We care about land plant, or embryophyte, evolution for many reasons:

• Plants are the primary producers for most ecosystems.
• Plants form the framework of the physical environment in which land animals live. In fact, in the history of life on land, plants have functioned as pioneers, evolving to live in new environments, and making those environments hospitable for animals.
• Plants influence sedimentation
• The carbonization and exhumation of plants influences the carbon cycle.
• Plants have a copious fossil record. Indeed, in the form of coal, they are economically important.
• Land plant fossils, both macro and microscopic, are ubiquitous and precise paleoenvironmental indicators - the forams of the land.

Adaptations to life on land:

The closest sister taxa to land plants are the paraphyletic grade group of multicellular green algae known as charophytes Charophytes:

• exchange gasses with the environment by simple diffusion,
• require no structural tissue to support their bodies,
• release their sperm (and in some, ova) directly into the water.

• A waterproof cuticle covering most of the external surface prevents desiccation.
• A vascular system of fluid conducting cells to move liquids through the plant.
• Stomata, controllable microscopic openings allowing gas exchange through the impermeable cuticle. (These open and close in response to illumination, humidity, and [CO₂].
• Gametes no good for dispersal on land, hence dispersal through spores with durable walls to resist desiccation.
• Were do we get spores? Alternation of generations in which the parent gametophyte shelters and supports a growing sporophyte.

• Adult diploid sporophytes produce haploid spores. These usually develop in capsules called sporangia which break open, allowing the spores to disperse.
• Spores grow into haploid gametophytes. These produce male and female gametes. In more primitive embryophytes, regions of the gametophyte known as antheridia and archegonia produce sperm and ova respectively. The sperm swim to the through a film of water on the plant's surface ovum to form a diploid zygote.
• The need for the sperm to reach the ova under their own power imposed a powerful constraint on the earliest embryophytes: Their gametophyte generation had to grow in moist environments.
• The zygote growing from the archegonium, develops into a new diploid sporophyte. Sporophytes generally differ from gametophytes in possessing tall stems. Why? Presumably to elevate the sporangia for better spore dispersal.

Tracheophyta:

The tracheophytes, or "vascular plants" include the majority of land plants. As the name implies, they are characterized by vascular tissue reinforced by lignin, a durable substance contributing to vascular tissue and call walls. The presence of this substance facilitated the growth of taller, stronger stems. This was a vital prerequisite to their second major adaptation: The emphasizing of the sporophyte generation over the gametophyte generation.

Modern ferns give a good idea of the ancestral tracheophyte reproductive system. Their gametophytes are generally only a few millimeters across, whereas their sporophytes can be quite large. By means of this innovation, ferns and other tracheophytes can break their dependence on moist environments for most of their life cycles. Although their gametophytes still require moisture, they are small enough that they can develop from spores quickly when favorable conditions occur.

The bodies of vascular plant sporophytes are generally differentiated into:

• rhizomes which run along the ground, connecting individual plants,
• roots, which obtain minerals from the soil, and
• stems, which produce glucose through photosynthesis above the ground.
• In all but the most primitive, leaves increase the amount of sunlight that can be captured.

"Seedless vascular plants": The first major radiation of vascular plants occurred in the Silurian and Early Devonian, giving rise to groups that dominated land floras for most of the Paleozoic This radiation consisted primarily of plants that reproduced in the manner described above:

• Rhyniophytes:" Sil. - Dev. The paraphyletic group containing the earliest and most primitive kown tracheophytes is named after a well-known genus, Rhynia. These plants consisted of rhizomes and leafless stems with small simple roots, were generally less than 10 cm. tall, and bore sporangia at the tips of their stems. The leafless modern Psilotum superficially resembles them, and was once thought to be a surviving member of this early radiation, but it is now thought to be a highly derived fern.

