|Plant Physiology (Biology 327) - Dr. Stephen G. Saupe; College of St. Benedict/ St. John's University; Biology Department; Collegeville, MN 56321; (320) 363 - 2782; (320) 363 - 3202, fax; firstname.lastname@example.org|
The Plant Way of Life or, On Being A Plant
(or Plants are smarter than you think!)
I. What is a plant?
Recall - by most definitions, a plant:
- is multicellular;
- is non-motile
- has eukaryotic cells
- has cell walls composed of cellulose
- is a photosynthetic autotrophic; and
- exhibits alternation of generations - has a distinctive diploid (sporophyte) and haploid (gametophyte) phase.
Examples include the angiosperms (flowering plants), gymnosperms
(cone-bearing plants), ferns, and bryophytes (mosses & liverworts). Recent
classification systems suggest that these organisms, in addition to the red algae and
green algae, should be classified in the Plant Kingdom (Plantae).
II. What is the single most important characteristic that distinguishes plants from
Autotrophism! Yup, that's my guess, too. We should recognize that a systematist (someone who studies classification systems) familiar with the most recent notions of classification might disagree since members of a "new" kingdom, Chromista, are also photosynthetic autotrophs. Nevertheless, both of these groups are closely related so we can still safely agree that autotrophism is important to the plant way of life.
A. Take-Home-Lesson 1: An autotroph makes its own food (energy-rich organic compounds) from simple, inorganic materials in the environment. Plants use light as their energy source, hence they are photosynthetic (vs. chemo-synthetic for certain bacteria). The general equation for photosynthesis is:
CO2 + H2O + light → (CH2O)n +O2
In contrast, animals are heterotrophic, meaning that they must obtain their food (pre-fabricated organic compounds) from the environment. They cannot manufacture their own food. Examples of heterotrophs include mycotrophs (plants that obtain their nutrient source from a fungus like Indian pipes (Monotropa), decomposers (fungi, bacteria), carnivores, and herbivores. Some parasitic plants (holoparasites like dodder (Cuscuta) and dwarf mistletoe that lack chlorophyll are obligate heterotrophs that can only obtain their nutrients from another plant. Others parasites, like mistletoe (Phoradendron) and Indian paintbrush are green and can make their own organic compounds but obtain water and minerals from a host plant (Hershey). Finally, some plants, like the carnivorous species, feed both autotrophically and heterotrophically.
B. Take-Home-Lesson 2: The autotrophic mode of nutrition evolved early in the evolution of life, ca. 3 billion years ago. This event set in motion the evolutionary events that culminated in modern plants. Therefore, modern plant characteristics can be explained as a direct or indirect consequence of the autotrophic mode of nutrition. Plants colonized land about 440 million years ago. The transition from water to land required the evolution of (in approximate sequence): cuticle (to resist drying out), stomata (gas exchange), and vascular tissue (for water/nutrient transport).
III. Consequences of autotrophic nutrition
Plants required specialized structures adapted for the autotrophic mode of nutrition. Specialization occurs at all levels of biological organization (e.g., organ, tissue, cell, organelle). Specific problems, and their solutions, related to autotrophic nutrition are:
Problem: Photosynthesis is a complicated biochemical process.
Problem: Photosynthesis requires efficient light harvesting.
Leaves are perfect solar collectors. These organs are broad and flat to allow for efficient light harvest. The leaves are broad to maximize surface area for light harvest and they are thin since light cannot penetrate too deeply into the leaf (the amount of light decreases exponentially with distance). As an aside, although the majority of light is absorbed near the leaf surface, in some situations plant tissues act like fiber optic cables that can funnel some light deeply into the plant body (Briggs et al). The window plant in the Namib desert funnels light through translucent cell into the photosynthetic tissue that is buried in the soil (Attenborough)
Even within the thin leaf, most chloroplasts are found in the upper layer of cells, the palisade layer, which is the tissue layer just beneath the upper epidermis. This makes "sense" since these cells will be receiving the greatest amount of light of any region in the leaf. Thus, this is an example of specialization at the tissue level.
Problem: Photosynthesis requires an apparatus for gas exchange.
Leaves double as a means to exchange photosynthetic gases (take up carbon dioxide and get rid of oxygen) with the environment. Leaves have pores in the surface (stomata) that regulate the entry/exit of gases and prevent the loss of excessive water.
The spongy layer of the leaf acts like a "lung" increasing the internal surface area and provides for more rapid diffusion within the leaf. Note again that leaves are thin - this avoids the need for lungs or other type of pump to move gases. Since diffusion rates are inversely related to distance, diffusion can account for gas movements into/out of a leaf. As a consequence, no cell if more than 2 or 3 cells from the air. An added advantage of having large leaves for light harvest is that they provide lots of surface area for absorption of carbon dioxide.
