The Archaeplastida is a monophyletic lineage of eukaryotic photoautotrophs. They have plastids acquired by primary endosymbiosis with a cyanobacterium. Members of this group have supplied plastids by secondary endosymbiosis to other photoautotrophic eukaryotic lineages (Euglenids, Alveolates, and Stramenopiles).
- Glaucophyta – glaucophytes
- Rhodophyta – red algae
- Chlorophyta – green algae
- Charophyta – charophytes (includes some groups previously classified as green algae)
- Embryophyta – plants
This is a small (about a dozen species) and obscure group of unicellular freshwater algae. They are of interest to evolutionary biologists in that their chloroplasts possess an outer layer of peptidoglycan that is interpreted as being a remnant of the peptidoglycan cell wall of the cyanobacterial endosymbiont. We will not be examining glaucophytes.
Rhodophyta (Red Algae)
This is primarily a marine group, although it contains a small number of fresh and brackish water species. There are about 7,000 named species. The oldest member of this clade is the fossil Bangiomorpha isolated from rocks dated to 1.2 billion years ago. Current and fossil red algae are reef builders, predating the corals in early Paleozoic reefs. The modern “coralline” red algae form a calcium carbonate layer on their cell walls, and are still significant reef contributors (you may observe coralline red algae in our saltwater tank in the third floor foyer). Modern red algae are unicellular to multicellular, with most filamentous or multicellular. Many larger multicellular species are familiar as the reddish to blackish “seaweeds” (note: seaweed is a descriptive term that is not systematically useful. Other seaweeds include some of the green algae – Chlorophyta, while the most prominent seaweeds are usually kelps and other brown algae – Phaeophyta).
The most distinctive feature of red algae is their red-purple secondary photosynthetic pigments, produced by the reddish to bluish phycobilin pigments (phycocyanin, allophycocyanin, and phycoerythrin). These are the same secondary photosynthetic pigments found in cyanobacteria. Their reddish pigments allow red algae to live in deeper waters than most photoautotrophs. Red algae have a glycogen-like storage polysaccharide (floridean starch) and cellulosic cell walls. The cell walls of some red algae are the source of the commercially useful colloidal compounds agar or carrageenan. Red algae are used in some traditional cuisines including the familiar nori (Porphyra) that has been cultivated as a food source for centuries in Japan.
Red algae have a complex alternation of generations (a variation on a sporic life cycle) with up to three distinct generations: a haploid gametophyte (with male and female individuals), a diploid carposporophyte generation, and a diploid tetrasporophyte generation. These three generations may be isomorphic (appearing similar) or heteromorphic (appearing different). The gametophyte is the sexual generation, forming the haploid gametes. The diploid carposporophytes and tetrasporophytes reproduce by forming spores (diploid carpospores and haploid tetraspores). Red algae do not form flagellated cells at any stage in their life histories.
Chlorophyta (Green Algae)
This is a morphologically diverse and widespread clade of organisms often recognized at the rank of phylum. It includes about 4,500 species that may be free-living, mutualistic, commensal, or parasitic. Mutualistic unicellular green algae may be found as phycobionts in lichens or in associations with animals such as corals. The green algae includes unicellular, filamentous, colonial, and multicellular members. Some are coenocytic (not subdivided into cells). Green algae are common members of marine, brackish water, fresh water, and terrestrial ecosystems. They may be found in hot springs or hypersaline environments. One species of unicellular green alga is responsible for the reddish snow sometimes seen in Utah. Most form flagellated cells that are biflagellate (although the number of flagella ranges from 0 to many). Flagellar position is apical. All three life cycles (gametic, zygotic, and sporic) may be found in this clade.
Biochemically the green algae are very similar to terrestrial plants and charophytes, and these three groups are often placed in a single (unranked) clade, the Viridiplantae. All have similar secondary photosynthetic pigments (chlorophyll b, carotenoids, and xanthophylls) that give all three groups a similar green color. All have cellulosic cell walls and use starch as an energy storage molecule. Viridiplantae is generally recognized as being monophyletic.
Charophyta (charophytes, stoneworts)
Charophytes are a group of mostly freshwater unicellular, filamentous or multicellular algae that are closely related to terrestrial plants. Some (Chara and Nitella) have striking macroscopic organization such that they may easily be mistaken for members of the plant kingdom. They are sometimes grouped with the green algae, but have some cytological features (such as the formation of a phragmoplast during cytokinesis) that show their close affinity with plants. Charophytes and land plants are generally placed together in the (unranked) clade Streptophyta. Charophytes usually form biflagellate sperm (similar structurally to those of terrestrial plants) and have a zygotic life cycle.
