Chapter: Plants

Ecosystem Services

Of all the eukaryotic lineages, the most important for terrestrial life on Earth. Where phytoplankton, are the key of the food chain for the ocean ecosystems. Plants are the organisms that are the base of the food chain for all terrestrial life. They produce oxygen that all eukaryotes need to breathe, animals, fungi, and protists. Their roots form an interconnected network of fibers that hold soil together, inhibiting soil erosion. Plants hold water in their cells in huge vacuoles, helping to regulate the water cycle. They produce nearly all of the food for terrestrial life as we know it. Herbivores (such as rabbits) eat vegetation. Carnivores, like coyotes, eat rabbits. And even though they are not directly dependent on the vegetation for food. They are indirectly dependent on the vegetation. Plants also moderate the local and global climate. Not only do they protect from the shade and the wind. They also regulate global climate. Tropical rainforest produce  most of the oxygen of all the terrestrial plants, and have been called the lungs of the Earth.

Primary producers

Primary producers are the beginning of the food chain. They are organisms that convert physical energy to chemical energy which living organisms that use to go through life functions. The most important primary producers on earth are photosynthetic. They produce chemical energy from light energy. And they are the dominant force of carbon and energy in the world. During photosynthesis, plants reduce carbon dioxide to make sugars. Plants provide humans with food for eating, fuel for cooking, fibers for clothing, building materials for ….well….building, and medicine for healing.

 Plant Domestication

The earliest human attempts at plant domestication occurred in South-Western Asia around 11,000 BC. There is early evidence for conscious cultivation and trait selection of plants (Rye specifically) by pre-Neolithic groups in Syria. By 10,000 BC the bottle gourd plant, used as a container before the advent of ceramic technology, appears to have been domesticated. The domesticated bottle gourd reached the Americas from Asia by 8000 BC, most likely due to the migration of peoples from Asia to America.Cereal crops were first domesticated around 9000 BC in the Fertile Crescent in the Middle East. The first domesticated crops were generally annuals with large seeds or fruits. These included pulses such as peas and grains such as wheat. The Middle East was especially suited to these species; the dry-summer climate was conducive to the evolution of large-seeded annual plants, and the variety of elevations led to a great variety of species. As domestication took place humans began to move from a hunter-gatherer society to a settled agricultural society. This change would eventually lead, some 4000 to 5000 years later, to the first city states and eventually the rise of civilization itself.Continued domestication was gradual, a process of trial and error that occurred intermittently. Over time perennials and small trees began to be domesticated including apples and olives. Some plants were not domesticated until recently such as the macadamia nut and the pecan. In other parts of the world very different species were domesticated. In the Americas squash, maize, beans, and perhaps manioc (also known as cassava) formed the core of the diet. Potatoes were cultivated in the Andes mountains and were the primary source of calories which led to the rise of the Incan Empire. In East Asia rice, and soy were developed and still form the basis of all Asian cuisine. Some areas of the world such as Southern Africa, Australia, California and southern South America never saw local species domesticated.

Green Algae

If plants are the masters of the land, green algae are the primary photosynthetic organisms in freshwater. Originally considered to be protists, because they are single-celled eukaryotic organisms. They have been reclassified to be a part of the plant kingdom. They are considered to be the closest living relative to the terrestrial plants. And this gels well with genomic phylogenetics.

Algae also share many morphological characteristics with plants. Unlike most eukaryotes they have chloroplasts and a cell wall. Unlike plants they can be unicellular or colonial. They can also be multicellular, and they are most common in the water, either in fresh or salt water.

The transition between water-bound and terrestrial living represented a major, major transition in life on Earth. Of all the plants living on Earth today, these two plants are hypothesized to best represent this transition. On the left is a member of the coleochaetes. These algae grow in the discs that represent the basic morphology of all and plants, and one of the few algae that have this growth form. The stoneworts are often considered to be the first to become land plants. They are the most primitive plant that has an upright stem, and were the tropical rainforest trees of their day, towering millimeters above the competition.

