Simplified plant evolution – From a tiny to a giant

The yellow meranti, a flowing plant in Southeast Asia reaches a height of 331 feet easily. The giant sequoia tree, a conifer species in California stands 314 feet tall and is over 36 feet in diameter at the base. The tallest tree, named Hyperion, a redwood species in California is 380 feet tall. How could a tree grow this huge? About 500 million years ago when the first land plants start to emerge, plants were as small as a tiny dot. Charles Darwin explained evolutionary phenomena and characteristics with taxonomic and morphological data, leading to his primary theory which is the natural selection of best adaptation. Herbert Spencer described as “survival of the fittest” in his book of Principles of Biology. A classic example of natural selection is the long neck of giraffe as only those giraffes with long necks are able to survive while short neck ones were eliminated. Natural selection could explain the emerging of new species by allowing increment in the matching between the organisms and their environments. Upon changes in the environment or group migrating to a new environment, the adaptation to the new conditions eventually gives rise to new species. Even though 99.9% of species went extinct throughout the time, some species managed to survive including some bryophyte and early vascular plants such as Equisetum (horsetail) and early gymnosperm like Ginkgo biloba, which are estimated to be about 270 million years old.    

Green algae are considered as the ancestor of land plants because they share the most similarities in chloroplasts and photosynthesis. However, algae have a much greater diversity in plastid types. Multicellular green algae (Chlorophyta) contain chlorophyll that performs photosynthesis in the presence of light. Whereas, non-green algae such as red algae (Rhodophyta) and brown algae (Phaeophyta) carry out photosynthesis by absorbing blue-green light energy. Algae-like plants might have evolved as early as 1 billion years ago from a branched, filamentous alga dwelling in shallow freshwater. Algae have neither roots nor stalks but they are able to live in freshwater or saltwater without any substrate. Molecular phylogenetic data provide the framework for reconstructing this evolutionary history and sheds light on the origins of a group that contains nearly all levels of vegetative morphology, from single cells to complex terrestrial plants.

Fossil records suggested that plants colonized terrestrial environments as early as five hundred million years ago. The rising levels of atmospheric oxygen might have triggered a series of life blooming events on earth including the adaptation from aquatic to a terrestrial environment. Plants need only some essential elements which are water, carbon dioxide, nitrogen, magnesium, phosphorus, potassium, some trace elements, photons, as well as the various biochemical pathway that responsible for photosynthesis. The successful terrestrialization was facilitated by the improved cellular structure. One of them is the cuticle layer, which is a waxy layer on most of the leaf surfaces that reflects the light and reduces water loss at the same time. The other is the emerge of stomata that are the tiny pores on leaves that control the gaseous exchange through transpiration.

Mosses, liverworts, and hornworts are simple plants known as bryophytes that absorb moisture from the air. They have no roots but rhizoids which not able to absorb water from the soil but just a filament that anchors the plant to the ground. Both mosses and liverworts have two reproductive generations, known as alternation of generation. They can either reproduce through sexual reproduction via gametes in gametophyte generation or produces spores asexually in sporophyte generation. However, sexual reproduction still needs the involvement of water as the medium. In arid condition, the sporophyte generation would produce spores but the sporopollenin exposed to the air for regeneration only when there is moisture in the air. The extremely pivotal evidence was the formation of desiccation-resistant spores in response to the periodic drying. This alternate between the sexual and asexual phases allows reproduction flexibility in response to different environmental conditions.

After bryophytes, specialized cells for water and nutrient uptakes, which are vascular systems take a central role. In aquatic conditions, plants have a short distance from the source of water and nutrients. To make the transition to land, they have to evolve from floating in the nutrients stream to carrying the nutrients stream inside. Those without vascular system but colonized at a terrestrial environment are commonly found at moist, damp, and shaded regions. With the development of vascular tissues, about twelve thousand species of ferns (Pteridophytes) started to emerge simultaneously throughout the land. Vascular tissue also provides mechanical support as well so that plants can stand tall. Other early vascular plant species such as lycophytes like clubmosses and horsetails are observed at one thousand three hundred species and sixteen species respectively. Ferns reproduce by spores, which are produced in sporangia, the spore case. And these cases grow in rows on the lower epidermal surface of the leaves known as sporophylls, a leaf that bears sporangia.

