How Plants Colonized Land

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Vascular tissue allowed vascular plants to grow tall.

Seedless vascular plants have flagellated sperm and are usually restricted to moist environments.

Slide 29

Origins and Traits of Vascular Plants

Fossils of the forerunners of vascular plants date back about 420 million years.

In contrast with bryophytes, sporophytes of seedless vascular plants are the larger generation. The gametophytes are tiny plants that grow on or below the soil surface.

Vascular plants are characterized by:

Life cycles with dominant sporophytes

Vascular tissues called xylem and phloem.

Well-developed / true roots and leaves.

Slide 30

Life Cycle of a Seedless Vascular Plant - Fern Dominant Sporophyte

Key

Haploid (n)

Diploid (2n)

MEIOSIS

Spore

dispersal

Sporangium

Sporangium

Mature

sporophyte

(2n)

Sorus

Fiddlehead

Spore

(n)

Young

gametophyte

Mature

gametophyte

(n)

Archegonium

Egg

Antheridium

Sperm

FERTILIZATION

New

sporophyte

Gametophyte

Zygote

(2n)

Slide 31

Transport in Vascular Tissue: Xylem and Phloem

Vascular plants have two types of vascular tissue: xylem and phloem.

Xylem conducts most of the water and minerals and includes dead cells called tracheids.

Phloem consists of living cells and distributes nutrients: sugars, amino acids.

Water-conducting cells are strengthened by lignin and provide structural support.

Increased height was an evolutionary advantage.

Slide 32

Evolution of Roots and Leaves

Roots are organs that anchor vascular plants and enable plants to absorb water and nutrients from the soil.

Roots may have evolved from subterranean stems.

Leaves are organs that increase the surface area of vascular plants for capturing more solar energy used for photosynthesis.

Slide 33

Hypotheses for Evolution of Leaves

Vascular tissue

Sporangia

Microphyll

(a) Microphylls - single veined leaves

(b) Megaphylls - branching leaf veins

Overtopping

growth

Megaphyll

Other stems

become re-

duced and

flattened.

Webbing

develops.

Slide 34

Most seedless vascular plants are homosporous, producing one type of spore that develops into a bisexual gametophyte.

All seed plants and some seedless vascular plants are heterosporous, producing megaspores that give rise to female gametophytes, and microspores that give rise to male gametophytes.