Plantae (terrestrial plants, embryophytes)
This is the most familiar and diverse group in this supergroup with over 300,000 species. This group includes flowering plants, conifers, ferns, and mosses. This clade is customarily placed at the rank of kingdom (Plantae), but has also been placed at lower ranks (e.g. division Embryophyta). This group is primarily terrestrial (with some marine and freshwater members that are secondarily aquatic). All have a sporic life cycle, waxy spores (or pollen) that are waterproofed with sporopollenin, and an embryo that develops within a sterile jacket of cells (hence embryophytes).
Plants are one of the two most familiar groups of organisms (the other, of course, being the animals). More than any other clade, our concept of “what is a plant” has changed over the course of the last century. By the mid twentieth century the algal groups (“thallophytes”), fungi, and bacteria were placed elsewhere, with the remaining group, the embryophytes (originally the class Embryophyceae) being the sole recipients of the title “plant.” For the last five decades the embryophytes have been placed in their own kingdom (Plantae). As has been discussed briefly in Lab 7, the embryophytes nest within the Viridiplantae clade of the Archaeplastida. Their sister group within the Viridiplantae are the charophytes.
All plants have a sporic life cycle with multicellular sporophyte and gametophyte generations. The cycle is summarized below. Variations on this cycle are useful in the recognition and description of different plant clades.
The plant kingdom is a terrestrial clade, and its most distinctive features are those that evolved during the transition to life on land. Key plant symplesiomorphies include:
- green pigments, cellulosic cell walls, starchy polysaccharide energy storage molecules (these features shared with other Viridiplantae)
- sporic life cycle with specialized reproductive structures to protect reproductive cells from desiccation:
- sperm and egg cells formed by the gametophyte within reproductive structures (gametangia) that have a “sterile jacket” of cells to protect the developing gametes.
- sperm formed within antheridia
- egg cells formed within archegonia
- zygote develops into a young sporophyte (embryo) protected by the remnants of the multicellular archegonium of the gametophyte (hence embryophytes)
- zygote and young sporophyte nourished by the gametophyte via placental transfer tissue
- spores formed by the sporophyte within sporangia that have a sterile jacket of cells.
- spores (and pollen of seed plants) waterproofed with the complex waxy compound sporopollenin.
- sperm and egg cells formed by the gametophyte within reproductive structures (gametangia) that have a “sterile jacket” of cells to protect the developing gametes.
- formation of a waxy cuticle to reduce water loss through the epidermis
- formation of stomata for gas exchange through the cuticle (absent from liverworts)
- growth by an apical meristem or meristematic cell (a feature shared by some Viridiplantae)
Derived features that occur within some (but not all) lineages of the plant kingdom include:
- vascular tissue (xylem and phloem) for rapid conduction of fluids (absent from mosses, liverworts, and hornworts)
- differentiation of the plant body into roots, stems, and leaves (absent from mosses, liverworts, and hornworts)
- reproduction by seeds (absent from the free-sporing plants: mosses, liverworts, hornworts, lycopods, and monilophytes)
The plant kingdom, as currently conceived, contains 10 clades recognized at the rank of phylum. These may be subject to a reduction in rank in future classifications:
- Kingdom Plantae (embryophytes)
- Bryophyta (mosses)
- Hepatophyta (liverworts)
- Anthocerotophyta (hornworts)
- Lycophyta (lycopods)
- Monilophyta (ferns)
- Pinophyta (conifers)
- Cycadophyta (cycads)
- Ginkgophyta (ginkgoes)
- Gnetophyta (gnetophytes)
- Magnoliophyta (flowering plants)
The first five phyla listed above have “free-sporing” life cycles in which the spores are released from the sporangia, and the gametophytes are (therefore) free-living. This lab is concerned with these divisions.
The second five divisions are seed plants in which the spores are retained (within sporophytes tissues) and develop into gametophytes within sporophyte tissue. These divisions will be discussed and observed in next week’s lab (Lab 11).
A summary of the five free-sporing clades is provided below:
The mosses are a large and diverse group of plants with 10,000-12,000 species. Mosses are generally considered to be “non-vascular” (lacking the specialized conducting tissues xylem and phloem), although some mosses contain functionally similar conducting cells (hydroids for conducting water and leptoids for conducting photosynthates). Because of their general lack of “true” tissues and (consequently) organs, the body of a moss is traditionally referred to as a thallus. That said, the thallus consists of stem-like and leaf-like portions, and these terms are usually applied for convenience. Mosses are anchored to their substrate by strands of cells called rhizoids.