With vascular tissue, plants further evolved into shrubs and non-flowering trees. Gymnosperm is known as a vascular plant with “naked seed,” in their seed are exposed without a coat and no flower. Currently, there are more than 800 species of gymnosperms. Typical gymnosperm such as conifers is monoecious, which means a plant has both male and female sexual organs.  Their seeds are held between the scales of a structure called a cone.

As the adaption proceeding further. Then the angiosperms which are flowing plants emerged. There are more than three hundred and fifty thousand species of angiosperms on the earth that produces seeds through self- or cross-pollination with the help of pollinators. Angiosperms grow in a wide range of habitats and show significant variation in appearances. First of all, pollinator like to fly between plants with similar flowers. And then isolation allows plants to evolve in different directions, which eventually contribute the diversities.  There are monocotyledon, basal angiosperms, magnoliids, and eudicotyledons.

Monocotyledons are characterized by their seeds that have only one cotyledon (seed-leaf) with most species having parallel veins. Their flowers are usually in three or the multiple of three. Generally, grasses with seven hundred and fifteen genera and ten thousand five hundred and fifty species as well as orchids with seven hundred and seventy-nine genera and twenty-two thousand five hundred species.

Basal angiosperms can be classified by trees, shrubs, and some aquatic plants such as water lily. It contains a few hundred flowering species that are showing the fused margin of the carpel. There are at least two hundred and fifty thousand angiosperms, making it the most diverse of all plant phyla. They share several morphological features that distinguish them from other plant groups. They have a network of veins with the floral structure usually in five or more petals that different in terms of shape, size, numbers as well as colors. Magnolias are the trees, shrubs, or lianas which have alternate or opposite leaves with branching veins. Their inflorescence typically has several to numerous parts that are commonly arranged in whorls of three. The last category of angiosperm is eudicotyledons, commonly known as dicotyledon which consists of two hundred and sixty thousand species that typically have broad leaves with a branching network of veins, and many have woody stems. The floral parts are usually in fours or fives or multiple of four or five.

The flowers carry male and female reproductive structures either on the same plant (monoecious) or separate plants (dioecious). Angiosperms carry out double fertilization in which two male gametes (or called as sperm nuclei) are released from the male reproductive organ, anther that is containing pollen, into the female reproductive organ, the ovule. One of the male gametes will fuse with two polar nuclei to form the triploid endosperm and another male gamete will combine with an egg cell to form the diploid zygote. The endosperm provides the nourishment or nutrients for embryo development. The other feature that angiosperm differed from gymnosperm is that the angiosperm embryos are protected by an ovary wall which then develops into a fruit after fertilization has taken place. On the other hand, the embryos of gymnosperm have no protective layers on the surface of the ovule-bearing scales in the female cones.

Land environments are much more complicated than aquatic environments, plants face different humidity, temperature, light intensity, and soil conditions. in order to survive and thrive, plants have evolved with a more sophisticated structure and complicated physiology and biochemistry pathways to achieve multiple functions. Namely, plants have in-built mechanisms for coping stressful environment including seed dormancy, leaf-shedding, die-back as well as the ability to regenerate. Plant regenerate new cells and tissues after damage such as wounds and injury. The ability to return to a pluripotent state is called dedifferentiation which is important for germplasm conservation. They have diversified the way to reproduce as well as the traits that attract pollinators for seed dispersal.

Among all plants, also thanks to human cultivation, grasses had expanded rapidly and became the dominant plants on the earth. They are crucial to human well-being as approximately fifty-four percent of the food eaten by people is provided by the seeds of the grasses, the grain that obtained from cultivated varieties of just three main grasses, mainly rice, wheat, and corn.

Further readings:

Henry, R. J. (2005). Plant diversity and evolution: genotypic and phenotypic variation in higher plants. Cabi Publishing.

Motley, T. J., Zerega, N., & Cross, H. (Eds.). (2006). Darwin’s harvest: new approaches to the origins, evolution, and conservation of crops. Columbia University Press.

Britannica, I. E. (2008). Britannica Illustrated Science Library Plants, Algae And Fungi.

Willis, K., & McElwain, J. (2014). The evolution of plants. Oxford University Press.