Botany Gymnosperms

Similarities with seedless vascular plants

Possess vascular tissue (xylem and phloem), true roots, stems, and leaves.

Advancements over seedless vascular plants

reproduction using seeds, water not required for fertilization

Advantages of reproducing via seeds

Contain well-developed multicellular your plant as compared to single cell spores, contain an abundant food supply, surrounded by a multicellular seed coat providing protection.

Gymnosperms ‘naked’ seeds

No ovary wall (fruit) surrounding seed: produced on surface of sporophytes, often on the scales of a cone.

Angiosperms seeds

Ovary wall (fruit surrounding the seed)

Gymnosperm seed anatomy

outside to inside: seed coat (from parental sporophyte), nutrition/food supply (from gametophyte), embryo (daughter sporophyte)

Gymnosperm leaves

megaphyll (many veins)

Gymnosperm phylums

Pinophyta, Ginkgophyta, Cycadophyta, Gnetophyta

Pinophyta- conifers

Most diverse phylum of gymnosperms, Linus is largest genus (dominant trees in northern hemisphere in coniferous forests), includes: bald cypress, yew, spruce, firs, Douglas firs

Pinophyta-leaves

Needle like arranged in clusters of two to five, cluster = fascicle, have resin ducts (resin is antiseptic and aromatic prevents fungi and insects), hypodermic located below epidermis, thick cuticle, recessed or sunken stomata.

fascicles

clusters of phinophyta leaves, short shoots, have restricted growth

Pinophyta-leaves-evergreen

held for 2-5 years (up to 30), water conserving adaptation, resins to deter herbivory and fungal attack.

Phinophyta-leaves-deciduous

bald cypress, dawn redwood, larch

Pinophyta- woody stems

Wood consists entirely of tracheids

Conifer wood

softwood- thick-walled cells (vessel elements) are absent

Broadleaf tree (dicot) wood

hardwood- thick-walled vessels and fibers present.

pinophyta- reproduction

Most are monoecious, generally borne as cones (strobili)

Monoecious

male and female reproductive structures present on different part of the same plant.

Pollen cones

male strobili, small, contain scales called microsporophylls, each microsporophyll has two microsporangia at the tip, inside microsporocytes undergo meiosis and produce 4 HAPLOID microspores which later develop into pollen grains.

pollen grains

immature male gametophyte, 2 prothallial cells, 1 generative cell, 1 tube cell, air sacs for wind dispersal.

Seed cones

larger than pollen cone, scales called megasporophylls, two megasporangia (ovules) at the base of each megasporophyll, megasporangium surrounded by integument (has a pore called micopyle), megasporocytes undergo meiosis and produce 4 haploid megaspores, 3 degenerate, 1 develops into female gametophyte with archegonia at micropyle end, nourished by the nucellus (provided by parental sporophyte)

seed cone maturation (year 1)

Pollen grains catch on sticky pollen drops from the micropyle, pollen grains produce pollen tube that grows through nucellus, pollen-sperm stuff, megaspore develops.

mature male gametophyte

germinated pollen grain with pollen tube and two sperm (sperm form from generative cell), sperm have no flagella and no antheridium is formed.

seed cone maturation (year 2)

Female gametophyte and archegonium mature, pollen tube arrives at archegonium, one sperm unites with egg, forming a zygote (other sperm degenerates), embryo nourished by female gametophyte, integument becomes seed coat, seed cones open to release the seeds.

Pinus life cycle

Yew (Taxus) and California nutmeg (Torreya)

produce ovules singly at tips of shoots, each at least partially surrounded by fleshy cupcake aril.

Junipers (Juniperus)

have seed cones with fleshy scales (appear as berries) (juniper berries)

Bristlecone pines

Oldest known living organisms (some over 4,000 years old)

Coast redwoods

tallest known organisms (some over 100 m tall)

Giant sequoias

Most massive living organisms (volume of 1,487 m³)

Gymnosperm evolution

Phylum Ginkgophyta

One species: Ginkgo bioloba, only exists in cultivation, deciduous, Dioecious, fleshy cones with a single seed, up to 19 species in fossil record going from 270 mya. Common in urban settings, tolerates air pollution, soil pollution, salt, and compaction, long-lived species.

Ginkgobiloba leaves

Broad, fan-shaped, notched, borne on short, slow-growing spurs, no midrib, dichotomous venation

Dioecious

male and female structures on separate trees

Phylum Cycadophyta

slow-growing plants of tropics and subtropics, tall unbranched trunks, crown of large pinnately divided leaves, similar life cycle to conifers, pollination mostly by wind sometimes beetles, dioecious (has pollen and seed strobili), sperm swims to female gametophyte, more prominent 250 mya, currently has around 140 species

Phylum Gnetophyta

Unique in having vessels in the xylem, more than half are species of joint fires (Ephedra) (shrubby plants of drier regions of southwester North America, South America, Asia, and Europe)

Phylum Genotophyta- Welwitschia

one species (Welwitschia mirabilis), in deserts of southwestern Africa, short stem, long taproot, only two traplike leaves that become tattered and split, dioecious, can be very long-lived (1000-2000 years), cones are insect pollinated (flies, bees, wasps)