Mosses, liverworts, and hornworts have a similar variation of the sporic life cycle in which the haploid gametophyte stage is perennial (long-lived), photosynthetic, and free-living. For this reason the gametophyte is described as the dominant generation. The sporophyte generation is much reduced (small), short-lived (in mosses and liverworts living just one growing season), and nutritionally dependent on the gametophyte. The sporophyte of mosses and liverworts lives just long enough to form and release the spores, then dies. The spores are released directly to the environment (hence these plants are free-sporing).
Although mosses are common in most habitats they are small and easily overlooked. They are familiar in shady, moist habitats, but are also very common in dry, often exposed habitats. Some are common components of cryptobiotic crusts in our deserts of southern Utah. Other moss species are pioneer species that grow on bare rock, sand, or clay (or fence posts, roofs, or brick walls). They are often commensals on tree trunks of branches. Mosses tend to be substrate specific and may be used in ecological studies as indicators of microclimates.
Life Cycle: A generalized moss life cycle is summarized below. Liverworts and hornworts have similar life cycles.
Gametophytes may be either monoecious (one individual forming both sperm cells and egg cells) or dioecious (male individuals and female individuals). Sporophytes are homosporous (form just one type of spore).
The images (below) show late stages in moss sporophyte development:
There are about 9,000 species of liverworts. Liverworts are often mistaken for mosses due to their similar small size and morphology. They also are abundant in the same moist, shady habitats favored by many mosses. Liverworts share additional features with mosses including: lack of true tissues and organs, life cycle with a dominant gametophyte generation and a reduced sporophyte that is nutritionally dependent on the gametophyte.
Liverwort gametophytes may be “leafy,” with stem-like and leaf-like portions or “thallose” with a flattened thallus. Leafy liverworts may be difficult to distinguish from mosses, but are usually dorsoventrally flattened with their leaves in three rows, the bottom row of leaves smaller than the other two. Gametophytes are attached to the substrate by unicellular rhizoids. Liverworts form a thin cuticle, but are the only members of the plant kingdom that lack stomata. Some thallose liverworts form air chambers on their upper surfaces that are connected to the outside environment by tiny pores. Air chambers contain short filaments of photosynthetic “cactus cells.”
Gametangia are usually borne on the surface of the thallus or leafy stem, but in some are raised above the thallus on antheridiophores or archegoniophores. Sporophytes develop within the archegonium and consist of a foot, seta, and a simple sporangium. At maturity the seta elongates, the spores are released, and the sporophyte rapidly withers.
The four images (below) show features of a thallose liverwort gametophyte:
Hornworts represent a relatively small clade of plants (120-180 species) that are morphologically similar to thallose liverworts. Their life cycle is similar to that of mosses and liverworts with a long-lived dominant gametophyte and a smaller sporophyte that is nutritionally dependent on the gametophyte. Archegonia and antheridia are sunken in the thallus. Unlike mosses and liverworts, the sporophyte of a hornwort is perennial and will live and continue to grow as long as the thallus stays alive. The sporophyte is cylindrical and anchored in the gametophyte tissue by its foot. New sporophyte tissue is formed from a basal meristem located just above the sporophyte foot. Even while the older, distal portions of the sporophyte are releasing spores, the newly formed proximal portions of the sporophyte are still developing.
The lycopods include a small group of modern plants (between 950 and 1,200 species) that have a long fossil history. Within modern lycopods, there are three major clades (often recognized at the rank of order) that bear the common names clubmosses, spike mosses, and quillworts.
Unlike the mosses, liverworts, and hornworts the dominant generation of a lycopod is the sporophyte. Lycopod sporophytes have true tissues (including the vascular tissues xylem and phloem), and are thus described as vascular plants. They have true stems, leaves and roots. Lycopod leaves have a different evolutionary history than all other leaves. They evolved from extensions of the stems (enations) that became vascularized. Such leaves are called microphylls.
Lycopod sporophytes reproduce by forming spores in sporangia that are situated on the upper (adaxial) surface of special leaves (sporophylls), near the leaf base. Sporangia are borne one sporangium per sporophyll. Sporophylls may be morphologically identical to vegetative microphylls, or alternatively may be very small (reduced) and clustered at the tips of stems to form a “cone” or strobilus. Plants may be homosporous (forming just one type of spore) or heterosporous (forming two types of spores: smaller microspores and larger megaspores). Spores are released to the environment (lycopods are free-sporing), and gametophytes live independently from the sporophytes. Homosporous lycopods form bisexual gametophytes while heterosporous lycopods form male gametophytes (from microspores) and female gametophytes (from megaspores).
