|EXERCISE 10 – Kingdom Plantae: Bryophytes Seedless Non-vascular & Vascular Plants
I. THE NON-VASCULAR SEEDLESS PLANTS
Kingdom Plantae: Division Bryophyta (the mosses, liverworts, and hornworts)
Bryophytes exhibit the following characteristics:
The plant body does not possess true stems, roots, or leaves, but organized tissue is evident.
There is no real need for vascular tissue since they are commonly immersed in water or found in moist environments.
They possess multicellular sex organs.
True alternation of generations is present.
The gametophyte generation produces gametes (i.e., sperm and eggs) and the sporophyte generation produces spores. It is important to remember that the sporophyte remains attached to the gametophyte.
For additional information refer to the text found in Chapter 6 and review the figures given in the photographic atlas (i.e., Figures 6.1 through 6.28 on pages 66 to 72). Pay particular attention to the life cycles give on pages 68 (liverworts) and 73 (mosses).
Observe fresh and/or preserved liverwort material try to identify a gametophyte plant with an archegoniophore (female gametophyte). Identify a gametophyte plant with an antheridiophore (male gametophyte). Identify a gametophyte plant with gemmae cups. Observe a prepared of Marchantia (archegoniophore). Notice the archegonia in an inverted position. In a good section, the egg will be quite obvious. Study a prepared slide of Marchantia (antheridiophore). Note the antheridia with sperm inside.
Observe a prepared slide of Marchantia (sporophyte). Identify the foot, seta, and capsule of the sporophyte. Can you see the spores? Obtain a prepared slide of Marchantia showing a gametophyte plant with gemmae cups containing gemmae. Gemmae can be splashed out of the cups and grow into new gametophyte plants, allowing the gametophyte a means for asexual reproduction.
Observe fresh and/or prepared moss material and identify gametophyte plants. Young specimens that are in good shape usually appear green due to the many leaflets. Identify gametophyte plants with the sporophyte attached. At maturity the sporophytes usually appear brown. Observe a prepared slide of Mnium (gametophyte plant with archegonia). Notice the egg. In a prepared slide of Mnium (gametophyte plant with antheridia) notice the sperm inside the antheridia. Examine a prepared slide of Mnium (sporophyte plant). Notice the capsule, seta, and peristome teeth. Have you seen any capsules with little straw hats? These form the “lid” of the sporangium which is known as the operculum. Removal of this lid allows the spores to escape into the air and become dispersed. If the spores find an appropriate environment, these will germinate and form a moss protenema. The moss protenema becomes the new thallus or body of the multicellular male or female gametophyte.
THE VASCULAR SEEDLESS PLANTS
Vascular plants possess a well developed vascular system which provides for conduction of food, minerals, and water throughout the plant. Algae, fungi, mosses, liverworts all lack conducting tissue. The seedless vascular plants have distinct alternation of generations, where the gametophyte plant (haploid generation) alternates with the sporophyte plant (diploid generation). In mosses and liverworts which generation was the dominant generation?
For additional information refer to the text found in Chapter 7 and review the figures given in the photographic atlas (i.e., Figures 7.1 through 7.56 on pages 76 to 91). Pay particular attention to the life cycles give on pages 77 (whiskferns), 80 (club moss - Lycopodium), 82 (club moss – Selaginella), 84 (horsetails) and 87 (ferns).
Kingdom Plantae: Division Psilophyta (the whiskferns)
The most primitive of the vascular plants are members of the divisions Psilophyta. These plants are mainly tropical with one genus (Psilotum) occurring as far north as Florida. Observe a living specimen of this plant. Which you seed represents the sporophyte generation. This generation produces spores by meiosis. Notice the small, scale-like leaves and the sporangia occurring on the stems. The gametophyte plant is cylindrical, subterranean, non-photosynthetic, and less than a centimeter in length.
