Seed Plants – Gymnosperms www.onacd.ca

Sub-Phylum Spermosida – Seed Plants Class: Coniferae Characteristics gymnosperms (gymno = naked; sperm = seed) Sporophyte larger than gametophyte Most are trees or shrubs with woody tissue Leaves are often needles or scales Produces cones with two types of spores Seeds usually produced on surface of scales of cones

Most ancient surviving seed plants. Three Classes: Gymnosperm Evolution Most ancient surviving seed plants. Three Classes: Cycadae Ginkgoae Conifernae Leaves evolved into specialized male and female reproductive structures called scales. Scales are grouped into larger structures called male and female cones. Male cones  gametophytes called pollen Female cones  gametophytes called eggs Once fertilization occurs the zygote grows into a seed that is held on the scales. The seeds are protected by a seed coat but the seed is not covered by the cone. Because the seeds sit ‘naked’ on the scales, these plants are called naked seed plants  gymnosperms (gymno – naked; sperm – seed).

Gymnosperm Diversity - Cycadae Cycads – palm-like plants but have naked seeds. First appeared in the fossil record 225 million years ago - only 9 genera remain today Habitat – Sub-tropical/tropical

Gymnosperm Diversity - Ginkgoes Ginkgoes – common when dinosaurs rules but only one species remains today – Ginkgo biloba Ginkgo biloba may be the only seed plant species alive today, thanks to Chinese gardeners

Gymnosperm Diversity - Conifers Most bear seeds or cones Needle-like or scale-like leaves with thick cuticles Pines, spruce, firs, cedars, junipers, larches Some live over 4000 years old and 1000 m tall

Old Trees Methuselah, a 4,845-year-old Great Basin bristlecone pine (Pinus longaeva) in the White Mountains of California

Reproduction in Gymnosperms FEMALE CONES Called seed cones House female gametophytes Stays inside tissue of sporophyte (cone) Produces ovules (plural = ovum) Inside the ovule = an unfertilized egg Very small MALE CONES Called pollen cones Produce male gametophytes Haploid spores: pollen grains Inside pollen grains = 2 sperm (1N) Very small

Alternation of Generations In the spring, the male cone releases millions of dust like pollen grains that are carried on the wind to the female seed cone. (not all make it to the seed cone) Once a pollen grain lands near a female gametophyte, it produces a pollen tube that grows into the female gametophyte until it reaches the egg.

Alternation of Generations inside the pollen tube are 2 sperm (1N), one sperm fertilized the egg, the other disintegrates. These sperm cells burst out of the pollen grain and fertilize the ovule (egg).

Alternation of Generations After fertilization – the zygote grows into an embryo encased in what later becomes the seed. Embryo + stored food + coat = seed Mature seeds drops out of cone onto the ground.

Alternation of Generations Seed germinates and seedling grows into a mature plant. When mature, the adult plant produces cones

Wood Xylem tissue conducts water and dissolved materials (called sapwood) Xylem forms outer rings of stem – “wood” Growth rings Xylem produced usually in late spring and early summer (thick at this time of year) During fall and winter thin xylem is produced (therefore you can count the rings)

Wood Heart wood: older, darker wood at the center of the tree trunk that is full of resins, gums and pigments. May rot without affecting the functions of the tree. Resin ducts – secrets resin which stops growth of fungus and invasion of insects. When distilled, resin  turpentine

Wood Rays Radiating out from the center of xylem and phloem, they transport nutrition and water radially across cells

Wood Cambium Thin sheets of unspecialized cells between central xylem of stem and phloem of inner bark Cells of Cambrian divide by mitosis If near phloem  becomes phloem If near xylem  become xylem Cambium layer adds new cells that increase the width of the stem As xylem expands it destroys phloem, therefore there is no increase in the thickness of phloem

Meristematic Tissue Cells capable of dividing through out the life of the plant (= growing) Only plant tissue that produces new cells through mitosis As new cells mature they differentiate into many different kinds of tissue Apical meristems allow the tips of stems and roots to grow in length Cork cambium produces outer covering of stems Vascular cambium produces vascular tissues (xylem and phloem) and increases the thickness of stems over time Pericycle allows roots to grow thicker and branch out

Surface Tissue Forms the outer or surface layer of leaves, stems and roots Surface tissue in tree bark protects against water loss Surface tissue in roots helps in the absorption of water