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Primitive Land Plants

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Primitive Land Plants Bryophytes: primitive land plants These are the plants that were present soon after land was colonized, over 400 million years ago. A few plants living today are closely related to those ancient plants, and we often call them “living fossils”. Two major lineages of plants evolved in these early times. Plants without conductive tissues occurred, whose descendents are the mosses and liverworts—or bryophytes. Plants with conductive tissues (xylem and phloem), called vascular plants, also appeared at this time. Both of these groups of plants had life cycles, involving two generations. One generation, the gametophyte, produced sexual organs and male and female gametes, or small moving spermatozoa and larger immobile eggs. A separate generation, the sporophyte, produced haploid spores after meiosis. This life cycle was derived from their algal ancestor. Here we describe those bryophytes and ferns that you will encounter on the FIU campus, and also common in South Florida. In a generalized life cycle, gametophytes, which are haploid (N), alternate with sporophytes, which are diploid (2N). Antheridia (male) and archegonia (female) are the sex organs (gametangia) produced by the gametophyte; they produce the sperm and eggs, respectively. The sperm and egg fuse during syngamy (fertilization) to produce the first diploid cell of the sporophyte generation, the zygote. Meiosis occurs within the sporangia, which are the spore-producing organs of the sporophyte. The resulting spores are haploid and are the first cells of the gametophyte generation. Bryophytes The bryophytes, division or phylum Bryophyta, are land plants that are non- vascular (no real conducting tissues) with a life cycle that is different from that of vascular plants. The green leafy structures are the sexually-reproducing generation (gametophyte). The capsule- like structure sticking above is the spore-producing generation (sporophyte), that is mainly dependent upon the gametophyte for its nutrition. The bryophytes are split into three classes: (1) the mosses; (2) the liverworts; and (3) the hornworts. We will observe the first two groups, by far the most important, from plants growing on campus and in the Wertheim Conservatory. Phylum Bryophyta: Mosses These small plants grow in very damp and protected places in south Florida, but we don’t know very much about the plants living here. You will see moss gametophytes growing on the protected trunks of some palms and trees on campus, such as the date palms outside of the Conservatory. You will also see mosses growing on the rocks by the waterfall in the conservatory. Mosses are dioecious, meaning that they have male and female plants. At certain times of the year these mosses will produce tiny sporophytes. Moss sporophytes consists of capsules located atop stalks, or seta, that extend upward the moss gametophyte. A sporophyte is attached to the gametophyte be a structure called a foot (see figure next page). Prior to this generation, the tiny plants produced sexual organs (archegonia and antheridia) at the tips of the shoots. The archegonia produce single eggs and the antheridia produce many motile swimming sperm. Mosses can also reproduces asexually by fragmentation. Procedure – examine mosses Procedure – water absorption by 1. Observe the living moss on display. moss 1. Weigh 3g of Sphagnum moss and 3g 2. Make a wetmount of one leaflet of of paper towel Polytrichum and examine it with low 2. Add the moss and towel to separate magnification. beakers each containing 100mL of water. 3. After several minutes, remove the Questions materials from the beaker. 1. How many cells thick is the leaflet? 4. Measure the amount of water left in each beaker by pouring the water into a 100 mL graduated cylinder. 2. Is there a midrib vein? Remember that 1 mL of water weight 1g. Record your data Questions 3. Are stomata or pores visible on the leaf surface? 1. How many times its own weight did the moss absorb? 2. How does this compare with the 4. How does the symmetry of a moss paper towel? gametophyte compare with that of a liverwort gametophyte? (come back to this question and answer after 3. Why is Sphagnum often used to ship observing liverworts) items that must be kept moist? Phylum Hepaticophyta: Liverworts Liverworts have a similar life cycle to that of mosses, only their spores are different. We will examine Marchantia, in which the gametophytic thallus grows as a large, flat photosynthetic structure on the surface of the ground. Asexual reproduction: Liverworts can reproduce asexually via fragmentation. In this process the older, central portions of the thallus die, leaving the growing tips isolated to form individual plants. In another form of asexual reproduction, structures called gemmae cups occur on the dorsal surface of some thalli near the midrib (see figure below). Gammae cups represent another means of asexual reporduction by liverworts. Inside the gemmae cups are lens-shaped outgrowths called gemmae (sing. gemma), which are splashed out