Gametophytes are morphologically variable, but are usually small and short-lived.
The monilophyte clade is large (with about 12,000 species) and diverse. It includes the familiar “filicalian” ferns, as well as tree ferns and heterosporous water ferns. This group also includes the whisk ferns (12 species) and horsetails (15 species). Monilophytes have a long fossil record, second only to the lycopods. Like the lycopods, these are free-sporing vascular plants with a dominant sporophyte generation and a free-living but reduced gametophyte division. Most monilophytes have stems, leaves, and roots.
Monilophyte leaves are more complex than the simple lycopod microphylls, and are considered to have had a different evolutionary origin. Monilophyte leaves (and the leaves of all vascular plants other than lycopods) evolved from lateral branching systems that became overtopped, planated (i.e. flattened), and webbed. Such leaves are called megaphylls.
Most familiar fern sporophytes have stems (usually called rhizomes) that bear roots and megaphyll-type leaves (called fronds). The rhizomes may be subterranean or above ground. The fronds may be small and simple, or large and complex. Fern fronds often “unroll” as they mature (developing leaves are often referred to as fiddleheads). Many fern fronds are subdivided into smaller units (leaflets). Ferns are often grown ornamentally for their attractive foliage.
Sporangia are often formed on the lower (abaxial) surface of sporophylls. Sporophylls may be morphology similar to vegetative leaves, or strikingly different. Numerous sporangia are often clustered into a zone called a sorus (plural sori), and the sori are often protected by a flap of leaf tissue called the indusium (plural indusia). In some ferns, the margin of leaflets is rolled or folded over the sporangia to make a “false indusium.” Most ferns are homosporous. Spores are released from the sporophyte and develop into small, free-living gametophytes.
Gametophytes are variable in their features, but are commonly small, flattened, photosynthetic and roughly heart-shaped. Fern gametophytes attach to the substrate by rhizoids, and form the antheridia and archegonia on their lower surfaces (typically the archegonia close to the “notch” of the heart, and antheridia closed to the base). Syngamy results in the formation of new sporophytes within the archegonia. Initially the sporophytes are nourished by gametophyte, but soon put out their own leaves and roots, causing the destruction of the tiny gametophyte.
The following images (below) show fern fronds and abaxial sporangia in sori:
Whisk ferns are a small group of simple monilophytes consisting of 12 species in two genera. The only common species is Psilotum nudum. The sporophyte of these plants lacks leaves and roots, consisting entirely of stems that bifurcate at regular intervals. Stems bear small scale-like appendages (often called prophylls), but because they lack vascular tissue they are not considered to be true leaves.
Sporangia are borne along the aerial stems in groups of three that are fused into a single synangium. Whisk ferns are homosporous. Whisk fern gametophytes are subterranean and cylindrical and form mutualistic relationships with mycorrhizal fungi
There are 15 species of extant horsetails, all belonging to the genus Equisetum. Although a small clade, they have a long fossil history extending back to Devonian time. Horsetail sporophytes have roots, stems, and leaves. The stems are hollow and roughened with crystals of silica oxide. Like ferns and whisk ferns, these plants spread by subterranean stems (rhizomes) and often form dense stands of individuals. Leaves are tiny, often not photosynthetic, and fused together to form a grayish band around each node. The stems are the photosynthetic organs of these plants. All parts of horsetails are whorled, meaning that numerous leaves (fused into a band, as mentioned above) or branches are formed at each node. Sporangia are borne on structures called sporangiophores that form a strobilus at the tip of some branches. All modern horsetails are homosporous. Horsetail gametophytes are very similar to those of ferns.
Equisetum sporophytes fall into two morphological categories. One group of species forms unbranched, photosynthetic, aerial stems. These stems are perennial, and stay green through the winter. Pioneers referred to these as “scouring rushes.”
A second group of species is dimorphic, forming separate reproductive and vegetative stems that appear quite different. The reproductive stems are formed early in the spring, and lack photosynthetic pigments. They are short, unbranched, and each bears a strobilus. They die shortly after releasing their spores. The vegetative stems are formed a little later and bear numerous smaller branches at each node. This results in a “plume-like” growth form that gives rise to the common name horsetails. Plants with this mode of growth die back to the ground during the winter.