Geography 140
Introduction to Physical Geography

Lecture: Classifying Plant Life Structurally

III. Structural classification.  Plants enjoy a structural classification 
     system, in addition to the genetic taxonomies above.  It is called the 
     life form classification.
     A. It has to do with the appearance or structure of an individual plant 
        (and nothing to do with its ancestry per se).  The reason for 
        using a structural classification is that many biogeographers are 
        interested in how plants affect the appearance of a particular 
        landscape, giving a certain character to an area or region.  Mastering 
        the taxonomy of all the species in the area (not to mention dealing 
        with the controversies in taxonomy) is not going to give you that 
        sense as well as knowing the vegetation structurally. 
     B. I'll just run through the categories in a list here:
        1. Vascular plants are plants that have an internal system of tubular 
           cells or vessel cells for conducting water and dissolved minerals 
           around within the plants.  This internal system has two basic 
           parts: Xylem (tubular cells or vessel cells, which are themselves 
           dead) and phloem (or sieve cells for moving food around, which are 
           fully alive).  Because of this internal division of labor, the 
           vascular model allows for the growth of really large plants (e.g., 
           those giant sequoias in the Sierra).  Non-vascular plants are 
           limited to small size.  So, vascular plants tend to dominate most 
           landscapes visually.
           a. Tree (awesome scientific word, hard to pronounce <G>):
                i. Trees are woody vascular plants.
               ii. They generally have a single main trunk, with branching 
                   away from this main trunk.
              iii. Examples:
                   a. Pines 

                      

                   b. Oaks 

                      

                   c. Sycamores (which often branch off pretty low to the 
                      ground) 
                   d. Palms (the tree definition maxxed out!)
           b. Shrub:
                i. Shrubs are also woody vascular plants.
               ii. Unlike trees, however, they have several major stems 
                   branching out from the ground or very close to it.  
              iii. Most shrubs are smaller than most trees.
               iv. Examples:
                   a. Rose 
                   b. Manzanita (common on the hills of California)

                      

                   c. Gardenia
                   d. Hibiscus
                   e. Chamise (another common California hillside native)

                      

           c. Liana:
                i. Lianas are also woody vascular plants.
               ii. They have a climbing habit, seeking support from trees, 
                   rocks, hillsides, fences, what have you.
              iii. Examples:
                   a. Poison Oak (leaflets three, let it be!)

                   b. Star Jasmine (often used as an aromatic ground cover in 
                      landscaping)
                   c. Lilac
           d. Herb:
                i. Herbs are tender vascular plants, with little to no wood 
                   development. 
               ii. Without the support of a woody internal framework, they are 
                   small in size.
              iii. There are some sub-types: 
                   a. Forbs are broadleafed forbs, such as dandelions, pothos, 
                      philodendron (and a lot of common indoor house plants)
                   b. Graminoids, a fancy way of describing grasses, which are 
                      also herbs: linear-bladed, the lower part of their 
                      leaves wrapping around the stem.  Examples include 
                      wheat, barley, corn, rye, oats, fescues, and other 
                      common lawn grasses.
                   c. Ferns or pteridophytes are herbs. Their leaves share a 
                      lot of characteristics with branches and are called 
                      fronds.  They reproduce with spores, not seeds, and do 
                      not flower.  Spores have only half the genetic material 
                      that a seed would (kind of like sperm and eggs in 
                      humans). What spore reproduction means is that the fern 
                      plant we're all familiar with generates a fine dust of 
                      spores.  In the right conditions, the spore develops 
                      into a tiny plant-like entity (0.6 cm) called a 
                      prothallium that generates eggs and sperm.  This 
                      prothallium is the sexually-reproducing phase of the 
                      fern life cycle.  The eggs and sperm find one another in 
                      water and their union produces the fern plant we know, 
                      with a complete set of genes, which gets their spores 
                      out and about.
                   d. Club mosses or microphyllophytes (micro-dinky leaf 
                      plants, rendered loosely) are very small plants a few 
                      centimeters high, though they have horizontal stems that 
                      may sprawl out something like 15 meters.  They look 
                      almost like miniature pine trees, complete with scaly 
                      leaves, stems, and "cones" (filled with spores not 
                      seeds).

                      

                   e. Horsetails or equisetophytes have jointed stems that 
                      makes them look a little like miniature bamboo. There 
                      are small scaly leaves that grow in whorls around the 
                      stem at the joints. Like the club mosses, the horsetails 
                      have spore-producing cones at the tops of their stems. 

