Vascular plants - Gymnosperms

benefits of seeds for survival

• Protection from desiccation and environment

• Stored food for development

are seeds heterosporous or homosporous

heterosporous - megaspores give rise to megagametophytes and microspores give rise to microgametophytes

how do seeds differ from spores

• seeds production is a form of extreme heterospory

• formation of an ovule that develops into a seed containing an embryo

ovule before maturity

Before maturity, the ovule consists of a megasporangium surrounded
by one to two layers of tissues (integuments)

events that led to the production of an ovule

• Megasporangium no longer releases spores

• Reduction in the number of megaspore mother cells to on

• Survival of only a single megaspore

• Female gametophyte no longer free living, retained within

• Development of the embryo within the female gametophyte megasporangium

• Formation of the integument which encloses megasporangium

• Modification of the megasporangium apex to receive microspores

when did progymnosperms originate

Late Paleozoic (290 Mya)

progymnosperms represent the transition from…

seedless vascular plants to seed plants

how are progymnosperms more advanced than seedless vascular plants

• Bifacial vascular cambium

• Eustele

Eustele

Vascular tissues in discrete strands in a ring around pith

progymnosperms

Woody plants, produced secondary xylem and phloem and reproduced
via spores

2 groups of extinct gymnosperms

• Pteridospermales

• Cordaitales

Pteridospermales

• seed ferns

• diverse

cordaitales

primitive and conifer-like

gymnosperms have what kind of seeds

naked seeds - not protected by an ovary or fruit

4 phyla in Gymnosperms group

• Coniferophyta

• Cycadophyta

• Ginkgophyta

• Gnetophyta

what do the microgametophytes develop into in gymnosperms

• develop into pollen grains

how does pollen grain dispersal differ in gymnosperms

• no more reliance on water for fertilization (like in ferns with flagellated sperm)

• dispersal by wind - passive towards the vicinity of a megagametophyte

pollination

• Pollen grain produces a tubular outgrowth called a pollen tube

• Penetrates archegonium (except in cycads and Ginkgo)

• Grows in tissues of the nucellus where it absorbs nutrients, bursts, and releases multiflagellated, swimming sperm

  
  

polyembryony

• Megagametophytes of gymnosperms produce several archegonia

• More than one egg may be fertilized with several beginning development

• In most cases, only one embryo will survive within a single ovule

• Few seeds contain more than one embryo

  
  

most numerous phylum in the gymnosperms

Coniferophyta (70 genera, 630 species)

tallest vascular plant

• Redwood

• from Coniferophyta

• Up to 120m tall with trunk diameters of 11m

when did conifers originate

Extend back to the Late Carboniferous (300 Mya)

Importance of Needle like leaves in group Coniferophyta

• drought resistant properties

• May be related to the diversification of this phylum during the dry and
  cold Permian period (290-245 Mya)

  
  

Pinus genus habitat

• Dominant in many areas of North America and Eurasia

• Cultivated in the Southern Hemisphere

leaf arrangement of Pinus Genus

• Spirally arranged and singly borne on stems

• After 1-2 years of growth, pines produce leaves in bundles of 1-8

fascicles

• short shoots with apical activity restricted

• A determinate branch

• wrapped by small scale-like leave

how are Pinus leaves suited for dry environments

• A thick cuticle reduces evaporation from leaf interior

• A hypodermis of thick-walled, compact cells below

• Stomata are sunken

• Mesophyll often has two resin ducts

vascular system in Pinus leaves

• One to two vascular bundles are surrounded by transfusion tissue

• Surrounded by a single layer endodermis

Pinus stems (vascular system and growth)

• Secondary growth begins early

• Leads to a large amount of secondary xylem

• Xylem is primarily tracheids

• Phloem is sieve cells

• The epidermis is quickly replaced by periderm

how do other conifers differ from Genus Pinus

• Lack the clusters of leaves

• shorter reproductive cycle (3 days - 4 weeks) (Pinus: 15 months)

• large diversity of cones

aril

• fleshy cone found in Taxus (yew) trees

• attracts birds for dispersal

cycadophyta timeline

• Appeared during the Permian period around 250 Mya

• Hugely abundant in the Mesozoic

• Often called the “Age of Cycads and Dinosaurs”

cycadophyta habitat

Palm-like plants found in the tropic and subtropic regions

only species of cycadophyta found in America

Zamia integrifolia

Cycad unique features

• large (18m)

• distinct trunk - covered with the base of shade leaves while functional leaves are only found at the top

• often toxic - produce neurotoxins and carcinogenic compounds

unique traits of cycad roots

• Have specialized roots that branch dichotomously, with upward branches near the soil surface called coralloid roots

• these roots host cyanobacteria that fix atmospheric nitrogen

reproductive structures in cycads

• Reduced leaves with attached sporangia clustered into cone-like structures near the apex

• Plants host either pollen or ovulate cones (dioecious)

how is sperm released in cycads

• Growth of the pollen tube causes destruction of the nucellar tissue

• Microgametophyte swells and ruptures releasing two multiflagellated sperm

what organism is associated with cycad reproduction

• Beetles often associated with male cones

• Less so female cones

• Some beetles are pollen-consuming

• Current cycads are mostly insect pollinated

only surviving species of Ginkophyta

Ginkgo biloba

why are Ginkophyta great for urban areas

resistant to air pollution

how is Ginkophyta different from most gymnosperms

deciduous

is Ginkophyta monoecious or dioecious

dioecious- Bears ovules and microsporangia on
different individuals

Ginkophyta ovules

• Ovules in pairs on short stalks and ripen to fleshy-coated seeds

• The rotting flesh of these seeds is foul, producing butanoic and hexanoic acids

• Same compounds as rancid butter and stinky cheeses

• seed is a delicacy in china and japan

• males planted in urban areas because no foul smell produced

Ginkophyta reproduction

• Fertilization may not occur until the ovules are shed from the tree

• Otherwise, reproduction is similar to the cycads

• Consumes nucellus and ruptures to release sperm

3 living genera of Gnetophyta

• Gnetum

• Ephedra

• Welwitschia

Gnetum

• found in moist tropics

• Around 35 species, trees and climbing vines with large, leathery leaves

• Similar in resemblance to eudicots

Ephedra

• found in arid desert regions

• Around 40 species, profusely branched
  shrubs with small, scalelike leaves

Welwitschia

• found in sandy desert soils of southwestern Africa

• One species, Welwitschia mirabilis

angiosperm-like traits found in Gnetophytes

• Their strobili clusters resemble angiosperm inflorescences

• Vessels in their xylem – independently derived