Land Plant Morphologies

Bryophytes represent the first true terrestrial plants. Bryophyte is a traditional name used to refer to all (land plants) that do not have true vascular tissue and are therefore called 'non-vascular plants'. Some bryophytes do have specialized tissues for the transport of water; however since these do not contain lignin, they are not considered to be true vascular tissue. Currently bryophytes are thought not to be a monophyletic group; however the name is convenient and remains in use as a collective term for mosses, hornworts, and liverworts. Bryophytes produce enclosed reproductive structures), but they produce neither flowers nor seeds, reproducing via spores.

As plants became terrestrial, they covered the land. At this point, light became the single most limiting factor affecting the fitness of plants. So growing above your competitors became one of the most important evolutionary adaptations that drove the diversification of plants on land.

The development of vascular tissue was the single most important factor that allowed plants to grow upright. You can basically think of vascular tissue as straws that send water up from the roots (to hydrate the leaves) and sugars down from the leaves (to feed the roots). In addition, seedless vascular plants reproduce using spores instead of seeds. They fall into the following categories or phyla: Psilophyta, Lycophyta and Phenophyta, and also include Pteophyta, or ferns. Common types of seedless vascular plants include spike and club mosses, horsetails, whisk ferns and quillwort.

Seedless plants were vascular, but didn’t produce seeds. Rather they produced tiny gametes call spores. These plants diversified to store much, much more nutritive tissue into their embryos (in what we know as seeds). Also, these seeds have a protective layer on the outside of them reducing dehydration. Seed-bearing plants are the most recent group of terrestrial plants. They are broken up into two groups, angiosperms (or flowering plants) and gymnosperms (naked seed plants). Seed plants include many groups that are well known.

Seed Plant Groups

Gymnosperms are known as non-flowering plants. Plants in this group are made of the following groups. Cycads are a subtropical and tropical group of plants with a large crown of leaves and a stout trunk. Ginkgo is a broad-leafed tree that produces seeds not in a cone, and not in a fruiting body. There is only a single known species.  Conifers are cone-bearing plants and include pines, spruces, and firs.

Angiosperms are the most famous plants in the world. They form all the plants we depend on for food: from corn and rice to tomatoes and sweet potatoes. They are commonly known as flowering plants, and all have flowers and fruits. Angiosperm literally translates to vesseled seeds, which means that seeds are stored inside a carpel. The carpel is the protective structure of a flower. Think of a peanut. The carpel is the part you have to get through to get to the nut. 

Origin of Land Plants

The transition from water to land required and very important evolutionary adaptation, the cuticle. Cuticles are a water-tight tissue on the exterior of all land plants on earth. Think of it kind of like skin on yourself. It allowed plants to take water from the soil and into the leaves and keep the water there to be used as needed. Before the cuticle, plants could live on land but require constant 24/7/365 access to water. So plants were typically bound to living close to existing water bodies. Now plants could leave the confines of water bodies. And take over the world. Plants also developed spores. You can think of spores are really, really tiny seeds. But the most important thing is that these new land plants can reproduce sexually. And their zygote is encased in a protective coating that resists dehydration. Now plants can not only live away from land, their offspring can be easily spread across the land and live for extended periods of time before germinating.

 Silurian-Devonian Radiation

These major adaptations in plant morphology is what defines the boundary of the Silurian and Devonian periods. Following the beginning of the Devonian there was another major radiation of plant species diversity when they were finally able to conquer the virgin lands of the continents. They spread like wildfire.

Plants required several physical adaptations in order to to explode onto land. Not only did they require cuticles. They required a way to move water from the ground up to the leaves. This require more than simple capillary action that was required in the bryophytes. These plants required water-conducting tissues (called vascular tissues). The picture on the left is a cross section of a leaf. That pink spot in the middle is the main vein of a lear. That is vascular tissue. Once vascular tissue came to be, so did roots and leaves. And so did stomata. Stomata are special cells in the leaves that open and close allowing air to get into the leaf. Recall that photosynthesis converts water (H2O) and carbon dioxide (CO2). Water comes from the roots and carbon dioxide comes into the leaves through the stomata. Plants were set to rule the world.

