Pre-reading and reading tasks.

1. Make sure you know the following words and word combinations:

2. Read and translate the text.

PLANT KINGDOM: PLANTAE

In most classification systems the plant kingdom Plantae includes several groups of simple photosynthetic organisms, sometimes known as eukaryotic algae, a reference to when people still grouped the prokaryotic blue-green bacteria with the algae. Euglenophyta, Chrysophyta, and Pyrrophyta, three phyla of unicellular eukaryotic algae often called plants, were described in the text Protista. The term true algae is no longer a technical word since there are so many algal groups that are not directly related to one another. Nevertheless, the common terminology is presented in conjunction with the formal classification because it continues to be used in most texts. Of the following eukaryotic algae, Chlorophyta (green algae) are thought to be the ancestors of most modern plants because they contain the photosynthetic pigments chlorophyll a and b, as well as beta-carotene; they store their food reserves as starch; and their cell walls are composed of cellulose. They are mostly freshwater organisms, though some are marine.

Phaeophyta (brown algae) are almost all marine and are common in cooler oceanic regions. Rhodophyta (red algae) are mostly warm-water, marine species, though some are freshwater. Most of the common species of seaweed are members of the brown and red algal groups.

Each of the three preceding phyla vary according to: 1) types of photosynthetic they contain, 2) type of food reserves stored internally, and 3) components found in their cell walls.

These algae are not included in the plant kingdom merely because they photosynthesize, since many protists and monerans also have that capacity. Nor is tissue differentiation always a key factor in determining where to draw the line. Multicellular algae have no true roots, stems, or leaves. Their simple body form is termed a thallus, which is why they are sometimes called thallophytes. Many biologists call algae plants, but only lower plants.

The reason such matters are complex is that these organisms represent many different lineages and many different steps in a continuum connecting the most primitive forms of life, such as bacteria, to the most complex multicellular organisms. It isn’t always clear just where one should draw the somewhat arbitrary lines that artificially separate each of the five described kingdoms—Monera, Protista, Fungi, Plantae, and Animalia.

A reason the algae have been kept from the plant kingdom in some classifications is their reproductive structures. Higher plants have reproductive structures encased inside a protective wall of sterile cells that protect the developing zygotes before they are released from the female reproductive organs, where they were produced. Algae lack a protective wall of nondividing (sterile) cells, and their zygotes do not develop into embryos until after having been released from the female reproductive organs.

When lower plants and higher plants are placed in separate kingdoms, then the higher plants are sometimes called the Embryophyta since the female reproductive organs retain the zygotes until after they have developed into embryos. In addition to the lower plants, the major groupings in the plant kingdom include the Bryophyta (mosses, liverworts, and hornworts) and the Tracheophyta, which include all the vascular plants (psilopsids, club mosses, horsetails, and ferns) and seed plants (gymnosperms and angiosperms). However, the name Tracheophyta, which means tube plant, has since been eliminated from the classification.

True algae are mostly restricted to aquatic environments. The move to land was accompanied by many adaptations to what would otherwise have been a hostile environment. Out of water, plants were met with dry conditions, ultraviolet light, and nutritional problems. In addition, in the air these plants no longer benefited from the surrounding water's buoyancy, so some structural support became necessary.

Terrestrial plants had to obtain water by a new means, since they were no longer bathed in it. The water had to be both procured and then moved from its point of uptake to the other structures. In addition, the photosynthetic products had to be transported from their specialized photosynthetic structures to the other parts of the plant. Vascular tissues evolved that solved all these problems.

Excessive water loss from evaporation had to be curtailed, while the moist tissues necessary to allow gaseous exchange for metabolic and photosynthetic purposes had to be maintained. This led to highly evolved mechanisms that controlled overall water loss, while enabling structures to remain moist.

Reproductive needs also had to be modified in terrestrial environments, where flagellated sperm cells no longer had the surrounding water in which to swim. And special structures evolved that protected the early stages of embryonic development from desiccation.

The modifications that differentiate many terrestrial plants from their aquatic counterparts are understood only in terms of the factors affecting plants living in terrestrial versus aquatic environments. The following mechanisms have helped make it possible for embryophyte plants to inhabit terrestrial habitats:

1. A waxy cuticle usually covers the aerial parts of the embryophyte plants, acting as waterproofing and preventing excessive water loss.

2. All embryophytes are oogamous. That is, they have two types of gametes, one of which, the female, is typically the large, non-motile egg cell, the oogamete.

3. All embryophytes have multicellular sex organs covered with a layer of protective cells that are sterile. The male sex organs are known as antheridia, and the female are archegonia. Within the sex organs the gametes are protected from desiccation.

4. All embryophytes have egg cells (oogametes) that are fertilized within the archegonia.

5. While inside the archegonium, the zygotes develop into multicellular diploid embryos that obtain some of their water and nutrients from the parent plant.

6. In addition to producing gametes, embryophytes produce spores, reproductive cells that develop directly into full-grown plants without first having to undergo fertilization by joining with another cell. The embryophyte sporangia produce spores that are covered with a protective jacket of sterile cells.

COMPREHENSION CHECK