ADAPTATIONS TO LIFE ON LAND
Making the transition from a small, aquatic, Chlorophyta alga to the large, multicelled terrestrial plants of today required a number of structural and physiological changes as outlined below. Recent* findings support the view that the most ancestral green flagellate was biflagellate and asymmetric, and possessed an eyespot, square scales and multilayered structures. *Lemieux, C., Otis, C.,and Turmel, M. Nature 403,649-652 (2000). This led to a number of evolutionary dead ends possessing many of the characteristics present in the higher plants.
Conditions of Land Life
To maintain itself a living cell must obtain water and other materials by absorption through the cell membrane. Materials must be available fairly consistently throughout all cells of an active organism. Diffusion can be used to distribute materials if the organism is small or consists of only a few cells. Difficulties arise in larger organisms because some cells are internal - out of direct contact with the environment. These problems are aggravated when some parts extend into the air and are distant from a source of water and dissolved materials. Water must be passed along from cell to cell by osmosis and diffusion. These are short distance processes which are slow and ineffective over distances inside the plant.
Movement of materials across great distances is the first barrier to any plant attempting to extend itself into the air. Development of vascular systems somewhat analogous to circulatory systems in animals has overcome this problem in higher or vascular plants.
Specialization at the level of the tissues and organs was also necessary. Systems to gather in nutrients and water (roots) and collection and conversion systems to utilize the power of sunlight (leaves)and systems to hold the plant upright and increase its competitive ability to battle for sunlight (stem) were required. Changes in reproductive structures and behaviors led to protected male gametophytes (pollen) and embryos that were protected and nurtured (seeds).
A COMMON SET OF FEATURES EVOLVED BY GREEN PLANTS
IN RESPONSE TO THE LAND ENVIRONMENT
- Cuticle - waterproofing of outer layers of leaf and stem
- Pores or Stomates surrounded by Guard cells and regulated by K+ levels - aided in water balance within leaf and entire plant body through transpiration
- Rhizoids or roots - provided anchorage, absorption, support
- Lignin - strengthening between cells and within tissues
- Vascular tissue - xylem and phloem; move water and nutrient
upwards and food downward
- Resistant spores and seeds - prevent dessication, contain inhibitory substances; ensure survival of the species
- Gamete forming cells enclosed within a jacket of sterile cells provides protection from water loss
- Embryos formed and held within parent tissues - provides
protection and a food supply to developing plant
- Root systems show a highly refined geotropic response.
EVOLUTIONARY TRENDS ACCOMPANYING THE TRANSITION
- The sporophyte has replaced the gametophyte as the dominant stage in the life history of the plant. During this shift,
the size of the sporophyte progressively increased, while that of the gametophyte decreased.
- The increase in the size of the sporophyte was accompanied by an increase in strength and efficiency of its vascular tissue.
- As tissues became more specialized, new organs were added
progressively: simple bodies with rhizoids ---> stems ---> stems with roots and tiny leaves ---> many-veined leaves ---> woody tissues
- The male gametophyte evolved into a waterproof pollen grain, which traveled to the female gametophyte before releasing the sperm. Sperm no longer faced a long, dangerous swim to the
- The female gametophyte of all land plants retains the egg and protects the zygote as it develops into an embryo; in higher land plants, the female gametophyte itself is retained on or in the sporophyte parent, which contributes food and protective coatings to the seed, a new dispersal structure containing the embryo of the next sporophyte generation.
- In the higher vascular plants, the spore, a single haploid
reproductive cell that is the dispersal stage of lower land
plants, has become differentiated into megaspores, which give rise to female gametophytes, and microspores which give rise to pollen grains (male gametophytes). Instead of being shed, the megaspores remain in the sporophyte, which protects the
female gametophyte and plays a major role in the production
of the seed. Seeds replaced spores as the dispersal stage in the life history.
