Bryophytes: primitive land plants

Transcription

1 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.

2 Bryophytes The bryophytes, division or phylum Bryophyta, are land plants that are nonvascular (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 capsulelike 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.

3 Procedure examine mosses 1. Observe the living moss on display. 2. Make a wetmount of one leaflet of Polytrichum and examine it with low magnification. Questions 1. How many cells thick is the leaflet? 2. Is there a midrib vein? Procedure water absorption by moss 1. Weigh 3g of Sphagnum moss and 3g of paper towel 2. Add the moss and towel to separate beakers each containing 100mL of water. 3. After several minutes, remove the materials from the beaker. 4. Measure the amount of water left in each beaker by pouring the water into a 100 ml graduated cylinder. Remember that 1 ml of water weight 1g. Record your data 3. Are stomata or pores visible on the leaf surface? Questions 1. How many times its own weight did the moss absorb? 4. How does the symmetry of a moss gametophyte compare with that of a liverwort gametophyte? (come back to this question and answer after observing liverworts) 2. How does this compare with the paper towel? 3. Why is Sphagnum often used to ship items that must be kept moist?

4 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 structures called antheridiophores. (b) A thallus bearing upright female reproductive structures called archegoniophores. Detail of Gemmae cups splash cups 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?

5 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.

6 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 worldwide, 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?

7 Fern reproduction Fern spores germinate and form a threadlike protonema. Subsequent cellular divisions produce an independent, heart-shaped prothallium. Rhizoids and male and female reproductive structures occur on the underside of the prothallium. However, a prothallium rarely fertilizes 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 sperm to swim to the archegonia. Archegonia are vase shaped and are located near the cleft of the heart-shaped prothallium. The zygote develops in the archegonium and is nutritionally depend on ton the gametophyte for a short time. Soon after, the sporophyte becomes leaflike and crushes the prothallium. Fronds of the growing sporophyte break through the soil in a coiled position called a fiddlehead. The fiddlehead then unrolls to display the frond, which is a single leaf. Fiddleheads are considered a culinary delicacy in some parts of the world. Procedure examine prothallium 1. Observe archegonia and antheridia on living prothallium. 2. Also observe archegonia and antheridia on prepared slides. Questions 1. Is the prothallium haploid or diploid? 2. Is the prothallium sporophyte or gametophyte? 3. What is the adaptive significance of having the archegonia and antheridia on the lower surface of the prothallium rather than on the upper surface? 4. What is the adaptive significance of having sperm and egg produced at different times?

8 Pleopeltis polypodioides--resurrection fern. This small fern grows as an epiphyte on trees in south Florida and south into the Caribbean region. It is particularly common on our native live oaks. The rhizomes grow into the bark and produce the small green fronds. Notice the circular sori on their undersurface. A close look with a magnifying lens will reveal the individual sporangia. These plants are called resurrection ferns because a long dry period causes the fronds to dry up, but subsequent rains cause them to quickly resurrect themselves, and become green and healthy again. This is an amazing process that is poorly understood. You can see these ferns on live oaks on campus and on the trunks of the date palms growing by the Conservatory. Phymatodes scolopendria--wart fern. This handsome fern is native to southeast Asia, but is commonly cultivated in south Florida and has escaped from cultivation as well. It grows from a thick and hairy horizontal rhizome which produces the erect and lobed fronds at intervals. These fronds produce large circular sori on their undersurfaces, which make an impression on the upper surface as well. These ferns are actually related to the resurrection fern. Some of these plants have established on the date palms near the Conservatory, but are most common in the atrium of OE, just outside of the laboratory where you meet inside.

9 Thelypteris palustris--marsh fern. This is a ferns with rather delicate and soft fronds produced from a dense web of rhizomes. The small sori are protected by an indusium on the undersurface of the fronds. This grows in damp areas in South Florida, and often is found in wet and disturbed areas. It grows in the OE Atrium and in the Ecosystem Preserve as well. Nephrolepis biserrata--boston Fern. A strange name for this fern. It is a tropical fern, not native to New England at all, but was sold as an ornamental plant even in the 19 th century when it got its name. It grows rather tall (1 m) and thick. It has a wide distribution in the tropics, including south Florida. Horticultural varieties have been selected and it is sold in nurseries as well. It grows in the OE Atrium and in other areas on campus. It is probably the most commonly cultivated fern in Miami. A related species grows as an epiphyte on the date palm trunks near the Conservatory. It has conspicuous sori with indusia on the undersurface. It is called biserrata because the teeth on the margins of the pinnae have teeth themselves.

10 Adiantum cappillus-veneris--maiden hair. This beautiful and delicate fern (where it gets its name) shows up anywhere conditions are very moist and protected. So, it has established itself all over the rocks on the waterfall in the Conservatory. You will see it in similar places at Fairchild Tropical Garden and elsewhere in Miami. The frond is compound with a black and wiry midrib, and its pinnae (leaflets) protect the sori with an infolding of the blade margin. This is a character of the genus. Blechnum serrulatum--saw fern. This is a common and rather weedy fern. It is native to south Florida and elsewhere in the Caribbean. It grows from a horizontal rhizome, and the margins of the pinnae have fine teeth (like a saw blade). The sori are produced by the midrib of each frond, and protected by leaf tissue growing over it. This grows in the Ecosystem Preserve and in the OE Atrium by your laboratory.

