NOTES FOR BIOLOGY 1002

SECTIONS 004, 005, 006

Spring 2006

DR. STEVEN POMARICO

CHAPTER 23

PLANTS

The main distinguishing characteristics of the Kingdom Plantae are

         -Eukaryotic

         -Multicellular

         -Photosynthetic autotrophs

         -Varied and complex life cycles

         -Cell walls made of cellulose

         -Food reserve stored as starch or oils

In addition to these characteristics, the plant kingdom shows a range of traits that appeared as plants made the transition from their origins in water onto the land.

Some of these new traits are:

         Anchoring mechanism                                  Roots

         Conducting vessels                                      Xylem and Phloem

         Increased support mechanism                     Lignin

         External protection                                       Cuticle

         Mechanism for gas exchange                      Stomata

         Xylem and Phloem are parts of the plants vascular tissue system. They are vessels that transport water, minerals, nutrients (especially sugars) and hormones throughout the plant. Plants with this trait are vascular plants.

         Lignin is a material that is added to the cell walls of “woody” plants. It increases the stiffness of the cells and allows better support for the leaves.

         Cuticle is a layer of waxy material on the epidermal layer of plants that helps prevent water loss or desiccation.

         Stomata (stoma singular) are openings in the surface (usually underside) of a leaf, which allow gas exchange but limit water loss.

When plants transitioned to land they also had to develop different methods for reproduction for the movement of gametes (zoospores) and for the dispersal of spores and seeds (zygotes).

>THE LIFE CYCLE IN PLANTS

Most plants go through a life cycle that involves an alternation of generations (See fig. 10.7).

In this life cycle the diploid phase produces a sporophyte that produces haploid spores by meiosis.

The spores grow into the gametophyte that produces gametes. These gametes fuse during fertilization to form a diploid zygote that then grows into another sporophyte.

As plants moved to land two main categories of land plants arose (see fig 21-2):

         Nonvascular - Bryophytes

         Vascular -Tracheophytes

The bryophytes did not develop all of the characteristics required for a complete terrestrial existence and therefore remain closely linked to water.

The bryophytes include (See fig. 23.7) :

                   Mosses

Liverworts

                   Hornworts

The characteristics of these nonvascular plants are:

         -No vascular tissue

-No true root but they have rhiziods

-No true stems or true leaves

         -Semi-aquatic (freshwater)

                   Require water for sexual reproduction

         -Cuticle and stomata in some moss species

         -Protective structure for gametes

                   Archegonium and antheridium

Since these plants lack vascular tissue they rely on diffusion to move water, nutrients, etc. This limits their size and where they can grow.

During the haploid phase of the life cycle the gametophytes produce archegonia (singular archegonium) and antheridia (singular antheridium)

These structures produce and protect the gametes until mating. During mating the sperm swim (or are splashed) from the antheridia to the archegonia.

The remaining main category of land plants, the vascular plants, show additional characteristics:

         -Roots

         -Vascular tissue

                   Xylem and Phloem

         -Lignin

The first vascular plants to evolve were the seedless vascular plants (See fig 23.8).

This group includes:

         Whisk ferns

         Lycophytes or Club Moss

         Horsetails

         Ferns

The seedless vascular plants share some common characteristics:

         -Produce spores (not seeds). This isn’t anything new!

         -Sporophyte is the dominant form in the life cycle (See fig. 23.9)

         -Still require water for reproduction

Of these three divisions of seedless vascular plants only the ferns have well developed leaves.

The next groups of land plants to evolve were the seeds plants. The new characteristics that showed up with this group are:

         -No longer require water for reproduction

         -Gametophyte greatly reduced

                   Male gametophyte becomes pollen

         -Seeds replace spores.

         -Heterosporous (two types of spores: Male and Female)

The ability of pollen to be dispersed by wind or animals allows the seeded vascular plants to break their ties to water.

The seeded vascular plants can be divided into two major groups:

These groups are not taxonomic groups but like the preceding groups (seedless, seed plants) they are helpful in understanding the evolutionary steps that seem to have occurred in the Plant Kingdom.

                   Nonflowering Plants - Gymnosperms

                   Flowering Plants - Angiosperms

In addition to the difference of flower production, these major groups show differences in production of seeds.

The overall seed structure is similar for both groups with:

                   -Embryo

                   -Stored food

                   -Seed coat

The seeds of the Angiosperms have an additional covering.

The category of Gymnosperms includes:

         Conifers - Evergreens including Pine, Fir, Spruce,

                                                 Cypress, Redwood and Cedar

         Cycads

         Ginkgo

         Gnetopytes

Of these three Divisions the Conifers are currently the most successful because of their ability to live under a wide variety of conditions (including very harsh).

The life cycle of the Conifers (See fig. 23.17) show two of the key characteristics of seed plants:

                   -Pollen

                   -Dominant Sporophyte

One division represents the remaining land plants:

                   Angiosperms

The main characteristics include:

                   -Flowers

                   -Seeds enclosed in a fruit

                   -Broad leaves

                   -Advanced vascular tissue

The Angiosperms are divided 3 major groups:

                   Magnoliids

Monocots

Grasses, Grains, Lilies, and Palms

                   Dicots