BIOL 371: Theme 3 - Adaptation of Land Plants

expected origin of Earth

~4.6 Ga (giga annum/billion years)

expected time of origin of life

~3.7-4 Ga

brief history of life

origin of Earth —> origin of life —> colonization began with fungi, early embryophytes —> animals (arthropods) —> vascular plants —> tetrapods (first to come on land) —> seed plants arise and arthropods and amphibians dominate animals on land, reptiles begin to radiate (last common acnestor between reptiles and mammals ~285 Mya

colonization of land

life on land only the last 500 million years, although life evolved in the seas 3,5 bya

multiple challenges on land

desiccation, respiration, reproduction, locomotion, senses

desiccation

need protection from drying out, a coat or skin to prevent body fluids from evaporating

respiration

in water dissolved oxygen and carbon dioxide are exchanged, organisms had to invent new structures to breathe, eg. stomata, lungs

reproduction

in water, eggs and sperms are released into the water, land organisms had to evolve to become successful reproducers under desiccating conditions, eg. alternation of generations in plants, evolution of amniotes, pollen, ovule)

locomotion

needed to get new structures for locomotion in animals and to defy gravity in plants, eg. roots, limbs/appendages, rigidity

senses

organisms on land had to adapt to the changes in light, sound, and smell which were perceived very differently in water

algal mats

were the first to come out and lived on the water edge for a long time

non-vascular plants

the first true fully terrestrial organisms around ~450 Mya, land offered unlimited mineral resources, unrestricted sunlight and no competition from other organisms that crowded the ocean

arthropods

the first terrestrial animals in the silurian period (~440-420 Mya), all bugs, ancestors of centipedes, millipedes, arachnids, and insects, they ate the plants that were established and also other bugs

mollusca

animals dominant in the ocean while arthropods dominated land

why is inhabitation of land by plants an important evolutionary milestone?

sustainability (oxygen, drugs, food, fiber, timber, ecosystem service), presence of plants on land has allowed other life forms and terrestrial animals to survive on land

phylogeny of green plants

first green plants —> non-vascular plants (475 mya) —> vascular plants (420 mya) —> seed plants (300 mya)

environmental pressures of movement from water to land

1. mineral and nutrients from water —> has to be obtained from soil

2. sunlight through water —> protection from direct sunlight and desiccation

3. gases diffuse from water —> need to acquire gas exchange mechanism

4. weight is supported by water —> requires weight supporting strategy (lignin for rigidity)

   1. spores and sperm can swim to their destination —> less or no water available for spores and sperm

major plant adaptations

1. waxy cuticles - to prevent them from drying out (desiccation)

2. stomata - pores to control gas exchange and water loss

3. vascular tissue - conducting cells for transport of water, structural support

4. root system - absorption of water and minerals

eg. algae would dry out if no water

waxy cuticle

skin of the plant, a major innovation, layer above epidermal cells that prevents water loss from stems and leaves, protects the plants from drying out and also from pathogen attack, fossil record shows presence of this in early land plants encasing spores (reproductive cells)

stomata

pores on the surface of land plants, allow gas exchange in photosynthetic tissues, plants can absorb/respire even with tightly sealed waxy coat, opening and closing of the pore is controlled by specialized guard cells

found in earliest branches of land plants (non-vascular plants)

guard cells

specialized cells that control opening and closing of stomata

vascular tissue

plant circulatory system, transport water with elongated conducting cells, maintains rigidity with a rigid polymer ring (lignin) that thickens cell walls, resists gravity, evolved gradually…

• fossils and non-vascular plants - simple water conducting cells

• fossils - first vascular tissue

• vascular plants - tracheids

• angiosperms - vessels

increasing level of structural support and efficient water transport (xylem)

tracheids

in ferns and conifers, closely packed elongated cells, cells die after maturity, thickened secondary cell walls with lignin deposits, have gaps in them, better structural support

vessels

in angiosperms, similar to tracheids, but shorter and wider, both walls have gaps in the them for more efficient water transfer

root system

lignified vascular tissue underground, evolved from belowground parts of ancient vascular plants

rhizoids

root like structures for anchoring, found in algae, non-vascular plants (mosses) and fern gametophytes, evolved to roots

pollen

evolved in gymnosperms, non-swimming sperm

flowers and ovaries

evolved in angiosperms (vascular seed flowering plants)

sequence of morphological adaptations to land

cuticle —> stomata —> vascular tissue —> roots —> tracheids —> vessels

reproductive adaptations on dry conditions

alternation of generations (gametophyte and sporophyte), before all reproduction was water-mediated - sperm can swim in water

alternation of generations

gametophyte and sporophyte (haploid and diploid), embryo retained and nourished by parents, an adaptive strategy to protect the gametes and embryo in a non-aquatic environment

alternation of generations vs. animal life cycles

one free living diploid individual, gametes (haploid) are formed through meiosis and are not free living

alternation of generations in the algal ancestor of land plants

long gametophyte (haploid) phase and short diploid phase (zygote), zygote undergoes meiosis to produce spores, haploid spores give rise to gametophytes which produce haploid gametes through mitosis, zygote represents the only diploid stage

evolutionary time line regarding alternation of generations

• water availability: high to low

• free living zygote: only in algae, all true land plants have zygote attached to female gametophyte

• length of gametophyte (n) generation: long to short

• length of sporophyte (2n) generation: short to long

• relative size of gametophyte (n): large to small

• relative size of sporophyte (2n): small to large

• protection of zygote/embryo (2n): low to high

gametophyte dominated cycle, free living gametes and zygote no retained on parent —> retention of gametes/zygote/embryo on parent —> sporophyte dominated cycle —> pollen —> seeds —> flowers