The Theory of Evolution

The plants that live and grow on the earth today have not always existed. Modern science believes that evolution is the path that leads back from today's plants to the first forms of organic life on earth. (Note: Evolution does not confine itself to the development of plants. For the purpose of this website, only plants will be discussed.) Studies in evolution also look forward to hypothesize how plants will exist and adapt in the future.

Charles Darwin was one of the first to document evolutionary theories, natural selection and adaptation. He applied these theories to the study of plant life. Neo-Darwinism, a modern evolutionist concept, states that mutations and variations necessary for natural selection occur due to random and non-random forces. These two forces involve the needs of the plant as opposed to the changing conditions of its environment. Adaptation defines the effect of the interactivity between random and non-random forces.

The theory of evolution, plant classification, and our knowledge of plants are expanding constantly through ongoing scientific research and discoveries. Allan Gardens has been a valuable resource for this type of research. For many years, both educators and students have visited Allan Gardens Conservatory to better understand the plant kingdom.

Some examples of the existing evidence supporting the theory that modern plants, and the entire plant kingdom, came from the Green Algae phylum are as follows:

It is because of these similarities, among others, that the lineage of today's plants can be traced back to the green algae phylum.

Originally, the earth had no atmosphere. Due to gravity, the earth became more and more compressed, causing heat and pressure to build up until volcanoes erupted all over the earth. When the lava hardened and the gases cooled, precipitation occurred. At this point, the earth's surface was not protected from the sun's deadly ultraviolet rays as in the case of bodies of water. Underwater, small unicellular organisms formed, developed and grew. They reproduced with one set of chromosomes by splitting one cell into two equal and identical halves that would then grow into cells large enough to divide again. Later, they became multicellular organisms with more complex methods of reproduction, involving two sets of chromosomes. This new method of reproduction made adaptations possible. (FOR MORE INFORMATION, SEE THE PLANT REPRODUCTION SECTION)

One of the by-products of their existence was oxygen, which they released into their environment. The oxygen floated to the surface of the water and was released into the newly forming atmosphere.

Approximately 410 million years ago, the oxygen in the atmosphere had reached levels greater than 2 percent and produced a by-product of its own: ozone. This blanket of gas was enough to protect plants from the sun's UV (ultra violet) rays and created a hospitable environment for organisms headed for land. Also, due to the tides, water levels would rise and fall and mud flats started to occur in more shallow areas. Sometimes they would be dry and at other times, they would be wet, creating ideal grounds for the development of land plants.

Water-dwelling plants had large surface-to-volume ratios. They were supported by water and had a thin construction for easy transfer and diffusion of gases and minerals throughout their entire surface. Now aquatic plants required a new construction more suitable for habitation on land.

The varying changes in plant adaptations to their new environment led to the first major split in the plant kingdom: vascular and nonvascular plants.

Once on land, some of the green algae developed ways to adapt to their new environment. Through natural selection, plants became more rigid in order to support themselves out of water. They also developed a more compact shape and stronger cell walls with hard outer layers to protect them from losing moisture and suffering from the other drying effects of the wind. In order to continue breathing, they developed small openings called stomata. Plants also began to absorb water and minerals in specific places rather than using their entire surface for that function. The plants in this group include mosses, hornworts and liverworts. Until 1995, all nonvascular plants were classified as bryophytes. The current view is that only mosses are a division of bryophyta and that liverworts are a division of hepatophyta and hornworts are a division of anthocerophyta.

Mosses, which continue to exist, are the largest group of the three major nonvascular plants. In their gametophyte stage they look like algae. Otherwise, they have stems and a type of leaf that makes them look more like vascular plants. These hardy plants can grow in polluted areas, synthetic habitats (e.g. cracks in sidewalks) and in environments where the geology or weather would make it impossible for other plants to grow. They, like liverworts, can be found in many places throughout the continents. Liverworts come in many different shapes and sizes, but never more than a few centimetres high. Hornworts are the simplest of the nonvascular plants and, like the other two, are small but obvious gametophytes, which produce smaller dependent sporophytes.

Only small plants are able to retain the surface film of water necessary for the free-swimming sperm to reach the egg cell, which remains attached to the parent plant. At Allan Gardens, there are many types of nonvascular plants that can be viewed much like they were centuries ago.

Vascular plants were the result of further adaptations that occurred due to the need for plants to transport food, water and minerals from areas of plenty or production to areas of need. Xylem tissue for transporting water and minerals, and phloem tissues for transporting food were developed. The principal water-conducting element of the xylem that evolved in the Paleozoic era was tracheid. Plants could now grow larger than before. They also became even more rigid in order to grow upward, so they could better compete for sunlight to be used in photosynthesis.

The sporophyte plant "Cooksonia" is the earliest and simplest type of vascular land plant with abundant fossil evidence as documentation.

Zosterophyllophytina started as leafless stems that grew 20 centimetres high. Later, as they grew taller, spiny precursors to leaves grew from the now thicker stem, thus providing more surface area for better photosynthesis. Eventually, as the spines got larger and flatter they developed more complex xylem and phloem tissues.

The trimerophytina, which evolved from the rhyniophytina, would be responsible for developing the first form of simple leaves. Trimerophytina is also the direct ancestor of ferns, horsetails and gymnosperms, and therefore, all seed ferns, cycads, conifers and flowering plants as well. Although actual trimerophytes, rhyniophytes and zosterophyllophytes have long since died out, it is their ancestors which rule the plant kingdom today. The invasion of land by plants was complete approximately 400 million years ago and made land hospitable for animals, amphibians and insects.

Many other adaptations had to be made, however, to create the vast number of species that currently cover the earth. Spines on cacti were once leaves that were modified to defend against predators. Allan Gardens has a wide selection of cacti in the Arid House.

Spines on cacti are modified leaves designed as a defence against predators. Before this change could take place, the stems of the cacti had to evolve first. They had to take over the major function of the leaves: photsynthesis. French botanist, E. A. O. Lignier hypothesized in 1903 that roots are simply branches that bent over and penetrated the soil.

After many years of living underground, through adaptation and natural selection, the root systems of a plant differ greatly from the shoots and aerial branches. Complex leaves also were yet to develop. Spores would grow to be so large that it would only take one to fill an entire spore case. Today, they are known as seeds. Eventually, plants would grow so tall that an even stronger structure would be required for support, causing the formation of the woody tree. Plants would grow taller in order to protect themselves from the growing number of ground-dwelling foraging animals, such as early insects and amphibians.

Today, at Allan Gardens, human intervention attempts to reduce many of the risks caused by animals, insects and environmental issues. Broken windows are replaced to protect plants from renegade raccoons and squirrels who cause damage to plants. The Integrated Pest Management system (IPM) is used by staff to control damage caused by insects. Humidity and temperature are also controlled from room to room. Many of the plants at the gardens come from other areas of the world, which differ greatly in geography and climate. These types of plants would not be able to survive in the cool temperature of southern Ontario if this controlled environment were not provided. Humans, rather than nature, also manage the watering and feeding of plants inside the conservatory. The soil at Allan Gardens is specifically chosen and mixed with compost. This kind of human intervention is helpful for the plants that live at Allan Gardens.

	Green algae had, and still has, photosynthetic and accessory pigments.
	Green algae use starch as their principal storage carbohydrate.
	The cell walls of green algae are mainly composed of cellulose.
	Some green algae have bulky three-dimensional bodies.

	early rooting structures.
	two separate stages in a more complex form of reproduction (the sporophyte and gametophyte stages) in which the plant looked very different.
	the sporangia became more egg-shaped rather than round.