Kingdom Plantae: Division Lycophyta (the club mosses, quillworts, & spike mosses)
The next division of the lower vascular plants is Lycophyta. There are three genera from this division found in Arkansas. The common name for these plants is club moss, a name which comes from the characteristic structure, the strobilus, which bears sporangia. A strobilus is an aggregation of modified leaves called sporophylls and these sporophylls bear sporangia. Club mosses like Lycopodium produce spores of one size, a spore condition called homosporous (homospory). The spores are produced by meiosis in the sporophyte plant. The gametophyte plant witch develops from the spores is small and may or may not have chlorophyll. Usually a fungus is associated with the gametophyte plant.
Segainella, another club moss genus, produces two different spore sizes, a condition called heterosporous (heterospory). The spores are produced by the process of meiosis in the sporophyte plant. In certain sporangia, all the spore mother cells are functional and a large number of small-sized spores are produced and these are called microspores. Such sporangia which bear microspores are called a microsporangium. In certain other sporangia, one spore mother cell functions to produce spores and these will total one in number. In these sporangia the spores are large and called megaspores. Sporangia borne by megasporphyllls are called the megasporangium. The modified leaves making up strobili that bear the micro- and megasporangium are called microsporophyllls and megeagsporophylls. The megaspores, after leaving the sporangia, become female gametophytes which possess archegonia and will produce eggs. The microspores become the male gametophyte plant which develops antheridia that produce sperm or male gametes. After fertilization, the diploid plant formed becomes the new sporophyte and allowing the life cycle to begin anew.
Kingdom Plantae: Division Spenophyta (horsetails)
Another division of lower vascular plants is the division Sphenophyta. These plants have many common names (horsetails, scouring rush, snake grass, joint grass). Equisetum is the only genus in this division. Some species are tropical while others are temperate (several species occur in Arkansas) the sporophyte plant may reach heights of six feet in temperate areas (20 feet for one tropical species), whereas the gametophyte plant is small (f few millimeters) and green. Horsetail sporophytes are characterized by small, whorled leaves, grooved internodes, and terminal stobili. The strobilus in horsetails is as aggregation of sporangiophores which bear 5-10 small sporangia. The spores are homosporous since they are all the same size. Each spore has four ribbon-like elaters which are sensitive to moisture and aid in dispersal. Observe the demonstration of a strobilus of Equisetum.
Kingdom Plantae: Division Pterophyta (ferns)
The final division of lower vascular plants is the division Pterophyta (ferns). Ferns are mainly tropical, but many occur in temperate areas. Arkansas has an abundant fern flora. The leaf of a fern is the most prominent part and is called there frond (the stalk is the stipe and expanded green part is the blade). Some tropical ferns are tree-like in size, reaching heights of 60 feet or more. Some ferns are homosporous and produce spores of the same size while others are heterosporous and produce spores of two sizes. In ferns, the sporophyte possesses an underground stem (rhizome) from which fronds and roots originate. Rhizomes provide a means of asexual reproduction as is evident by the “rabbit’s foot fern”. When a fern is mature, sporangia my be borne on the underside of the frond, at the tip of the plant in fertile spikes, or on separate fertile fronds (different from the sterile fronds). Sporangia occur in clusters. A single cluster is called a sorus, with two or more clusters being called sori. Many ferns possess an umbrella-like or blade-like structure which covers each sours. This cover, which is called an indusium, protects he young sporangia. Some ferns have sori on the margins of the blades and the edge of the blade my curve back over the sporangia and form a false indusium. Each sporangium has a stalk, annulus, and a lip cells. When the spores are mature, the annulus (looks like an ancient warrior’s helmet) may contract and rupture the sporangium in the area of the lip cells. In many cases this forcibly ejects spores from the sporangium and thereby aids in spore dispersal.
When a spore lands on a suitable substrate, the gametophyte plant develops. The gametophyte generation is called the prothallium in ferns. This generation is usual small, heart-shaped, has rhizoids, and is photosynthetic. Archegonia originate near the apex while antheridia are borne among rhizoids. The egg and sperm unit to form the zygote, which develops into an embryo, which grows to become the sporophyte plant. The fist frond is called the primary leaf and can usually be seen attached to the prothallium. A root system develops and young sporophyte becomes independent. As frond they are initially curled up, hence the name “fiddleheads” or “fiddle necks). Study the life cycle of a fern (in the Photographic Atlas in Figure 7.36 on page 87).