Rhodophyta is the sister taxon to Green Plants, and both groups are included in the putatively monophyletic taxon Archaeplastida, formerly known as Plantae.
Green Plants Green plants include all organisms commonly known as "green algae" and all land plants. Their common ancestry is reflected in these synapomorphies, unique to green algae and land plants...
- chlorophyll a and chlorophyll b
- carotenoids as accessory photosynthetic pigments
- double-membraned chloroplasts containing stacks of thylakoids (grana)
- energy stored as starch inside the chloroplast in which it is produced
- cell walls composed of cellulose and pectin
- cell wall construction via cell plate formation at the end of mitosis
- alternation of generations (more on this later)
- haploid gametophyte produces gametes
- gametes fuse to form a zygote
- zygote grows to become the diploid sporophyte
- sporophyte produces haploid spores
- spores grow into a haploid gametophyte
Remember that the above characters are synapomorphies with respect to the taxa which branched off earlier on the phylogenetic tree.
These same characters are considered to be symplesiomorphies with respect to the green algae and plants considered as a group.
These characters are thus not informative in determining further clades within the green plants.
The synapomorphies of land plants relative to their green algae relatives include:
- heteromorphic alternation of generations (In algae, alternation of generations is isomorphic
- gametophyte and sporophyte are physically indistiguishable)
In land plants, gametophyte and sporophyte look completely different (heteromorphy)
- Plants have multicellular sex organs (gametangia) (male = antheridium; female = archegonium)
- Plants have true tissues
- Plants have true organs (root, stem, and leaf)
- Plants have a waxy cuticle external to their tissues
- Plants have breathing structures called stomates
Many green algae exhibit a progression of complexity (e.g., the volvocine line of evolution). This should not be taken to represent a progression towards multicellular land plants. It merely shows that many algae appear to be genetically predisposed to have a division of labor among their cells. But this predisposition could be the reason that multicellular plants eventually did evolve
. Early Cousins of Land Plants: Zygnematales The Zygnematales are (haploid) green algae that undergo sexual reproduction via conjugation: haploid vegetative cells in the algal adult form non-flagellated gametes. Complementary (+ and - types) gametes fuse to form a zygote. There are many variations on the conjugation theme, but one example is nicely shown in Spirogyra:
Each zygote undergoes meiosis to produce four new haploid propagules, which are released from the cell wall of the former vegetative cell to grow into new, haploid filaments of algae.
Early Cousins of Land Plants: Charales The Charales, aslo known as stoneworts or brittleworts, are common in slow-moving, often eutrophic freshwater habitats, and are found on every continent except Antarctica.
The organism's body (a.k.a. thallus; a plant body not differentiated into roots, stems and leaves) consists of large cells that may be several centimeters long. The algae may branch at nodes made up of smaller cells.
As in land plants, growth occurs at the apex of each tip of the thallus. Translucent rhizoids, rootlike structures that are not absorptive, but primarily for anchoring the organism, sprout from the bottom of the plant.
Stoneworts have multicellular male and female sex organs (male = antheridium; female = oogonium), which grow at the nodes.
The name of these plants comes from the crust of white calcium carbonate seen in some species, which gives them a crunchy feel. Some species have a strong, musky smell, and are sometimes given the nickname "skunkweed" or "musk grass".
Chara is a species of particular interest in southern Florida, as it is a problematic invasive exotic that chokes canals and waterways throughout our area.
Early Cousins of Land Plants: Coleochaetales There are fifteen extant species of Coleochaete, of interest to systematists because they appear to be the closest living relatives of land plants. What characters do they share?
- Zygotes are retained in the oogonium, where they grow and develop into new plants.
- Oddly, they do not undergo an alternation of generations, which other green plants do.
- Cellulose synthesis takes place inside rose-shaped hexameric proteins known as rosettes anchored in the plasma membrane. (Land plant and charophyte cell walls contain more cellulose than do those of other green algae.)
- Their cell walls contain lignin-like compounds
- along with the complex carbohydrate cellulose (the most abundant organic molecule on earth), lignin is a major structural component of many types of plant cell walls.
- lignin is most often a component of woody plant bodies, and less common in herbaceous (i.e., non-woody) plants
- lignin is a very large organic molecule that confers compressional strength to plant parts.
- Zygote cell walls contain sporopollenin, once thought to be unique to land plants.
- Many algae (including charophyceans) live in ephemeral ponds that dry up.
- Natural selection favored individuals with traits that best enabled them to survive and/or produce offspring despite drought.
- Charophyceans produce a tough polymer, sporopollenin, that covers the zygotes (which are still attached to the mother plant) and protects them from desiccation.
- Sporopollenin is also found in plants, where it is a component of the protective covering of plant spores.
- Because sporopollenin allowed plants to survive in drier habitats, it facilitated their early colonization of land.
- Peroxisome enzymes
- Recall that peroxisomes contain enzymes that manufacture H2O2 from various substrates, and use it to oxidize large molecules (e.g., fatty acids) into smaller molecules that can be transported to the mitochondria for use in cellular respiration.
Only charophycean and land plant peroxisomes also contain enzymes that reduce the loss of the organic products via that old evolutionary relic, photorespiration.
- Similar flagellated sperm - its structure is different from that found in other algal groups.
- A phragmoplast forms near the end of mitosis.
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The phragmoplast is composed of microtubules and microfilaments, and appears late in mitosis. Vesicles derived from the Golgi gather at the equator of the nascent daughter cells, forming a network of tubules and vesicles filled with cell wall precursors. This "cell plate" gradually expands towards the perimeter of the cells, and joins with the original cell wall, separating the two daughter cells from each other.
Land Plants: Evolutionary History and Diversity Land plants all produce embryos that develop inside the tissues of a maternal plant. This is where the taxon gets its name. All embryophytes are primitively land-dwelling, though some are not entirely free of a need for water in the environment, especially for reproduction.
Why would a terrestrial existence be an advantage for a plant? Why didn't they just stay in the water?
- air filters less sunlight than water. There's more light for photosynthesis.
- air has more CO2 than water. There's more fuel for photosynthesis.
- early terrestrial habitats lacked pathogens or predators/herbivores.
- terrestrial soil is richer in nutrients than aquatic soil (why?)