Again, note the specialization of the leaf at the organ, tissue, and cellular levels for gas exchange.
Problem: Thin leaves, required for light absorption and gas exchange, need
This problem was solved by the evolution of the cell wall which provided for the support of thin structures without the need (or potential) for significant numbers of internal support structures. Leaves also have some internal "struts" (in other words, veins).
Problem: Photosynthesis requires a water supply.
With the exception of the algae and aquatic plants, plants obtain their water through the roots from soil. Essentially the roots "mine" the soil for water. Thus, photosynthesis and the transition to a terrestrial environment necessitated the evolution of a root system to obtain water (specialization at the organ level). And, it required the evolution of specialized transport tissue (xylem) to move the water from the roots to the leaves.
Problem: Photosynthesis requires a mechanism to transport end products throughout the
Once carbohydrate is produced during photosynthesis there must be a mechanism to transport it to other locations throughout the plant. The evolution of vascular tissue, specifically phloem, permitted movement of photosynthate from leaves to roots, fruits and other tissues where required.
IV. Consequences of autotrophic nutrition - Motility is no longer required; Or
One of the main reasons for motility is to obtain food. Since the nutrients required by plants are "omnipotent" there was never an evolutionary pressure for "motility." Lets quickly compare the nutrients used by plants and animals:
|Table 1: Comparison of Plant & Animal Nutrition|
|form of uptake||inorganic (CO2, water, ions)||organic (proteins, carbohydrates, fats)|
|concentration||dilute (i.e., CO2 = 0.03%)||concentrated|
: plants must be adapted for harvesting dilute nutrients that occur everywhere, whereas animals are adapted for searching out and trapping widely dispersed, concentrated packets of food.
Supportive Evidence: if this is true, then we hypothesize that animals with a nutrient source like a plant should have similar features to a plant. Check out corals, sea fans, and hydra. These are all non-motile animals that occur in aquatic environments which enables them to "feed like a plant" - food is essentially brought to them via water currents. Thus, they never had any pressure for motility and they have very similar lifestyles/forms as plants.
In addition, note that motility is really not possible for terrestrial plants. Once plants evolved roots it precluded movement. These evolutionary "choices" are closely connected.
However, being stationary has its own problems/consequences.
V. Consequences of a Stationary Lifestyle - The need to exploit a limited volume
of the environment for resources.
The problem: a fixed (stationary) organism must be able to continually obtain nutrients without using them up. Plants face the additional problem that their nutrients are "dilute." Thus, plants must be designed for collecting dilute nutrients in the environment. Plants have several solutions to this "problem":
A. Plants are dendritic.
In other words, the basic shape of the plant body is dendritic - which means "tree-like" or "filamentous". The advantage of this shape is that it provides a large surface-to-volume (s/v) ratio which enables a plant to exploit a large area of the environment. In contrast, animals are more compact (spherical) to minimize their s/v ratio. Among other things, this is an advantage for motility. Surface-to-volume ratios are very important in many areas of biology. In class we will investigate surface/volume ratios in more detail.
B. Plants have indeterminate growth.
Process by which a plant continues to grow and get larger throughout its life cycle. The advantage of this is that it allows the plant, especially roots, to grow into new areas. In contrast, determinate growth is where an organism or part reaches a certain size and then stops growing. This is characteristic of animals and some plant parts (e.g., leaves, fruits).
C. Plants have an architectural design.
In other words, the plant body is constructed like a building - modular (Silverton & Gordon). It is built of a limited number of units, each of which is relatively independent of the others and that are united into a single structure. Thus, just like a building is made of rooms, the leaves, stems and roots of a plant are analogous to a rooms in the building. Each room is somewhat independent, yet they all function together to make an integrated whole. You can seal off a room in a building, or remove a leaf or fruit, with little harm to the overall integrity of the structure. This is critical for plants to be able to add or remove parts (leaves, stems, flowers, fruits) as necessary. One conclusion is that because of their indeterminate growth and architectural design, plants are not limited by size. This gives plants the ability to colonize and exploit new areas for resources.
In contrast, an animal has a mechanical design. In other words, animals are built more like a machine, made of numerous, different parts that function together. The parts are highly integrated. Parts cannot be added or removed without reducing the efficiency of the operation of the whole. Animals are limited by size.
As a consequence, plants are not a static shape - plants constantly change shape by adding/loosing parts - by accumulating modular units. Animals dont change shape - they remain the same general shape throughout their life. Thus, growth in plants occurs by the addition of new "units" not enlargement.
D. Plants have a well developed ability to reproduce asexually.
This can be viewed as a quick and energetically inexpensive way to expand the influence of the parent into a new location. One testable prediction from this hypothesis is that plants under nutrient stress should increase their rate of asexual reproduction (see foraging data below).
