Land Plants, Evolution and Diversity

Outline of the Lecture (primarily from Wallace)

What are Plants
Alternation of Generations

Plant Divisions

  • Bryophyta: Nonvascular Plants
  • Vascular Plants
    • Club Mosses
    • Horsetails (Sphenophyta)
    • Ferns (Pterophyta)
    • Seed Plants
      • Gymnosperms
      • Angiosperms [link to Flowering Plant Diversity]
        • Monocots
        • Dicots


Plant Adaptations to Land

Special Adaptations of Angiosperms

Morphology of a Typical Angiosperm


Essential vocabulary

What are Plants

Plants are multicellular photosynthetic organisms that are believed to have evolved from green algae. Both groups have chlorophylls a and b and betacarotene as their photosynthetic pigments, both store reserve food as starch, and both have cellulose containing cell walls.


Distinguishing Features of all plants

  • multicellular
  • photosynthetic (chlorophylls a and b and carotenoids in thylokoid membranes.)
  • food storage molecule - amylose starch
  • structural polysaccharide - cellulose
  • life cycle - sporic alternation of generations
    • diploid sporophyte
    • haploid gametophyte
  • produce multicellular embryo protected in multicellular haploid (gametophyte -egg sac) tissue which differs from green algae.


The plant life cycle is sporic. This involves an alternation between a multicellular diploid sporophyte which, through meiosis produces haploid spores which in turn undergo mitosis to form a multicellular gametophyte which makes, what else, gametes.

The multicellular diploid embryo is housed within a multicellular gametophyte (in algae the embryo is separate from the haploid tissue).


Plant Divisions

Plants are divided into two major groups based on the presence (in vascular plants) or absence (in nonvascular plants) of an internal vascular system for transporting water and dissolved particles.

The nonvascular plants -- bryophytes -- require a constantly moist environment. They include:

Vascular plants or tracheophytes include:

seedless vascular plants such as -

seed- bearing forms. The success of seed plants (gynmosperms and angiosperms may be attributed to:

  • development of an extensive root system
  • an efficient vascular system (xylem and phloem)
  • reproductive structure in which the gametophyte is protected inside sporophyte tissue (the seed)

The seed plants include:

Plants - Adaptations to Land

Some of the adaptations of plants to a terrestrial existence include a waxy cuticle, surface pores (stomata) that enable gas exchange, protected reproductive structures, and the retention of the embryonic sporophyte within the female gametophyte. Review the table below for more adaptations for life on dry land.

Spatial Segregation of Resources
Minerals in the soil and water




Light and CO2 in the air





Regional Specialization


  • lack wax and chlorophyll
  • have large surfaces area (aided by mycorrhiza
  • have vascular tissue for conduction of food and water

Leaves and Stems

  • have waxy coverings to prevent water loss
  • chloroplasts can move to obtain the best arrangement for absorption of light
  • turgor pressure prevents wilting
  • stomata regulate water loss and CO2 absorption
prevents vertical growth


reinforces cellulose. This skeletal support allows turgor pressure to increase to help maintain rigidity.

Increase in Height
requires mechanism to prevent water loss and conduction of water more effectively



Vascular Transport System
allows plants to grow taller in order to reach light. The vascular system is made of microscopic pipes, the xylem and phloem. Xylem is lignified adding to the other supporting tissues.

Adaptations to Dryness











help disperse plants and are able to survive the harshest conditions. Some can survive for years waiting for just the right conditions to sprout.


Pollen, the male gametophyte, is designed to travel great distances by wind or insect

Dominance of Sporophyte Generation

The diploid condition exhibits all the specialized tissues described above.

Special Adaptations of Angiosperms

Flowering plants are the most numerous of the modern plants. Many of their flowers, designed to attract pollinators, are the product of coevolution with insects (and other animals) resulting in an efficient means of uniting sperm and egg. Their fruits are often designed to aid in the dispersal of their seeds.

Flower Specialization
How -
  • colors, nectar, and fragrances
  • radial vs. bilateral symmetry
  • incomplete vs. complete flowers
  • perfect vs. imperfect flowers
  • single vs. composite flowers

Why -

  • prevents self fertilization
  • reduces or minimizes energy needs
  • attracts insect pollinators


Pollen grains may be transferred from anther to stigma without the help of moisture.
  • wind and millions of tiny, buoyant pollen grains
  • coevolution between flowers and pollinators such as bees


Double Fertilization
  • A tube emerges from the coat of the pollen grain (male gametophyte) and
  • digests its way through the style.
  • Generative and tube nuclei become sperm.
  • The sperm enter egg sac via pollination tube through micropyle
  • one sperm unites with egg forming the zygote
  • the other sperm nucleus unites with a polar cell containing two nuclei- endosperm is formed (triploid tissue).

Morphology of a Typical Angiosperm

The plant body is divided into a root system and a shoot or stem system, connected by vascular tissue that is continuous throughout the plant. The root system of this dicot consists of a taproot and several lateral roots. Shoots consist of stems, leaves, and flowers. The blade, the expanded portion of a leaf, is attached to a stem by a petiole. Nodes, the regions of a stem where leaves attach, are separated by internodes. At a shoot's tip is the terminal bud, the main growing point of the shoot. Axillary buds are located in the upper angles of leaves. Most of these axillary buds are dormant, but they have the potential to develop into vegetative (leaf-bearing) branched or flowers.