11 Sphaeropteris cooperi--australian Tree Fern. This is native to the rainforests of Queensland, in the north of Australia, and is the only commonly cultivated tree fern in South Florida. You can find it planted commercially, but in protected areas. There is a fine specimen growing in the Conservatory. The trunk is actually a dense web of individual vertically growing roots. Angiopteris evecta Angiopteris. This is a most spectacular and primitive fern, native to tropical rainforests in southeast Asia. It has distinctive simple sporangia in rows under the pinnae of its huge fronds. These arise from a massive rhizome. This is the spectacular fern that arches over the pond in the Conservatory. It also grows in the Conservatory at Fairchild Tropical Garden. Ferns that are Not Ferns. Many plants are called ferns because they have fine, lacy and highly dissected leaves. However, they reproduce by seeds and not by spores and independently living gametophytes. The artillery fern (Pilea microphylla) is a tiny plant in the nettle family, known for expelling its seeds some distance when the fruits ripen. It grows as a weed in any wet-humid place. We constantly remove it from the Conservatory. The asparagus fern (Asparagus sprengeri) is a wiry and thorny vine that shows up in hedges. Its lacy foliage is pretty, but the plant is a pain and difficult to remove. The fern-leaf tree (Filicium decipiens, growing by Duplicating and near the Tower Dorm) has attractively divided foliage but is obviously not a fern.

12 Division Lycopodophyta: The Club Moss These plants do not grow outside in south Florida (although a couple of species grow in the scrub lands north of Lake Okeechobee), and the ones you see have been planted inside of the Conservatory. These plants are called club mosses because their sporangia are protected by special leaves (bracts) to form cone-like structures called strobili at the tips of the branches. Few of these plants grow very large and none are economically very important. There are two living families, the Lycopodiaceae and the Selaginellaceae. These differ in details of their life cycles and in their leaf arrangements. Look at the plants closely to see these differences. Selaginella These are sometimes called spike mosses. There are at least three species in the Conservatory. All share a flattened arrangement of two sizes of small leaves on their horizontal stems. They produce strobili on the tips of their branches (but you probably won t see them on our plants). The life cycle of these plants represents an evolutionary transition in origin of seed plants. They produce both megaspores and microspores (in the same or different strobili depending on the species), and the gametophyte generation is retained inside of the spore walls, the microspores producing sperm and the megaspores producing eggs. These are all tropical rainforest plants; those growing at higher latitudes (as Central Florida) are extremely drought-tolerant. Some desert plants can dry up completely and be resurrected when immersed in water. Two of the plants in the conservatory, the straggling Selaginella willdenowii and the smaller Selaginella uncinata, produce an iridescent blue leaf color in very shady spots. This is probably an adaptation to low light conditions in the understory of tropical rainforests. These are commonly called peacock ferns (but they are really not ferns).

13 Division Psilotophyta: The Whisk Ferns You will see Psilotum nudum, a cosmopolitan tropical plant, at the base of palms on campus. We didn t plant them; they arrived from spores on their own. We once thought that these plants represented the most primitive of vascular plants. They have no roots, no real leaves, and look similar to fossils of the most ancient of land plants. We now know that these plants are actually highly reduced descendants of the ferns

14 Lycopodium The ground pines grow in forests in temperate and tropical regions. All have a spiral arrangement of their small leaves with strobili at the tips of their branches. Again, these plants do not grow in south Florida and you can only observe them in our Conservatory or in the conservatory at Fairchild Tropical Garden. They grow as both epiphytes and terrestrial plants. Observe the Lycopodium plants growing near the waterfall. Lycopodium cernuum: a, mature plant, x3/4; b, strobilus, x71/2; c, sporophyll, abaxial surface, x50 Lepidodendron These plants once grew in the coal swamps of the Carboniferous era, some 300 million years ago. Pressure and temperature turned them into the coal deposits of West Virginia, Pennsylvania and other states. Fossils are commonly found of all parts, particularly the stems (Lepidodendron), leaves (Lepidophyllum), and strobili (Lepidostrobus). From these form genera, we know what the intact plant was like. Many of them were large trees. Observe the fossils of Lepidodendron and Lepidostrobus adjacent to the Lycopodium plants in the conservatory. Compare them to diagrams of the fossils and a rendering of the entire plant below.

15 Division Equisetophyta: The Horsetails This was once a very important group of plants that dominated the coal swamps of the Carboniferous along with Lepidodendron. Today it is only represented by a single genus of very few species, Equisetum. This plant is commonly called the horsetail (some of the species branch and look vaguely like tails) or the scouring rush (it contains silica in its stems and campers sometimes using them to clean their pans). It is rarely seen in south Florida, although a small clump occurs in the north end of the Ecosystem Preserve. Again, we have planted some examples in the conservatory. Equisetum Notice the general appearance of the plant. Vertical stems arise from an underground rhizome. These stems produce tiny scale-like leaves at jointed nodes on the stem. Briefly and early in the year the stems produce a club-shaped strobilus at their tip. This strobilus consists of hexagonal discs (sporangiophores) under which the sporangia are produced. Compare the plants you see to the diagram below. Calamites This was a treelike plant in the coal swamps of the Carboniferous. Thus it has left numerous fossils that can be found in the slag heaps of coal mines, along with those of Lepidodendron. Observe the fossil impressions of this plant adjacent to the living Equisetum, near the waterfall in the Conservatory. Compare these fossils to the diagram and rendering of what the intact plant probably looked like.