  Lycophyta: (Lycopods) Dev. - rec. Members of this group possess narrow pointed leaves that are vascularized by a single strand of vascular tissue. The leaves cover the stem. When they fall away, they leave the stem covered by characteristic diamond shaped scars. Modern representatives, the "club mosses," are a sad remnant of a once mighty radiation. During the Mississippian and Pennsylvanian, their tree-sized relatives, including Lepididendron and Sigillaria were the largest forest plants. The sporangia of lycophytes are born on the upper surface of specialized leaves which grow in groups, forming cones or strobili at the ends of stems. Earliest representatives of lycophyte lineage may be Zosterophyllopsida, (Zosterophyls) resembling rhyniophytes but with alternating sporangia on stem.

  Sphenopsida (horsetails) Late Dev. - Rec. Like lycophytes, these plants protect their sporangia in strobili that develop at the end of stems. They are distinguished by the unique morphology of their stems. These consist of cylindrical sections separated by nodes. Whorles of slender leaves radiate from the nodes. During the late Paleozoic and Triassic, some sphenopsids such as Calamites attained tree size, but they were never the primary flora of their environments. Modern sphenopsids are known for rendering themselves inpalatable by incorporating silica phytoliths into their tissues.

• Pteridopsida (ferns): Late Dev. - rec.
  

  Ferns were the first plants with large vascularized leaves. Most, but not all ferns have very short trunks that support large pinnate leaves. Many ferns have independently evolved the "tree-fern" habitus, growing to the size of small trees. Unlike lycophytes and sphenopsids, ferns bear their sporangia on the bottoms of their leaves.

Seeds: The second great radiation of land plants occurred during the Late Paleozoic, and was associated with the evolution of the seed. This occurred in stages.

• Ancestrally, gametophytes produced both sperm and ova.
• Some fossil as well as some recent non-seed bearing plants, display a dimorphism of between macrospores and microspores, which specialize in producing either ova or sperm, respectively and which resemble them in size dimorphism. This trend seems to develop primarily in plants with strobili which can, in principle, harbor the gametophyte as well as the sporangia.
• When we pick up this trend again, it is in plants that retain the female gametophyte inside special reproductive structures. The gametophyte is known as an ovule, and contains the ovum, nutritive tissue, a seed coat, and structures for conducting sperm to the ovum. Once the ovum is fertilized, the ovule becomes a seed.
• How does the sperm get to the ovum? When a microspore lands on the sperm-capturing portion of the ovule, it develops into a specialized male gametophyte called a pollen-tube which grows into the ovule. Sperm is conducted down the pollen-tube to the ovum. In such a set-up, we call the microspore a pollen grain.

Wood: A second major evolutionary novelty was the ability of seed plants to lay down secondary tissue. Ancestrally, land plants could create new tissue only in the apical meristem tissue at the tips of their stems. In seed plants, however, we see the appearance of vascular cambium, a sheath of tissue that generated new vascular tissue around the circumference of the stem. New tissue laid down by vascular cambuim appears as growth rings in cross sectioned stems. The ability to thicken existing stems allowed seed plants to attain greater size than their arborescent lycopod precursors.

"Seed ferns" Paraphyletic grade group including numerous Late Paleozoic land plants:

• Progymnosperms: (Dev - Miss) Intermediaries between spore and seed plants. (Paraphyletic?) Fern-like foliage, no seeds, but with woody tissue. E.G. Archaeopteris, the tree of the Late Devonian. Display distinction between mega and micro spores.
• Medullosales: (Late Paleozoic) Late Paleozoic plants superficially fern-like bore distinct ovules and pollen (though not in protective structures.)
• Glossopteris: (P - Tr) Major portion of Late Paleozoic and Triassic Gondwanan flora.

Conifers: P. - rec. carry their seeds on the scales of cones. Most conifers are tree-sized and produce both male (pollen bearing) and female (seed bearing) cones. In many cases, their leaves are reduced to needles, allowing them to withstand cold and dry conditions. This ability allows many to retain their leaves year-round. Earliest known representative were Late Paleozoic Cordaitales, trees with whorls of strap-like leaves. Living examples: Araucarians Podocarpus Yews Pines Cypress

Ginkgoales (Ginkgos) P. - rec. Common in the Mesozoic and Paleogene, they are now reduced to a single species, Ginkgo biloba. Like cycads, ginkgos are dioecious - have separate male and female individuals.