of the cups by falling drops of rain. If a gemma lands in an adequate environment, it can produce a new gametophyte plant. Sexual reproduction: Many species of Marchantia are dioecious, meaning that they have separate male and female plants. Gametes from each plant are produced in specialized sex organs born on upright stalks (see pictures below). Archegoniophores are specialized stalks on femal plants that bear archegonia. Eack flask-shaped archegonium consists of a neck and a venter, which contains the egg. Antheridiophores are specializes stalks on male plants that bear anteridia. Sperm form in antheridia. Flagellated sperm are released and washed from the antheridia during wet conditions and eventually fertilize the egg, which is located in the venter. The zygote remains in the venter and grows into a sporophyte. A B Marchantia. (a) A thallus bearing upright male reproductive Detail of Gemmae cups “splash cups” structures called antheridiophores. (b) A thallus bearing upright female reproductive structures called archegoniophores. Procedure – examine live Marchantia 1. Examine live Marchantia a using the naked eye or a dissecting scope. Be sure you can identify the archegoniophores, antheridiophores, and gemmae cups. Questions 1. How do the positions of the archegonium and antheridium relate to their reproductive function? 2. When would it be advantageous for the Marchantia to reproduce asexually through gemmae cups instead of through sexual reproduction? Seedless Vascular Plants These early land plants had specialized vascular tissue (xylem and phloem) in both roots and stems that conducted water and nutrients up, and translocated sugars and some amino acids down, the roots and stems. They had life cycles similar to the mosses and liverworts, but reversed in the relative importance of the two generations. In their life cycles the dominant green generation is the sporophyte, and the smaller less conspicuous generation is the gametophyte. All of the groups of seedless vascular plants seen below share very similar life cycles, the principal differences being in the structure and arrangement of the sporangia and the structure of the gametophytes. Sporangia is where spores are produced by meiosis. They are formed on sporophylles, which are leaf like structures of the sporophyte that bear spores. Sporophylls may be large megaphylls as on true ferns, or smaller micrphylls such as those on whisk ferns, scouring rush, and club mosses. We illustrate the life cycle of a typical fern as illustrative of all of these plants. We describe the major groups of these primitive vascular plants below, including plants that you will see on campus and in south Florida. By far the most important group of these plants is the ferns, but the other groups have wide distributions and are representative of groups that were once very important during the early history of land plants. Division Polypodiophyta: the Ferns This is by far the most important and diverse group of all of the primitive land plants. Ferns grow from the arctic to the equator, although they are most diverse in tropical forests, particularly at higher altitudes. Ferns may be extremely tiny, almost the size of mosses, and extremely tall, as tree ferns 15 m high. There are over 12,000 species world- wide, and some 150 species in Florida. All ferns share a general morphology. They produce leaves, called fronds, and underground stems, called rhizomes. Since these are sporophytes, the sporangia are produced in clusters, called sori, in the undersurface of their fronds. The nature and distribution of these sori help in identifying ferns. Usually the sori are protected by an indusium, which is a specialized outgrowth of the frond. The sporangia are produced on a stalk and include thickened cells, forming an annulus, that help them spring open and disperse the spores when mature and dry. Procedure – examine sori 1. Scrape a sorus into a drop of water and use the low power on your microscope to observe the sorus. Note the row of thick-walled cells along the back of the helmet-shaped sporantium. These cells are the annulus. Alternatively, you can observe sori on the back of a fern leaflet using the dissecting scope. Questions 1. What is the function of the annulus? 2. Are there any spores in the sporangium? Fern reproduction Fern spores germinate and form a Procedure – examine prothallium threadlike protonema. Subsequent 1. Observe archegonia and antheridia cellular divisions produce an independent, on living prothallium. heart-shaped prothallium. Rhizoids and male and female reproductive structures occur on the underside of the prothallium. 2. Also observe archegonia and However, a prothallium rarely fertilizes antheridia on prepared slides. itself because the antheridia and archegonia mature at different times. Globe-shaped antheridia form first, followed by archegonia. After producing sperm, the antheridia drop off, leaving Questions sperm to swim to the archegonia. 1. Is the prothallium haploid or Archegonia are vase shaped and are diploid? located near the cleft of the heart-shaped prothallium.
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