- As plant structure and reproduction became more and more
independent of water and moisture in the environment, plants underwent adaptive radiation and spread into more and more
Sexual reproduction (fusion of GAMETES) and Asexual reproduction (by means of SPORES), occur in ALTERNATING GENERATIONS. One generation, where the plant body is known as the GAMETOPHYTE is Haploid (n) and produces Haploid GAMETES, the Sperm and the Egg. These gametes then fuse to form a ZYGOTE in the process of FERTILIZATION. This Diploid Zygote develops into a SPOROPHYTE (also Diploid). The Sporophyte undergoes MEIOSIS and forms SPORES that develop into an adult organism called the GAMETOPHYTE This completes the cycle.
The zygote was formed as a result of the fusion of two cells, while the Gametophyte plant developed as the result of asexual reproduction (mitosis) of the spore.
METAPHYTA: NON-VASCULAR PLANTS - THE BRYOPHYTES
All available evidence seems to indicate that this division represents an evolutionary dead end. There is no evidence to indicate that this division is ancestral to any of the modern vascular plants. It has been suggested that differences between the three major classes within the division may necessitate reclassification into separate divisions at some time in the future.
These plants show many of the adaptations needed for a successful transition to a terrestrial environment. As a division these plants:
- lack specialized vascular tissue or supportive tissue
- possess a characteristic life cycle which follows the generalized life cycle where the SPOROPHYTE is attached to and dependent upon the GAMETOPHYTE for nutrition - the
GAMETOPHYTE is dominant, conspicuous, and longer lasting
In higher, vascular plants the relative importance of the SPOROPHYTE and GAMETOPHYTE are reversed.
Gametophyte plants may be MONOECIUS (one house) and contain both male and female organs or, DIOECIOUS (two houses) have separate plants for each. The sex organs are multicellular and bounded by a layer of sterile cells.
Bryophytes are usually small, ranging from 2-20 cm. They lack true roots, stems, or leaves although they have structures which accomplish many of the functions of these organs.
Leaf-like structures are usually a single cell layer thick and are called PHYLLIDS. Root-like structures, called RHIZOIDS, serve mainly for anchorage, and stems or stalks, called SETA or CAULIDS, serve for support.
There are three recognized classes of Bryophyta:
Liverworts, Hornworts, and Mosses
Class Musci - The True Mosses
This class is frequently confused with sea moss (red algae), reindeer moss (lichen), spanish moss (flowering plant), and club mosses (primitive vascular plant).
Mosses are distinguished from Liverworts and Hornworts by:
- an algal-like PROTONEMA
- radial symmetry of the plant body
- the elaborate capsule of the mature sporophyte
There are approximately 670 genera representing about 16,000
species of mosses. They are normally found in wooded, moist areas, usually in dense stands forming carpets or mats, and are seldom more than 6-8 inches tall. There are some species found in desert areas. They are sensitive to air pollution, especially sulfur dioxide.
Economically they are of importance as:
soil formers in the process of succession
retarders of erosion
a source of fuel (peat) in some parts of the world
a source of packing and/or aeration of soil, as with
Sphagnum moss, which has a high water holding capacity and
adds some acidity to the soil
The following characteristics of mosses distinguish them as transition plants between aquatic and terrestrial habitats.
- dependence on water for sexual reproduction
- vegetatively well adapted for life on land
- relative size and independence of sporophyte and gametophyte generations; situation reversed in higher plants
It appears that the sporophyte, which eventually evolved a vascular system in higher plants, is better adapted to life on
land (terrestrial), in that spore production is well suited to that type of habitat. By contrast, the biflagellated gametes of the gametophyte generation are better suited to an aquatic mode of life.
Mosses seem to retain a close algal relationship as evidenced by the structure of the PROTONEMA, similar to that of some green algae; possible evidence for evolution from green algal forms.
Mosses established at least two critical features which later were more fully developed by land vascular plants.
- retention and protection of the embryo within the female parent plant
- presence of a strand of tissue occupying the center of the stemlike axis used for support and conduction - may be prevascular tissue which evolved into true vascular tissue containing tracheids and sieve tubes; in mosses this central strand of tissue consists of cells called HYROIDS
ALTERNATION OF GENERATIONS in Mosses
Haploid spores form within the capsule which is covered by a lid called an OPERCULUM. As many as 50 million spores may be formed. When released the spores develop into a branched, filamentous PROTONEMA from which a leafy gametophyte develops (may be monoecious or dioecious). Biflagellated sperm are released from the ANTHERIDIUM and attracted to the ARCHEGONIUM by malic acid. Within the archegonium the sperm fertilizes the egg cell which develops into a zygote which divides mitotically to form the SPOROPHYTE. Typical structures form and spores develop within the capsule. Some species have a PERISTOME beneath the operculum (humidity sensitive hairs regulating spore dispersal (hygroscopic), e.g. the genus Polytrichum.
