Kingdom Plantae | Phylogenetic System of Classification |

Kingdom Plantae

Phylogenetic System of Classification

The system of classification which is based on the natural relationship among the organisms and their mode of origin is called the Phylogenetic system of classification. The biologists are trying to classify the living organisms into various groups on the basis of similarities and dissimilarities at various levels. These organisms show natural relationships among the organisms and their origin. Therefore, the biologists use phylogenetic system of classification.

Kingdom Plantae

There are about 360,000 species of plants. Kingdom Plantae mainly includes:

1. Eukaryotic autotrophic, multicellular and non-motile organisms.

2. They develop from the embryo.

3. They have cell wall outer to cell membranes. This cell wall is made up cellulose.

The kingdom Plantae can be divided into two broad categories:

➤ Bryophyta: These are non-vascular plants.

➤ Tracheophyta:  These are vascular plants.

This grouping is not according to the specific system of classification. It shows only similarities and dissimilarities among various groups of plants. Each category (division) is divided into Sub-divisions, Classes, Sub-classes and other taxonomic ranks. There is a brief outline of the classification of Plantae:

Division bryophyta

Bryophytes are defined as, plants without vascular system, gametophyte dominant, sporophyte attached to gametophyte and homosporous. Bryophytes show the following characters.

1. Bryophytes are the first land plant.

2. They were evolved from green algae.

3. They are poorly adapted for life on land. So they are present only in damp shady places.

4. Conducting and strengthening tissues (xylem and phloem) are absent in bryophytes.

5. The transport of water, minerals, prepared food and substances takes place by diffusion.

6. The plant body is with or without cuticle. Some bryophytes have a very thin layer of cuticle.

7. Water is absorbed by the general surface of the plants.

8. The bryophytes cannot live without water. So they are also called amphibious plants. They need water for their reproduction.

9. The gametophyte is dominant in them and the sporophyte is attached with the gametophyte.

10. Sex organs in bryophytes are antheridia and archegonia.

Life Cycle of Bryophytes

Bryophytes plants show regular alternation of generations. These generations are heteromorphic.

Gametophyte

The gamete producing generation is called gametophyte. It is haploid, dominant, independent, and free-living generation in bryophytes. There are two body forms of gametophytes in bryophytes.

➤ It may be thallus as in many liverworts.

➤ It is differentiated into structures resembling to stem, leaves and rhizoids. Rhizoids are absorbing and anchoring organs. Examples: mosses and some liverworts.

Sporophyte

The spore-producing generation is called the sporophyte. It is a less conspicuous generation in bryophytes. It partially or completely depends on the gametophyte for its nutrition. The sporophyte generally consists of foot, seta and capsule. 

The sporophyte is diploid (2n). It produces haploid spores by meiosis in sporangia (capsule). These spores are of one kind. So sporophyte of is homosporous. The spores germinate to produce gametophyte. The entire development of sporophyte takes place within the gametophyte generation.Even after full development, the sporophyte remains attached to gametophyte. It does not contain chloroplasts and cannot perform photosynthesis. So it depends on the gametophyte for nourishment. The alternation of generation is an important phenomenon. It produces variation and selects the best genetic makeup among the organisms. These variations are necessary for survival and adaptation in changing environments.

Reproductive organs

The gametophyte produces male and female reproductive organs. These male and female reproductive organs are present on the same plant or on different plants. The reproductive organs are protected by a covering of sterile cells. The sex organs produce gametes by mitosis.

➤ Antheridia (sing. Antheridium): Antheridia are male reproductive organs. Antheridia produce male gametes called antherozoids. The antherozoids are motile. They are produced in large numbers.

➤ Archegonia (Sing, Archegonium): Archegonia are a female reproductive organ. A single egg is formed in each archegonium.

Fertilization

Fertilization takes place in water. Antherozbids (1n) move towards archegonia chemotactically (chemical attractions). A single antherozoid (1n) fuses with an egg (1n) to form a diploid (2n) zygote.

Embryo formation

The zygote is retained within the female sex organ (archegonia). After resting period, the zygote divides by mitosis to form the diploid embryo. This embryo develops to from sporophyte. This sporophyte is also diploid.