                      

        2. Byrophytes are non-vascular and, thus, are unable to pull water and 
           nutrients up from the ground for any significant distance. It is 
           for this reason that they are considered to be rather primitive 
           plants, transitional from aquatic plants (such as kelp) and land 
           plants (such as trees).  Given their lack of a vascular system, 
           they are really dependent on a very moist environment.  Like the 
           ferns, club mosses, and horsetails (which are vascular), they 
           reproduce by spores, alternating generations the way I described 
           for ferns.
           a. Given their lack of a vascular system, bryophytes have to be 
              pretty small, a few centimeters, maybe up to a meter at most, 
              and so they live close to the ground or on tree trunks.
           b. Like the vascular plants, however, they do have the capacity to 
              photosynthesize (they are autotrophs).
           c. They can be subdivided:
                i. True mosses (including peat moss or sphagnum, much loved by 
                   gardeners for keeping moisture in the soil), with 
                   structures that resemble leaves, stems, and roots but no 
                   vascular tissue to conduct materials around within the 
                   plant.  So, like all bryophytes, they are pretty small, 
                   ranging from microsopic in size to maybe one meter tall. 
                   Interesting geotrivia:  Mosses are very old land plants 
                   (going back about 280,000,000 years) and were the first 
                   green plants to stand erect on the land.

                   

               ii. Liverworts or hepatophyta are weird plants that sort of 
                   look like leaves or chains of leaves on the ground.  They 
                   have no roots or stems, just these leaf-like structures.  
                   Photosynthesis goes on at the top surface of the plant, 
                   while storage of food goes on directly below.

                  

              iii. Hornworts or anthocerotophytes resemble liverworts but they 
                   have irregular and misshapen greasy-looking blue-green 
                   "leaves."  From this structure rises a long, slender stalk 
                   within which the spores are developing:  This structure is 
                   the "horn" in "hornwort.  It can get up to half a meter in 
                   height.

                  

        3. Epiphytes may be either vascular or non-vascular, so I couldn't put 
           them in the tidy classes above.  What differentiates them is that 
           they live on top of other plants without soil contact for at least 
           part of their lives.  Very commonly, you'll see them on tree 
           branches, especially in tropical rainforests, where small plants 
           would not get enough sunshine on the forest floor to 
           photosynthesize.  So, small plants take up life on top of other 
           plants to get access to the sunshine they need.  
           a. Most epiphytes seek only mechanical support from other plants to 
              have access to sunshine.  They may extract water from the pools 
              of water that may form in their own leaves or perch sites or 
              from moist litter that accumulates there.  Other epiphytes are 
              parasitical, sinking their roots into the vascular tissue of 
              their hosts to extract water and nutrients brought up by their 
              hosts.

             

           b. Epiphytes can be subdivided by just how long they live up on top 
              of another plant:
                i. Full epiphytes spend their entire lives as arboreal 
                   species, never once coming in contact with the ground.
               ii. Hemi-epiphytes spend only part of their lives without soil 
                   contact in a completely arboreal stage.  There are two 
                   variants on this:
                   a. A primary hemi-epiphyte starts life as an epiphyte until 
                      it can get aerial roots down from the treetops to the 
                      ground where they become rooted. The resulting rooted 
                      plant still leans on the plants they started out on and 
                      now sort of resemble lianas.  Note the Tarzanesque 
                      appearance of this forest because of these aerial roots!

                      

                   b. A secondary hemi-epiphyte starts life on the ground and 
                      then climbs up on a nearby tree like a liana but, unlike 
                      a liana, once part of it reaches a suitable perch, it 
                      abandons its soil-touching roots and spends the rest of 
                      its life as an epiphyte.
           c. Some examples:
                i. Bromeliads have many epiphytic species.
               ii. Orchids also have many epiphytic species.
              iii. Mistletoe is a parasitical epiphyte that grows on oaks and 
                   other trees.
               iv. Strangler fig is a primary hemi-epiphyte that starts out as 
                   an epiphyte on a tropical rainforest tree and then sends 
                   down aerial roots in such profusion that they strangle the 
                   host tree.  The tree dies and slowly rots out, leaving 
                   behind the huge, hollow cylinder formed by all those 
                   tangled fig roots, and the strangler fig is left for all 
                   intents and purposes a tree of sorts.
                v. Dodder weed in California is an obligate parasite on 
                   several chaparral brush species.  Though related to morning 
                   glory, it is incapable of photosynthesis, so it looks like 
                   this bright orange netting!  It has to sink modified roots 
                   into the stems of its host to extract water and food from 
                   it.  