Age of the Gymnosperms

Gymnosperms were the dominant land plants in the age of dinosaurs, the Cretaceous and Jurassic periods. Gymnosperms literally translates to “naked seeded” plants. They have true seeds (not spores). However, they aren’t enclosed in a vessel like fruit, like Flowering Plants. Common surviving gymnosperms are the Conifers, Cycads and Ginkgos and are similar in their woody habit and pattern of seed development but none of them have true flowers or fruits.

Cycads are every in Florida. The are short woody plants with long frond-like leaves (and have seeds produced in cones). Conifers (such as pines spruces and firs) all have short, scaly or needle-like leaves and seeds in a cone. Ginkgos are the only broad-leafed naked-seed tree that exists on Earth today.

And of all the ginkgos that were present, only one species still exists on this Earth, Ginkgo biloba. In fact, if it weren’t for some plant-loving monks in China, it is likely the ginkgo would be no more. A botanist traveling China found this strange tree growing in a monestary. It did not exist in nature. Since then it has been cultivated and remains a popular landscaping tree. Some efforts have been conducted to establish viable populations in its native China.

If ginkgos weren’t weird enough, the gnetophytes have got to take the cake. The gnetophytes have only three genera, but they are each highly variable in their own way. They do differ from other gymnosperms in that they have vessel similar to those found in flowering plants. So in a sense, these group of plants represent a morphological link between the gymnosperms and the angiosperms. And they are the stars of the age that we live in.

Age of Angiosperms

About 145 million years ago, the first true flowering plant came to the stage, and they have come to dominate as the most diverse group of land plants on Earth.Angiosperms are seed-producing plants like the gymnosperms and can be distinguished from the gymnosperms by a series of synapomorphies (derived characteristics). These characteristics include flowers, endosperm within the seeds, and the production of fruits that contain the seeds. Angiosperms can to dominate conifers as the dominant trees only around 60–100 million years ago. And still to this day gymnosperms are the sentinels of the high latitudes (especially in the north).

Land Plant Adaptations

Once plants developed the adaptations to be able to grown on land, their diversification (well….flowered). They had far greater access to two key components of photosynthesis highly limited in an aquatic environment: light and carbon dioxide. The two most important adaptations that allowed plants to become land bound were tissues that prevent the loss of water from within the plant, and tissue that acted a lot like plumbing to get water from the ground to the top of the plant.

Land Plant Adaptations: Cuticle

Perhaps the first adaptation was the cuticle. If you look at plants that live in water, they do not have a cuticle. They don’t need it. They are continually submersed in water. The cuticle is a waxy, watertight layer that reduces water loss. And just like any good idea, it has a downside. It inhibits the exchange of gas. And plants need gas (specifically CO2) to go through photosynthesis. So plants that developed cuticles also had to develop a way to get CO2 into their leaves. 

Land Plant Adaptations: Stomata

And that adaptation was stomata. These are pores in the leaf that promote gas exchange. Just like pores in a filter allow coffee to flow through it, yet retain the grounds to be disposed of. These pores are created by specialized cells that look a lot like lips. When the plant needs CO2 the lips open. However, when they open they also release water. So to conserve water these guard cells can also close, preventing water loss (but also not letting gas in). So, these cells are constantly opening and closing controlling the flow of gas and water.

 Land Plant Adaptations: Vascular Tissue

The major advantage of growing upright is getting a competitive advantage in the battle for light. Early on plants were really close to the ground (think of mosses), because as plants got taller the upper part of the plant would dry out. Therefore, transporting water against the force of gravity was essential in the development of upright growth. Vascular tissue allowed plants to break the force of gravity by drawing water up from the roots sort of like a straw in a soda. The biggest disadvantage of this is that these plants become floppy due to a lack of rigidity. However, they did overcome this (but more on that later).

