Having examined the distinguishing features of the four main groups of plants, namely bryophyta, ferns, conifers and flowering plants, we are in a position to understand more clearly the evolutionary progress that plants have made on adapting to life on land.
The problems faced by plants in adapting to land/terrestrial habitat
Probably the greatest single problem to overcome is making the transition from water to land is that of drying out, or desiccation. Any plant not protected in some way, for example by a waxy cuticle, would tend to dry out and die very rapidly.
Even if this difficulty is overcome, there remain other problems, notably that of successfully achieving sexual reproduction. In the first plants this involved a male gamete which had to swim in water to reach the female gamete.
The first to colonise the land are generally thought to have evolved from the green algae, a few advanced members of which evolved reproductive organs, namely archegonia (female) and antheridia (male), that enclosed and thus protected the gametes within. This, and certain other factors that helped to prevent desiccation, enabled some of them to invade the land.
One of the main evolutionary trends in plants is their gradually increasing independence from water.
The main problems associated with the transition from an aquatic to a terrestrial environment are summarised below.
- Desiccation: air is drying medium and water is essential for life for many reasons. Means of obtaining water and conserving it are required.
- Reproduction: Delicate sex cells must be protected and motile male gametes (sperm) require water if they are to reach the female gametes.
- Support: air, unlike water, offers no support to the plant body.
- Nutrition: plants require light and carbon dioxide for photosynthesis, so at least part of the body must be above the ground. Minerals and water, however, are at ground level or below ground, and to make efficient use of these, part of the plant must grow below ground in darkness.
- Gaseous exchange: for photosynthesis and respiration, carbon dioxide and oxygen must be exchanged with the atmosphere rather than a surrounding solution.
- Environmental variables: water, particularly large bodies of water like lakes and oceans, provides a very constant environment. A terrestrial environment, however, is much more subject to changes in important factors such as temperature, light intensity, ionic concentration and pH.
Adaptations of Liverworts and mosses
Mosses are well adapted to a terrestrial environment in their mode of spore dispersal, which depends on the drying out of the capsule and the dispersal of small, light spores by wind. However, they still show a great reliance on water for the following reasons.
- They are still dependent on water for reproduction because sperm must swim to the archegonia. They are adapted to release their sperms when water is available since only then do the antheridia burst. They are partly adapted to land because the gametes develop in protective structures, the antheridia and archegonia.
- There are no special supportive structures, so the plants are restricted in upward growth.
- They are dependent on the availability of water and mineral salts close to or at the surface of the soil, because they have no roots to penetrate the substrate. However, rhizoids are present for anchorage, an adaptation to solid substratum.
Adaptation of seed bearing plants – conifers and flowering plants
One of the main problems for plants living on land is the vulnerability of the gametophyte generation. For example, in ferns the gametophyte is a delicate prothallus and it produces male gametes, or sperm, dependent on water for swimming. In seed plants, however, the gametophyte generation is protected and very much reduced.
Three important advances have been made by seed plants, first the development of two types of spore (heterospory), secondly the development of non-swimming gametes and thirdly the development of seeds.
Heterospory and non-swimming male gametes. An important evolutionary advance was made when certain ferns and their close relatives developed two types of spore. This is known as heterospory, and the plants are described as heterosporous. All seed-bearing plants are heterosporous. They produce large spores called megaspores in one type of sporangium (megasporangium) and small spores called microspores in another type of sporangium (microsporangium).
When spores grow they form gametophytes. Megaspores produce female gametophytes and microspores produce male gametophytes. In seed bearing plants the gametophytes produced by megaspores and microspores are very small and never released from the spores. Thus the gametophytes are protected from desiccation, an evolutionary advance.
However, sperms from the male gametophyte still have to travel to the female gametophyte. This is made easier by dispersal of the microspores. Being very small they can be produced in large numbers and blown away from the parent sporophyte by wind. Being very small they can be produced in large numbers and blown away from the parent sporophyte by wind.
By chance they can be brought into closer proximity to the megaspores, which in seed plants stay attached to the parent sporophyte. This is the basis of pollination in seed plants, where microspores are in fact the pollen grains, inside the pollen grains male gametes form.
In seed plants another evolutionary advance has occurred. The male gametes no longer have to swim to the female gametes because seed plants have evolved pollen tubes. These grow from the pollen grains to the female gametes and deliver the male gametes. There are no longer any swimming sperm, just male nuclei.
Thus for the first time, plants evolved a mechanism for fertilisation which was not dependent on water. This is one of the main reasons why seed plants are so much more successful than other plants at exploiting dry land. Pollination was originally achieved by wind, a fairly haphazard process involving large wastage of pollen.
However, early in the evolution of seed plants flying insects appeared (in the Carboniferous era about 300 million years ago) bringing the possibility of more efficient pollination by insects. The flowering plants have exploited this method to a high degree, although conifers are still wind-pollinated.
Seeds. In the early heterosporous plants, megaspores were released from the parent sporophyte like the microspores. However, in the seed plants megaspores are retained on the parent plant within the megasporangium. This is known as an ovule in seed plants.
The ovule contains the female gamete. Once this is a fertilised ovule. The ovule/seed brings the following advantages.
- The female gametophyte is protected by the ovule. It is totally dependent upon the parent sporophyte and is not susceptible to desiccation as would be a free-living gametophyte.
- After fertilisation it develops a food store, supplied by the parent sporophyte plant to which it is still attached. The food will be used by the developing zygote (the next sporophyte generation) at germination.
- The seed is specialised to resist adverse conditions and can remain dormant until conditions and can remain dormant until conditions are suitable for germination.
- The seed may be modified to facilitate dispersal from the parent gametophyte.
The seed is a complex structure because it contains cells from three generations, a parent sporophyte, a female gametophyte and the embryo of the next sporophyte generation. All the essentials for life are supplied by the parent sporophyte and it is not until the seed is mature, containing a food store and an embryo sporophyte, that it is dispersed from the parent sporophyte.
Summary of adaptations of seed-bearing plants to life on land
The major advantages that seed bearing plants have over other plants are as follows.
- The gametophyte generation is much reduced. It is always protected inside a sporophyte, which is well adapted for life on land, and is totally dependent on the sporophyte. In other plants the gametophyte is susceptible to drying out.
- Fertilisation is not dependent on water. The male gametes are non-motile and carried within pollen grains dispersed by wind or insects. Final transfer of the male gametes after pollination is by means of pollen tubes.
- The fertilised ovule (seed) is retained for some time on the parent sporophyte from which it obtains protection and food before dispersal.
- Many seed plants show secondary growth with production of large amounts of wood. This provides support. Such plants become trees or shrubs and are able to compete effectively for light and other resources.
Seed plants have other features which are not unique to them as a group but which are also adaptations to life on land.
- True roots enable water in the soil to be reached.
- The plant is protected from desiccation by an epidermis with a waterproof cuticle (or by cork after secondary growth has taken place).
- The epidermis of aerial parts, particularly leaves, has many small holes, called stomata, which allow gaseous exchange between plant and atmosphere.
- Plants can show specialised adaptations to hot dry environments.