Amphibious Nature of Bryophytes: Adaptations to Land Habit

Plants grow in two well-defined habitats: water (aquatic) and land (terrestrial.). Between these two habitats is a transitional zone, represented by the swamps and the areas where land and water meet. This zone is called the amphibious zone. Inhabiting this zone are the moses, liverworts, and hornworts which constitute a group of non-vascular land plants called the bryophytes.

Evidence supports the view that these early land plants descended from the algal ancestors which were green. 

This movement of plants from water to land required them to make several adaptations to survive this new and strange environment. These adaptive features are as follows:

Development of organs for anchorage and absorption of water: With the change of habitat the bryophytes unlike the algae are not bathed in water. So, absorption of water and nutrients through body surface is not possible anymore. This led to the development of rhizoides which help in absorption as well as anchorage of the plant to the substratum. Two types of rhizoids are found: smooth and pegged. In addition to this, scales are developed in bryophytes with increases absorptive surface. For example, simple rhizoids are present in anthocerotopsida. In Riccia, both simple as well as pegged rhizoids are present. Septate rhizoids, which increases absorptive function, are present in mosses.

Rhizoids of Riccia

Protection against desiccation: A major problem due to change of habitat from water to land is desiccation. So, to protect it from desiccation, thick, compact, multicellular epidermis is developed in the bryophytes. In some liverworts the free surface of the epidermal cells is covered with waxy substances like cuticle which reduces water loss.

Gaseous exchange: In the terrestrial habitat the plants have to take carbon dioxide and oxygen from the atmospheric air. So numerous minute pores called airpores are developed on the upper surface of the thallus. These airpores facilitate gaseous exchange between atmospheric air and the interior of the thallus. In addition to this, air canals (vertical slits between pseudo-mesophylls) also help in gaseous exchange. For example, airpores are present in Marchantia. In Riccia, air chambers are present.

T.S. of Marchantia thallus showing air pores, air chambers and rhizoids

Protection of reproductive structures from drying and mechanical injury: As terrestrial habitat is more prone to mechanical injury and drying up the reproductive structures, i.e. sex organs in bryophytes become multicellular and jacketed. This gives additional protection to the eggs and the sperms.

The fertilized egg is retained within the archegonium where it obtains food and water and remains protected from drying as it develops into an embryo.  For example, in Marchantia, the antheridium is protected by antheridial wall while the archegonium is protected by archegonial jacket. The zygote is retained within the venter.

Marchantia thallus bearing reproductive structures

Development of vascular bundle-like structures in higher bryophytes: Primitive vascular system in the form of a conductive strand is present due to land adaptation. Bryophytes have been group into three main groups on the basis of water absorption:

Ectohydric: They can absorb water or solutes by body surface only. They do not possess well developed conducting strands. Example: leafy liverworts

Endohydric: They absorb water by rhizoids which are connected to well-developed conducting strands. Example: some moses and thalloid liverworts

Mixohydric: They absorb water and solutes both by body surface as well as rhizoids. Example: Funaria

Necessity of water to complete life cycle: Bryophytes cannot carry on their reproductive activities without sufficient moisture. Without water sex organs cannot mature and do not dehisce. Water is essential for transport of sperm to archegonium. This explains why the bryophytes usually inhabit moist shaded situations. 

Since the bryophytes usually grow in amphibious situations and cannot complete their life cycle without presence of external water they are called amphibians of the plant world.