Cycadales: (Cycads) Penn. - rec. Shrub-sized superficially fern or palm-like plants with short trunks and very tough pinnate leaves. They are very slow-growing. Like conifers, they produce male and female strobili. In this case, however, each individual plant is fully male or female, producing cones of only one sex. Cycads were very common understory plants during the Mesozoic but are uncommon today.

Anthophyta (Angiosperms) - flowering plants:



Simply to say that flowering plants are distinguished by flowers would be sort of dumb. In fact, flowers represent three major innovations:

• Unlike other seed plants, in which pollen and ovule producing structures are completely separate, flowers combine them in a single complex structure. In the flower, pollen is produced by stamens.
• The ovule is completely concealed by specialized leaves called a carpel. When the ovule is fertilized, the carpel of many species swells to form a fruit, whose primary purpose seems to be to intice animals into consuming the seed. This allows the seed to hitch a ride in the animal's gut and insures that it will be deposited with a blob of fertilizer. Indeed, some seeds can't germinate without first passing through someone's gut.
• The flower incorporates specialized leaves (petals, sepals) and secretes nectar whose function seems to be to attract pollinators, usually insects. In the latter two adaptations, the plant "pays" animals to transport pollen directly to other flowers, and to disperse seeds, resulting in an increase in reproductive efficiency.
  

• The anthophyte seed is derived in that the endosperm - the tissue that nourishes the embryo - must also be fertilized by sperm. After fertilization, it is triploid. In non-anthophytes, the nutrient tissue is haploid.

Additionally, Anthophytes tend to have broad leaves with branching veination.

Anthophytes appear in the fossil record in the Late Jurassic but don't become common until the Cretaceous. Today, they are the dominant plant type, with over 200,000 known species (compared to 550 species of gymnosperms).

Anthophyte diversity:

• Traditionally, flowering plants have been divided into Dicotyledonae and Monocotyledonae (commonly dicots and monocots).
• Phylogentic systematists have confirmed that Monocotyledonae is monophyletic, however dicots are not.
• Most "dicots" do fall within a monophyletic "Eudicotyledonae"
• Other groups, including water-lillies, star anise, Amborella, and magnolias fall outside these groups.
• Monocotyledonae appear in the Cretaceous and include lillies, orchids, grasses, palms, etc. Note that grasses appeared in the Eocene and proliferated during the Oligocene, transforming much of the world by converting arid and semi-arid regions to grasslands.

C₃ and C₄ plants: Depending on the plant, two distinct photosynthetic pathways are employed by plants, termed C₃ and C₄. Most plants use C₃. The C₄ pathway conserves water and it present in arid adapted plants and grasses.

Origin of Anthophyta: The presence of flower-like structures in certain gymnosperms presents a picture of the origin of flowering plants:

• Bennettitaleans: Sometimes called Cycadeoidea. Superficially resembling cycads with short trunks covered by leaf scars and pinnate leaves. Their reproductive structures are different. First, like most flowering plants, they are hermaphrodites. Second, some display flower like cones in which the ovule is contained in a central recepticle surrounded by specialized leaves (bracts) and microspore producing structures are arrayed radially around it, like stamens.

• Gnetales: (Ephedra, Welwitschia, etc.) Triassic-recent. Weird arid adapted plants with flower-like structures that take the bennettitalean pattern one step further by enclosing the ovule in an integument (carpel homolog?) that is open at the end.

• Final step: the complete enclosure of the ovule inside the carpel.

• Amborella, although weird for being dioecious, otherwise suggests the structure of the ancestral flower:
  • very small
  • petals and sepals undifferentiated
  • ovum develops indirectly, as in nonflowering plants

  • Finally water-lilies have endosperm that is merely diploid - a plesiomorphy or autapomorphy?

  To Syllabus.

  Last modified: 22 August 2008