Adaptation in bryophytes to land habitat

Adaptation in bryophytes to land habitat.

Bryophytes have developed following adaptive characters for terrestrial environments:

1. Bryophytes have compact multicellular plant body. It reduces the surface area exposed to dry land. So it helps in the conservation of water. They also have a layer of cuticle. It further reduces the loss of water by evaporation.

2. Their photosynthetic tissues develop special chambers for the absorption of CO₂. These chambers do not lose much water on exposure to light.

3. They develop rhizoids for absorption of water and anchorage (fixation).

4. They show heterogamy (different types of gametes). Their egg is non-motile and it contains stored food. Their sperm is motile.

5. They develop special multicellular reproductive organs called antheridia and archegonia. These reproductive organs produce and protect gametes.

6. They develop multicellular embryo. Embryo is retained and protected inside the female reproductive body during its development.

7. They show alternation of generation. They develop gametophyte and sporophyte generations. Gametophyte produces gametes and sporophyte produces spore. The alternation of generation produces best genetic combinations. It helps them to adapt in the terrestrial conditions.

Classification of bryophytes

Bryophytes are divided into three sub-divisions:

1. Hepaticopsida (Liverworts)

2. Bryopsida (Mosses)

3. Anthoceropsida (Horn worts)

1. Hepaticopsida (Liverworts)

The bryophytes belong to this sub-division are called liverworts. This class contains 900 species.

1. Liverworts are the simplest bryophytes.

2. They grow on moist rocks and wet soil. They live near water. So there they do not become dry.

3. Their plant body is gametophyte. There are two forms of gametophytes:

➤ Thallus: Thallus is flat or ribbon-like. It is branched dichotomously. It is attached to soil by means of rhizoids. Example: Marchantia.

➤ Plant like body: In this case, the plant body grows upright. It has false leaves. Its stem is differentiated into false stem and leaves. Examples: Porella.

4.The sex organs develop near the tips of the branches on the upper surface of thallus. Some time sex organ develop on special branches of gametophyte called antherididophores and archegoniophores as in Marchantia.

5. The sporophyte is dependent on gametophyte.

2. Bryopsida (Mosses)

1. Mosses live in damp places. They can also grow in dry places. However, water is essential for the reproduction of mosses. 

2. They grow to form cushions or mats on the rocks.

3. The adult plan body of moss is gametophyte. Gametophyte has stem and leaves like structures.

4. Their reproductive organs, antheridia and archegonia develop on the tips of different branches. These branches may be on the same plant as Funaria or on the different plants as in Polytrichum. The archegonia and antheridium form clusters. They are mixed with hairs called paraphyses.

5. They show alternation of generation. The gametophyte and sporophyte generations alternate with each other.

➤ Gametophyte: Gametophyte produces haploid gametes. These gametes fuse to form zygote. The zygote germinates to form sporophyte.

➤ Sporophyte: The sporophyte produces spores. The spore of mosses produces algae like structure called protonema. Protonema is absent in liverworts. A bud is formed on protonema. Each bud grows to form haploid gametophyte and complete the life cycle.

3.Anthoceropsida (Horn worts)

They are slightly advanced than the Hepaticopsida and Bryopsida.

(a)  Gametophyte

1. Their gametophyte is highly lobed. It has irregular outline.

2. Antheridia and Archegonia are partially sunken in the gametophyte tissues.

(b)  Sporophyte

1. The sporophyte does not depend on gametophyte for nourishment and protection, except in the early stage of development.

2. The sporophyte shows many advanced characters. So it can easily grow on land as compared to other bryophyte groups.

3. Sporophyte has stomata and chloroplasts in the epidermis. So it can prepare its own food by photosynthesis. It does not obtain food from gametophyte.

4. It also has waxy cuticle layer. It reduces loss of water (desiccation).

5. It also has a band of meristematic tissue at the junction of foot and spore producing region.

The meristematic tissues continue to form new cells towards the spore producing region during. While the spores are formed, mature and disperse from the other end There is fast growth rate of these meristematic tissues. So the length of the sporophyte continues to increase for an indefinite period of time. In this way, the sporophyte survives even after the death and decay of the gametophyte.

6. Example: Anthoceros. It is found in the hilly areas of Pakistan.