Mosses: Liverworts And Ferns (1969)

[Music] Most of the green plants we see from day to day, trees, shrubs, and grasses, are spermatapites, plants that produce seeds. A trip through the forest reveals little plant life besides spermatapites. Even in the shady depths of the forest where there's abundant moisture but little sunlight, seed plants appear to be the only plants on the scene. Yet here also live other plants, plants that do not produce seeds. Among them are mosses and liver warts, members of the bryophy film. And there are ferns, primitive members of the tracheopyte film. Interestingly, scientists believe that mosses, liver warts, and ferns were among the first kinds of green plants that lived on land. In the physiology and reproduction of these plants, they find clues indicating that the ancestors of land plants lived in a watery environment very much as modern-day algae. Members of the phalopyte group live in fresh water and in the sea. Microscopy reveals that certain algae of today resemble water plants that grew millions of years ago. Evolution has resulted in different species of algae with different appearances and all resemble the ancestral forms in this respect. They exchange water and waste products directly with the surrounding liquid. Algae are so dependent on their liquid environments that their bodies are limp, unable to support themselves when removed from water. Yet, from water plants like these, there may have evolved vast numbers of tall land plants. Scientists have reconstructed many extinct species and as they study other species that live today, they have come to believe that mosses were among the first green plants to live on land. In view of the primitive structure of these tiny plants, that conclusion seems logical. Mosses lack the complex roots of seed plants, yet they certainly are better adapted to life on land than most algae. And above the ground, some species reveal a significant hint about the watery origin of mosses. During dry periods, the leaves of these mosses retain water by folding closed, exposing less surface to the air. When moisture is available, the leaves unfold. Related reactions occur in some liver wart plants and many scientists believe that their ancestors were the first plants to live on land. The bodies of many liver warts are flat, well adapted to take moisture from the ground below. Some liver wart adaptations for life on land can be more easily seen through a magnifier. For example, on the unders sides are clusters of fine rootlike filaments, risoids, which absorb water, and on top there are pores through which oxygen, carbon dioxide, and water vapor may pass. Scientists speculate as to whether liver warts or their relatives, the mosses, were first to live on land. And they believe that ferns evolved more recently. Ferns have undergone more complex adaptations to an air and earth environment. For example, they have leaves, each known as a frond. Most fronds have smaller leaflets. A frond is supported by a pedio attached to a stem or a ryome. Roots are more complex than the risoids of a liver wart. In a fern's roots, stem, and pedole, we see an evolutionary advance that distinguishes ferns from bryophites. Cutting a pedole longitudinally reveals a vascular system that conducts water, dissolved minerals, and food to and from the fond. A cross-section cut of a ryome reveals that the vascular system has two kinds of tubes. those that make up xyllem, vascular tissue that carries materials toward the frond and those that comprise phe tissue that carries materials away from the fond. Fern plants with xyllem and pheum grow much larger than mosses or liver warts. In fact, some grow to the size of trees. Since mosses occupy a number of different niches throughout the world, it may be interesting to consider how they reproduce. At certain times of the year, depending on the species, there are changes in the tops or heads of the plants. A male head contains a cluster of sex organs known as anththeridia. And in the head of a female plant, invisible from the outside, are female organs, aragonia. These male and female sex organs may be dissected from the plants and we can observe that a male organ discharges sperm cells, male gamtes. In nature, rain or dew carries these to female organs and they travel until they reach egg cells, female gamtes. Fertilization occurs. The gamtes combine producing new individuals that begin their lives down inside the female head. Each new individual is a sporophyte and it grows on the gamt producing or gitaphyte body of the plant. When a sporophyte matures, it releases asexual reproductive bodies, spores. Spores that fall or are blown onto damp shaded soil germinate. They begin to grow into new young moss plants. Not into new sporopyes, but into leafy gitaphytes. When these gitaphytes mature, they will in turn give rise to new sporaites. The cycle continues over and over and is known as alternation of generations. Since alternation of generations is a continuous cycle, we may begin with either the gamitaphyte or the sporopy generation. Let's detail the cycle. The tips of the leafy gitaphyte develop anththeridia and aragonia sex organs. The anththeridia produce sperm cells. The aragonia egg cells. These male and female gametes unite to form a fertilized egg. This is sexual reproduction. The egg becomes a sporophyte, a plant that produces tiny asexual reproductive units, spores. Each spore can germinate to form a young gitaphyte which grows into a leafy moss plant. This asexual reproduction completes the moss life cycle with its alternating gamitaphyte and sporophyte generations. Reproduction in mosses is not very different from the same process in liver warts. These plants also are highly dependent on water. For example, a liver wart with male sex organs may grow some distance from one with female organs. If sexual reproduction is to occur, the sperms must travel through water. Liverward sporopites are also similar to those of mosses. They release spores that may be carried several miles by the wind. Each liver wart spore is a complete asexual reproductive unit which can grow into a new liverwart kitapy. Liver warts and mosses are somewhat less complex than ferns and there's an interesting difference between them. The leafy fern plant that we commonly see is the sporophyte among liver warts and mosses. Recall the leafy plant is the gitaphyte. On the underside of a frond are rounded organs, clusters of sporangia or sori. They contain the tiny spores so small that they're difficult to see even in a microscopic photograph. They have thick coatings, protection against dryness. Landing in a moist, adequately light environment, spores germinate, each growing into a small gitaphyte that reproduces sexually by means of gamts, giving rise to young fern sporopites. The sporopites mature eventually forming fronds that grow from stems in this case underground and pedoles. Let's review alternation of generations in a fern. As with a moss, we may begin our study with a gitaphyte or the sporopy generation. The mature gitaphyte called a prothalis has sex organs, male or female. These produce sex cells, sperms or egg cells which combine to produce a fertilized egg. From this a sporopy develops. It has developed through sexual reproduction. On the unders sides of the sporopy leaves are formed asexual reproductive organs sporangia. These release spores which germinate producing fern gitapytes. This asexual reproduction completes the fern life cycle with its alternating gitaphyte and sporopy generations. Ferns, mosses, and liver warts are less abundant than seed plants. Yet throughout the world, there are great numbers of mosses, including pin cushion moss and spagnum moss. There are thousands of species of liverarts. One of the commonest is marantia. And there are thousands of different ferns including the poly pody fern, Christmas fern, sensitive fern, cinnamon fern, and the walking fern. Thus they inhabit the land. ferns, liver warts, and mosses. Perhaps not so well adapted to land life as seed plants, yet certainly more fit for land than algae, the simplest green water plants. It is this very fact, the adaptation of ferns, liver warts, and mosses to life on land that makes these plants without seeds so meaningful to observe and to study.