Lecture 18: Seed-Plant Innovations

seed plants two large group

gymnosperms and angiosperms

Early seed plant evolution: extinct foosils

progymnosperms: trees that exhibited secondary growth- wood but lacked seeds

seed ferns: woody trees with fern like leave that bore seeds

innovations of seed plants arose in the MRCA

1. secondary growth

2. Reduced and dependent megagametophytes

3. seed

4. pollen

secondary growth

the ability to increase the girth of the growing plant by means of vascular cambium

primary growth

ability to increase length of the plant by apical( shoot and root) meristems

vascular cambium

innovation of seed plants that is comprised of a ring of meristematic cells btw the primary xylem and primary phloem

bifacila vascular cambium of seed plant

makes secondary xylem to the inside of the plant that gives rise to wood and secondary phloem to the outside of the plant(rise to bark)

Adaptive significance of secondary growth: origin of vascular cambium 4 benefits

-allows for taller growth of the sporophytes which increase sunlight and efficient dispersal of pollen and seeds

- constant rejuvenation of the xylem and phloem: gets clogged with resins over time

formation of bark prevent water loss, infections and protects against disease

the female gametophyte in seed plants are

reduced, embedded within and nutrionality depend upon the sporophytes: innovations enabled evolution of the seeds

seed type gymnosperms: 3 components

naked seed

- innermost component is the diploid embryo: the baby sporophyte

-middle is haploid nutritive tissue: from the megagametophytes

outermost layer:diploid seed coat: from the integument

seed type aniosperms

enclosed seed

A seed is major plant innovation that confer 3 benefits

Facilitates efficient dispersal of the next sporophyte generation

Provides protection of developing sporophyte plant

Allows for prolonged dormancy of the embryo so that development can resume when conditions are favorable, and is provisioned with nutrients to facilitate growth when development resumes.

pollen

highly reduced male gametophyte: microgametophyte of seed plant

pollen of seed plant components

4-8 cells

lack of a multicellular male gametangia: no antheridia

includes 2 male gametes: sperm cells

enclosed in sporopollenin, a resilient coat that protects the pollen from desiccation and may be elaborated to enhance dispersal

Pollen is an major innovation in Seed Plants that confers two major benefits:

-delivers sperm without water to megagametophytes

- facilitates efficient disperla of sperm to pollinate female; long distance