Evolution of Vascularity

It is thought that vascularity evolved in a series of steps. The first of these steps was developing vascular tissues that were structurally rigid. In other words, they had to be strong enough to withstand the pressure of pumping water. These vascular tissues developed cell walls made of a scaffold (or a framework) of a very strong substance, lignin. Lignin formed structural support that allowed stems that were rigid enough to withstand the effects of gravity. 

It is thought that the second step in vascularity was the development of the tracheid. Tracheids are long, thin tapering cells. They have two cell walls one made of cellulose (which all plant cells have) and another made of the super strong lignin. These tracheids also have pits in the end that act sort of like a sieve. These cells allowed water to be efficiently moved upward. These cells are currently found in all vascular plants. 

If tracheids were a tremendous improvement in design over the first vascular tissues, vessel elements are the Ferraris of the vascular world. They are shorter and wider than tracheids and have large gaps on both ends forming a VERY efficient pipeline for the plant to move water around. Unlike tracheids, these vessel elements line end to end, in effect, making a pipe. Tracheids on the other hand, are close to each other but don’t line up end on end. So they are not nearly as efficient as the vessel elements. These structures are found in all flowering plants (and the gnetophytes).

Vascular tissues

Vascular tissues are made of two different tissues: xylem and phloem. Xylem carries water up from the roots to the leaves and phloem carries sugars down from the leaves to the roots. Just remember: xylem up, phloem down. 

Reproducing in Dry Environments

When plants came onto land, they had to develop a way to protect their embryos from drying out. Spores were the first on the stage. They hold the embryo of seedless land plants and resist drying out due to a tough outer coat. Spores are currently found in mosses and ferns. These gametes eventually evolutionarily developed into seeds. Seeds hold gametes in complex, multicellular structures in which the embryos are retained and nourish by the parent plants.

Seed Dispersal

Once on land, plants quickly diversified many different forms of seed dispersal. The reason for this strong selection pressure is that it is evolutionarily disadvantageous for offspring of a plant to be directly under it. In order to avoid competition for light, water and nutrients, plants evolved several mechanisms for getting their offspring away from their mother plant. Seeds can be dispersed by wind in parachute like structures or helicopter-like structures. They can be dispersed by animals, either by sticking to fur or by enticing animals to eat them. Seeds can be dispersed by water, by floating. They can burst from their seed pods (shooting seeds away from the mother plant). And humans have also been an important dispersal mechanism when they began to cultivate plants.

Parts of a flower

One of the most important recent developments that help to diversify plants was pollination associations. This symbiotic relationship gives the animals very high-energy nectar and/or pollen for food. Of the strangest associations, some plants trick flies into pollinating them because they look like and smell like rotting meat. Hummingbirds can see in orange and red really well. Flowers have taken advantage of this by producing very red and tubular flowers with copious nectar. Bees see from blue to purple to ultra-violet and attracted to that color. Flowers that attract bees also need a landing pad because bees don’t have the power to hover. So they need a place to land. 

Monocots vs. Dicots

Flowering plants are broken up into two main groups: monocots and dicots. A cotyledon is the first leaf that comes out after a seed germinates. Monocots are called that because they have one cotyledon. Dicots (or Dicotyledons) have two cotyledons when they germinate. However, monocots and dicots have many other differences. Monocots have their vascular tissue scattered throughout their stem. In contrast, dicots have a circular vascular tissue arrangement, forming a ring of vascularity. Also, monocots have parallel veins. Dicots typically have a branching vein network. Maybe the most telling difference between the monocots and the dicots is the floral structure. Monocots almost always have floral parts (such as petals) in parts of threes. Whereas dicots typically have petals in multiples of 4s or 5s.