A plant growing on land
plant adaptation

Having examined the distinguishing featuresof the four main groups of plants, namely bryophyta, ferns, conifers andflowering plants, we are in a position to understand more clearly theevolutionary progress that plants have made on adapting to life on land.

The problems faced by plants in adapting toland/terrestrial habitat

Probably the greatest single problem toovercome is making the transition from water to land is that of drying out, or desiccation. Any plant not protected insome way, for example by a waxy cuticle, would tend to dry out and die veryrapidly.

Even if this difficulty is overcome, thereremain other problems, notably that of successfully achieving sexualreproduction. In the first plants this involved a male gamete which had to swimin water to reach the female gamete.

The first to colonise the land aregenerally thought to have evolved from the green algae, a few advanced membersof which evolved reproductive organs, namely archegonia (female) and antheridia(male), that enclosed and thus protected the gametes within. This, and certainother factors that helped to prevent desiccation, enabled some of them toinvade the land.

One of the main evolutionary trends inplants is their gradually increasing independence from water.

plant adapted to land
terrestrial plant

The main problems associated with thetransition from an aquatic to a terrestrial environment are summarised below.

  • Desiccation: air is drying medium andwater is essential for life for many reasons. Means of obtaining water andconserving it are required.
  • Reproduction: Delicate sex cells must beprotected and motile male gametes (sperm) require water if they are to reachthe female gametes.
  • Support: air, unlike water, offers nosupport to the plant body.
  • Nutrition: plants require light andcarbon dioxide for photosynthesis, so at least part of the body must be abovethe 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 indarkness.
  • Gaseous exchange: for photosynthesis andrespiration, carbon dioxide and oxygen must be exchanged with the atmosphererather than a surrounding solution.
  • Environmental variables: water,particularly large bodies of water like lakes and oceans, provides a veryconstant environment. A terrestrial environment, however, is much more subjectto changes in important factors such as temperature, light intensity, ionicconcentration and pH.

Adaptations of Liverworts and mosses

Mosses are well adapted to a terrestrialenvironment in their mode of spore dispersal, which depends on the drying outof the capsule and the dispersal of small, light spores by wind. However, theystill show a great reliance on water for the following reasons.

  • They are still dependent on waterfor reproduction because sperm must swim to the archegonia. They are adapted torelease their sperms when water is available since only then do the antheridiaburst. They are partly adapted to land because the gametes develop inprotective structures, the antheridia and archegonia.
  • There are no special supportivestructures, so the plants are restricted in upward growth.
  • They are dependent on theavailability 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 arepresent for anchorage, an adaptation to solid substratum.

Adaptation of seed bearing plants – conifers and floweringplants

One of the main problems for plants livingon land is the vulnerability of the gametophyte generation. For example, inferns the gametophyte is a delicate prothallus and it produces male gametes, orsperm, dependent on water for swimming. In seed plants, however, thegametophyte generation is protected and very much reduced.

Three important advances have been made byseed plants, first the development of two types of spore (heterospory), secondlythe 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 closerelatives developed two types of spore. This is known as heterospory, and the plants are described as heterosporous. Allseed-bearing plants are heterosporous. They produce large spores called megaspores in one type of sporangium(megasporangium) and small spores called microsporesin another type of sporangium (microsporangium).

When spores grow they form gametophytes.Megaspores produce female gametophytes and microspores produce malegametophytes. In seed bearing plants the gametophytes produced by megasporesand microspores are very small and never released from the spores. Thus thegametophytes are protected from desiccation, an evolutionary advance.

However, sperms from the male gametophytestill have to travel to the female gametophyte. This is made easier bydispersal of the microspores. Being very small they can be produced in largenumbers and blown away from the parent sporophyte by wind. Being very smallthey can be produced in large numbers and blown away from the parent sporophyteby wind.

By chance they can be brought into closerproximity to the megaspores, which in seed plants stay attached to the parentsporophyte. This is the basis of pollination in seed plants, where microsporesare in fact the pollen grains, inside the pollen grains male gametes form.

In seed plants another evolutionary advancehas occurred. The male gametes no longer have to swim to the female gametes becauseseed plants have evolved pollen tubes. These grow from the pollen grains to thefemale gametes and deliver the male gametes. There are no longer any swimmingsperm, just male nuclei.

Thus for the first time, plants evolved amechanism for fertilisation which was not dependent on water. This is one ofthe main reasons why seed plants are so much more successful than other plantsat exploiting dry land. Pollination was originally achieved by wind, a fairlyhaphazard process involving large wastage of pollen.

However, early in the evolution of seedplants flying insects appeared (in the Carboniferous era about 300 millionyears ago) bringing the possibility of more efficient pollination by insects.The flowering plants have exploited this method to a high degree, althoughconifers 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 withinthe megasporangium. This is known as an ovulein seed plants.

The ovule contains the female gamete. Oncethis is a fertilised ovule. The ovule/seed brings the following advantages.

  • The female gametophyte isprotected by the ovule. It is totally dependent upon the parent sporophyte andis not susceptible to desiccation as would be a free-living gametophyte.
  • After fertilisation it developsa food store, supplied by the parent sporophyte plant to which it is stillattached. The food will be used by the developing zygote (the next sporophytegeneration) at germination.
  • The seed is specialised toresist adverse conditions and can remain dormant until conditions and canremain dormant until conditions are suitable for germination.
  • The seed may be modified tofacilitate dispersal from the parent gametophyte.

The seed is a complex structure because itcontains cells from three generations, a parent sporophyte, a femalegametophyte and the embryo of the next sporophyte generation. All theessentials for life are supplied by the parent sporophyte and it is not untilthe seed is mature, containing a food store and an embryo sporophyte, that itis dispersed from the parent sporophyte.

Summary of adaptations of seed-bearing plants to life onland

The major advantages that seed bearingplants have over other plants are as follows.

  • The gametophyte generation ismuch reduced. It is always protected inside a sporophyte, which is well adaptedfor life on land, and is totally dependent on the sporophyte. In other plantsthe gametophyte is susceptible to drying out.
  • Fertilisation is not dependenton water. The male gametes are non-motile and carried within pollen grainsdispersed by wind or insects. Final transfer of the male gametes afterpollination is by means of pollen tubes.
  • The fertilised ovule (seed) isretained for some time on the parent sporophyte from which it obtainsprotection and food before dispersal.
  • Many seed plants show secondarygrowth with production of large amounts of wood. This provides support. Suchplants become trees or shrubs and are able to compete effectively for light andother resources.

Seed plants have other features which arenot unique to them as a group but which are also adaptations to life on land.

  • True roots enable water in thesoil to be reached.
  • The plant is protected fromdesiccation by an epidermis with a waterproof cuticle (or by cork aftersecondary 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 specialisedadaptations to hot dry environments.