E. Plants (may) exhibit heterophylly.
Heterophylly refers to leaves with different shapes. For example, the aerial leaves of aquatic plants are entire but the submerged leaves are dissected. Sun leaves tend to be smaller and thicker than shade leaves. Dandelions are toothier when grown in a carbon dioxide enriched (700 vs. 350 ppm) environment. The leaves of the vine Monstera are more or less heart-shaped and pressed to the trunk as the vine climbs into the tropical forest canopy. Once in the canopy, the leaves take on the mature form with slits and holes.
F. Leaves are arranged to minimize overlapping.
Phyllotaxy is the fancy term for leaf arrangement. Interestingly, phyllotaxy patterns have always been shown to be related to Fibonacci number series (1, 1, 2, 3, 5, 8, 13, 21....etc). For more on phyllaxy, visit this web site.
G. Plants can forage.
The growth patterns of plants, especially vines and plants with stolons (runners), are similar to the foraging tactics of animals. As an example, rhizomes of Hydrocotyle veer from patches of grass to avoid competition. A brief overview of the anatomy of a clonal plant like Glechoma hederacea (ground ivy): parent plant, stolon (internode), ramet (individual of a clone).
For a Case Study on Foraging, click here.
Thus, plant growth is essentially analogous to animal behavior. One of the first to express this idea was Arber (1950; The Natural Philosophy of Plant Form. Cambridge). She said, "Among plants, form may be held to include something corresponding to behavior in the zoological field...for most though but not for all plants the only available forms of action are either growth, or ascending of parts, both of which involve a change in the size and form of the organism."
VI. Consequences of a Stationary Lifestyle - Positioning in the environment.
Problem: a non-motile organism is unable to move to a more favorable location to carry out its vital functions. Thus, plants have at least three major problems to contend with:
A. Environmental Positioning/Location - or, Getting Started in the Right
Obviously a motile organism can move to a favorable location, but a plant is stuck once the seed germinates. For most plants getting started in the right place is a matter of luck. Thus, it is no surprise that plants exhibit a Type III survivorship curve (produce lots of offspring, few survive, no parental care of offspring - think oak tree and acorns). However, there are a few "tricks" that plants use to help increase the odds that seeds will germinate in a favorable environment:
Specialized Dispersal Mechanisms - some plants have specialized mechanisms for dispersal that will increase the odds of the seed getting into the proper place. For example, mistletoes have sticky seeds that often stick to the beak of a hungry bird. The bird will try to rub it off on a branch where the seed will adhere and germinate.
B. Axis orientation.
Once a seed germinates in a favorable environment it must determine which way is up/down to insure that the roots grow down and shoots up. Thus, gravitropism is a very important physiological response characteristic of all plants.
C. Fine Tuning.
Even non-motile organisms need to "fine-tune" their position in the environment. Thus plants have a variety of mechanisms that enable them to optimize their position in the environment including:
VII. Consequences of a Stationary Lifestyle - the need to sense & respond to the
Problem: plants, like other organisms, must be able to respond to changes in their environment. Plants respond to their environment in a variety of ways.
VIII. Consequences of a Stationary Lifestyle - need to protect themselves from physical and biological dangers in the environment.
Problem: a non-motile organism cannot flee when conditions get tough. It must "fight" it out. Both the physical environment and biological environment threatens the well being of plants.
A. Physical dangers - wind, water (flood), drought, cold (winter) are among the physical dangers that a plant faces. In general, plants cope with these (at least the predictable ones like winter and drought during summer) by dormancy, senescence, and even death. The evergreen and deciduous lifestyle are in part a response to adverse conditions. Evergreens are much better able to tolerate cold, dry conditions. They also do better in poor soil because they don't loose as many leaves. Plants also respond to enviromental challenges morphologically - for example, xeric plants reduce their S/V to minimize water loss. Arctic or montane herbs are small and hug the ground.
B. Biological dangers - predators (=herbivores) and competitors (=other plants). Plants have evolved:
- Anatomical weapons (thorns, hairs, thick cuticle). Some plants are even "smart" enough to stop producing defenses when they are out of range of a herbivore. For example, the upper parts of Acacia trees above giraffe height produce few thorns. Similarly, holly leaves have few prickles above herbivore (e.g., deer) height;
- Chemical weapons - produce toxic, unpalatable chemicals. These can be inducible (produced in response to attack) or constitutive (always present) (Karban & Myers. 1989. Ann Rev Systemat. Ecol 20:331); Allelopathy is chemical warfare between plants. Phytoalexins are chemicals produced by plants to resist microbial infection.