METAPHYTA: SEEDLESS VASCULAR PLANTS
Tracheophytes (vascular plants) are the largest group of plants on earth and are characterized by the presence of tracheary elements and sieve tube elements organized into xylem and phloem in the adult sporophyte. (see notes above on Plant Adaptations to the Land Environment)
All lines of evidence suggest that the tracheophytes developed directly from some complex, multicellular Chlorophyta. Unfortunately, an extremely large gap exists between the algae and vascular plants, and there are no living plants which bridge the gap. Bryophytes are somewhat intermediate but are closer to algae than even the simplest tracheophytes, and it appears that the intermediate plants have become extinct.
In addition, tracheophytes are identified by independent and dominant sporophytes and greatly reduced gametophytes. This is in direct contrast to the Bryophytes. The sporophyte of most tracheophytes are differentiated into roots, stems, and leaves. Also, vascular plants are typically land plants and, like bryophytes, have evolved terrestrial adaptations. Check out this comparative information on the lower tracheophytes.
Evolution in the Lower Tracheophytes (Psilophytes, Lycophytes,
Sphenophytes, and Ferns)
Transition from water to land was gradual over millions of years. Those plants which evolved adaptations permitting growth in a dry environment established themselves as the first land plants. The basic problem confronting early land plants was availability of water. The tracheophytes did not have the limitless supply of water which was available to the aquatic green algae. A successful land plant must possess an efficient mechanism for absorbing materials from the soil and transporting them throughout the plant. Once obtained from the environment the water must be conserved.
The evolutionary significance of the Psilophytes is that they are the most primitive known group of vascular plants and are considered to be a transitional group linking the aquatic algae with more advanced tracheophytes (club mosses, horsetails, and ferns). In some respects, the Psilophytes substantiate the hypothesis for the evolution of the root as proposed by Lignier. In other respects they are of interest because they suggest how club mosses, horsetails, and ferns may have originated by various leaf modifications and developments. Certain psilophytes with small leaves developed as emergences and may have given rise to the Lycophyta line. Others with whorled branches may have been the forerunners of the Sphenophyta line. The fern line may have developed from forms in which the branch tips were flattened, possibly leading to the evolution of large leaves. Two major genera represent this group, Psilotum and Tmesipteris.
Represented by Lycopodium and Selaginella,the lycophytes are believed to have evolved from the psilophytes. Although members of this group are of no real economic import, the group does illustrate some noteworthy evolutionary advances over the psilophytes. The presence of true roots, and leaves, increased devel
opment of the vascular tissue, and the organization of sporophylls into cones (strobili) are all viewed as advances over the psilophytes. It is not clear why the lycophytes have not given rise to more advanced groups of plants. Some have developed secondary growth and exhibit early stages of the seed habit.
These plants are of little economic significance, although together with the lycophytes, they contributed their vegetative parts to the formation of coal during the Carboniferous period. They are considered to be a separate line derived from the psilophytes which did not give rise to other plant groups. Major genus is Equisetum, known as horestails or scouring rushes. Some species accumulate silicaceous materials in their cell walls.
Division Pterophyta (Ferns)
Ferns and other pterophytes are believed to have developed from the psilophytes. Certain fossil ferns resemble the psilophytes rather closely. The seed plants are considered to have derived from certain of the extinct fern class. Modern fern groups are of little economic importance. From an aesthetic view, they are grown exclusively for decorations. The fiddleheads and young foliage of many species are edible and are used as green vegetables in the Orient. A drug is derived from the rhizomes (underground stem) of some ferns and is used to expel worms (vermifuge)
especially tapeworms, from the intestinal tract of man. Fossil ferns contributed to the deposits of the Carboniferous period.
Refer to the diagram of the Fern Life Cycle