                  

        4. Fungi are no longer, strictly speaking, classified as plants!  They 
           are incapable of photosynthesis and do not descend from a 
           photosynthetic ancestor. In fact, genetic evidence suggests that 
           they are, in fact, more closely related to animals than to plants, 
           probably sharing a common protist ancestor with animals!  
           Subjectively, we tend to think of them as plants because they are 
           immobile and because they have structures that vaguely resemble 
           stems and roots, sort of, and they do reproduce with spores.  
           a. Fungi, incapable of making their own food like plants and unable 
              to move and hunt for food like animals, make their livings as 
              saprophytes (they are very important detritus feeders), 
              parasites (think: athlete's foot, jock itch, yeast infections, 
              and ringworm, all commonly caused by fungi), and symbionts (more 
              on that in a minute).
           b. Examples:
                i. Mushrooms and toadstools (toadstools are poisonous 
                   mushrooms), morels and truffles
               ii. Yeasts, including those used to make bread rise 
              iii. Powdery mildew
               iv. Molds, including bread mold
                v. An assortment of blights, rusts, and diseases like the ones 
                   I mentioned above.
        5. Algae (except blue-green algae, which are also called 
           cyanobacteria) are very simple eukaryotic organisms that can 
           perform photosynthesis with chlorophyll.  Because of this, they are 
           often lumped in with plants and gave rise to plants.  They are a 
           very ancient lineage, going back about three billion years.
           a. Most are one-celled organisms, such as that slime that forms in 
              toilets if you're not into the Suzy Homemaker routine (science 
              comes alive in the privacy of your dwelling unit -- isn't Nature 
              beautiful!?).
           b. Sometimes they group together in colonies, such as algal mats.

              

           c. Some versions can group together to form organisms with simple 
              tissues, such as kelp (seaweed, which can get up to 65 m in 
              length!).  Here's an image of an underwater "forest" off the 
              coast of Alaska, made up of kelp -- isn't it gorgeous?  Looks 
              just like a forest on land, doesn't it?  

              

        7. Blue-green algae (aka cyanobacteria) and bacteria are prokaryotic 
           one-celled creatures, meaning their DNA is not housed in a nucleus.  
           Blue-green "algae" is now seen as a misnomer but, interestingly 
           enough, cyanobacteria are relevant to regular algae and plants:  
           The chloroplasts inside plants and ordinary algae are basically 
           endosymbiotic cyanobacteria.  This means that the eukaryotic cells 
           that make up algae and plants have internal structures and 
           organelles, one of which is the chloroplast.  The chloroplast so 
           strongly resembles existing cyanobacteria that we think an ancient 
           symbiosis was struck up between a cyanobacterium and some 
           eukaryotic cell, wherein the cyanobacterium photosynthesized food 
           for both cells in exchange for a safe shelter from the harsh 
           outside world, complete with a flow of nutrients needed for 
           photosynthesis, and this union became the template for regular 
           eukaryotic algae and plants.
        8. Lichens are that crusty or fuzzy stuff you find growing on rocks 
           and tree trunks.
           a. They are tiny compound creatures, representing the union of a 
              fungus with an alga and/or a cyanobacterium (sometimes all 
              three).
           b. This is about the perfect symbiosis:  A total win-win situation 
              for everyone involved:
                i. The alga and/or cyanobacterium provides food for everyone 
                   through photosynthesis, allowing the fungus to live in 
                   harsh, sterile environments (e.g., rocks) that it could not 
                   colonize before for lack of food (most fungi are parasites 
                   or detritivores and rocks can't play hosts and there's 
                   little dead organic matter to be had on them).
               ii. The fungus provides protection from drying out and is able 
                   to draw water and minerals into the compound, allowing the 
                   alga or cyanobacterium to live in dry habitats normally off 
                   limits. 
              iii. Lichenologist Trevor Goward put it this way: "Lichens are 
                   fungi that have discovered agriculture," which is a rather 
                   cool analogy.
           c. This symbiosis is so close that it models the far more ancient 
              symbioses between mitochondria and chloroplasts with some other 
              ancient bacterium to form the eukaryotic cell.
                i. Eukaryotic cells have this internal division of labor among 
                   organelles that strongly resemble existing species of 
                   bacteria and cyanobacteria.
               ii. This internal division of labor was later extended to 
                   coördinating an external division of labor among 
                   different cell types and tissues, which is the basis for 
                   all complex multicellular organisms today, including human 
                   beings. 
              iii. So, have some respect for that obscure crusty stuff on 
                   rocks, as it helps us understand the process by which the 
                   eukaryotic cell evolved, on which we're based!