- Mimicry - "tricking" predators. For example, lithops in S. African deserts look like pebbles - are stone mimics. NZ mistletoe leaves look just like the host tree leaves to avoid being eaten which is important because they contain even higher amounts of nitrogen. Alseuosmia is a non-toxic New Zealand plant that looks very similar to Wintera pseudocolorata (a toxic species). When young, NZ lancewood looks very unappetizing - much like a woody umbrella that is folded up. However, when it gets about 15 feet tall, above predator (e.g., moa) height, it branches out and has a more "traditional" appearance.
- Armed Forces - some plants, like wild tobacco, when attacked by herbivores release volatile chemicals that summon predatory insects to the damaged plants. These insects in turn, kill the herbivores.
Overall, volatile chemicals play a very important role in plant defense. These chemicals, released when the plant is attacked by an herbivore serve as signals that: (a) warn other plants that danger is imminent. For example, tobacco eaten by herbivores produces salicylate (similar to aspirin) that stimulates its own defense response and is converted into methylsalicyate which is volatile. This compound travels to other plants to induce their defense response; (b) tell other herbivores that it is being attacked and that they should look for another food source unless they want to battle it out (compete) with another herbivore; (c) alert predator insects that their are tasty herbivores in the area (see #4); (d) tell herbivores that the chemical defense system of the plant is ready for them and that should go pick on someone else; and (e) the volatiles themselves help to repel the attack.
Plants are more sensitive to being handling than perhaps we give them credit. Recent studies have shown that simply stroking the leaf of a plant just once will affect herbivory - some plants suffer greater damage while others less damage (Sci News 159: 119, 2001).
IX. Consequences of a Stationary Lifestyle -
Problem: A non-motile organism cannot seek a mate (for gamete transfer) or easily disperse offspring. Plants solve the gamete transfer problem by relying on various pollination vectors. Fruits/seed dispersal mechanisms help disperse offspring. Check out the Private Lives of Plants video, Volume 1 (dispersal) and Volume 3 (pollination) for some great examples. These document some absolutely fascinating stories about plants and their pollination and dispersal vectors. To add a recent story, the fragrance of a particular orchid changes after pollination. Prior to pollination the orchid released a fragrance that was an "aphrodisiac" for the male but once the flower was pollinated it produced a fragrance to repel him. And, when a few fruits of Hamelia patens, a neo-tropical tree, are removed it stimulates the rest of the fruit to ripen - it essentially tells them that a dispersal agent is in the area.
X. Additional Consequences: The consequences of the cell wall
Problem: As we mentioned, plants evolved cell walls: (a) as a means of support; (b) to prevent the protoplast from bursting in a hypotonic medium; and (c) some speculate that walls may even be a way to dispose of excess carbon. In any event, by surrounding their cells with a rigid box, this imposed certain limitations. These include:
XI. Plants are Smarter
than you Think!
For many years, I have joked that "plants are smarter than you think." Hopefully, our discussion of the plant way of life has led you to agree. Although my comment was somewhat tongue-in-cheek, there have been recent discussions about the intelligence of plants. Historically, plants have not been considered to be "intelligent" because this concept has been associated with movement. Thus, animals are smart, plants are not. But, if you agree with Anthony Trewavas (2002) who first argued in Nature that if "intelligence is defined as adaptively variable behaviour during the life of the individual, then, in plants, behavioural plasticity is where intelligence should be apparent." The examples cited in this document and lots of others from Trewavas, convince me that Trewavas is correct - plants are intelligent. For more reading, Trewavas has expanded his Nature article into a longer review which is excellent and available on-line. David Hershey agrees, and has also addressed this issue in an eloquent essay, "Plants are indeed, intelligent," that was submitted to the American Biology Teacher. Both of these articles contain lots of examples why plants are so "cool."
One caveat, don't associate the
concept of intelligence with the notion of intelligent design, which is an idea
that suggests God must have created the earth and its inhabitants because
everything is too perfect to have happened via the somewhat random or chancy
process of evolution.
XII. Plant Blindness/Animal
Chauvinism/Plant Neglect/Plant Prejudice
Hopefully, I've made a good case for why it's important to study plants and why are they are "cool." Which brings up a question - why don't more people like or want to study plants? Why is plant physiology normally a small class? Evidence shows that even grade school kids prefer zoological topics to botanical ones. But why?
Wandersee and Schussler (1999)
argue that the main problem is that by human nature we are "blind" to plants due
to limitations in our visual perception. Others, like Hoekstra (2000) and Hershey (2002)
argue that a more critical concern is that plants are neglected in the
curriculum and that botanical concepts are often either not taught, or not
presented well. Whatever the reason, this semester we are going to
be Plant Chauvinists practicing Animal Neglect, seeing clearly the beauty and
excitement in the study of plant physiology.
Test Your Understanding:
01/29/2009 © Copyright by SG