Environment
ESC 221 Dendrology and Autecology