     C. These basic life forms are often refined for particular purposes with 
        the following considerations:
        1. Size, relative to each life form category, of course, e.g.
           a. In trees: tall  might be >25 m; medium might be 8-25m; 
              while small might be <8 m.
           b. For grasses, it would be more appropriate to define tall as 
              > 1 m,  medium as 15 cm-1 m, and small as <15 cm.
        2. Coverage, that is, the percentage of ground covered by a given life 
           form and its pattern of coverage.
           a. You could describe a woodland as having 25% tree coverage, 25% 
              shrub coverage, 65% grass coverage, and 50% forb coverage.  Yes, 
              the percentages add up to more than 100% because the forb 
              coverage, for example, might well be below the tree coverage, 
              double-counting the ground cover.
           b. Pattern of coverage refers to its spatial distribution.
                i. Barren is the pattern that means members of a given life 
                   form are not present at all in a given vegetation (so 
                   ground coverage would be 0%).  Examples might be the tree 
                   layer in a short-grass prairie or in a desert or in a 
                   tundra or in chaparral. 
               ii. Discontinuous means that a given life form IS present in a 
                   vegetation, but it is rather sparse and open, with ground 
                   coverage below, say, 60%.  More than that, the spatial 
                   distribution of the individual plants is random or even 
                   close to uniform.  We're talking a thin but even 
                   representation of that life form.  
                   a. An excellent example is the shrub layer in a desert:  
                      They are found there, maybe with ground coverage of, oh, 
                      10-15%, but far apart from one another (several meters).  
                   b. When you see a discontinuous pattern in vegetation, this 
                      sparse and random to uniform pattern, you are probably 
                      looking at intense competition for a scarce and 
                      unpredictable resource (water, in the desert).     
                   c. This competition can even be played out with chemical 
                      warfare among the plants, which is called "allelopathy." 
                      Plants can release chemicals that kill or suppress 
                      competitors for water:  Volatile, aromatic compounds, 
                      such as terpenes, are used to pollute the local air 
                      against seedlings in an adult plant's root zone or the 
                      chemicals can be released from fallen leaves or from 
                      their roots.  And here you thought plants just sat there 
                      passively:  They are actually pretty active players in 
                      their environments.
              iii. Grouped or clumped means that members of a given life form 
                   are present in a vegetation, with, say, 10-50% ground 
                   coverage, but they occur in clusters, groups, or clumps.  
                   a. An example would be the oak tree coverage in a 
                      California oak woodland or oak park:  Groups of oak 
                      trees cluster here and there in a gently hilly 
                      landscape, with grasses in between.

                     

                   b. This is a very common pattern, and it reflects the 
                      concentration of plants where a necessary resource tends 
                      to concentrate, such as water in uneven terrain.
               iv. Continuous means that a life form is present in great 
                   density and forms a pretty much even pattern: random or 
                   uniform.  The coverage is very high, over 90%, so that 
                   there are hardly any breaks in the life form's ground 
                   coverage.  
                   a. An example would be the shrub cover in chaparral or the 
                      trees in a tropical rainforest or the grass in tall-
                      grass prairie.
                   b. This pattern and luxuriance indicates that the landscape 
                      provides a pretty even exposure to all resources needed 
                      by the species of a given life form.
        3. Periodicity is sometimes used to modify life form descriptions.  It 
           refers to the duration of active photosynthesis each year.  Are the 
           plants evergreen or are they deciduous?
           a. Plants face a basic cost and benefit dilemma in maintaining the 
              ability to photosynthesize all year round, when the declination 
              of the sun shifts from the northern to the southern hemispheres 
              each year, which affects sun angle, which affects the efficiency 
              of photosynthesis.
                i. Plants "want" to photosynthesize all year round, to feed 
                   themselves.
               ii. The problem is they may face a lean season of inefficient 
                   photosynthess.  This is a real problem, since their photo-
                   synthetic structures (e.g., leaves) transpire water and 
                   burn energy during respiration no matter what season it is.
           b. So, if you're a plant, you have to "decide" whether to dump your 
              photosynthetic structures (e.g., your leaves) during the lean 
              season to avoid drying out or starving.  The problem is that, if 
              you do so, it's going to cost you a lot to rebuild all your 
              leaves at the beginning of the good season.  What to do? You 
              have to "decide" whether the benefit of escaping dehydration and 
              starvation is higher than the cost of rebuilding all your leaves 
              each year.
           c. Some plants are evergreen:  They photosynthesize (or remain 
              capable of photosynthesis) all year round.
                i. Plants can get away with this if they live in the humid 
                   tropics and subtropics, where moisture, temperature, and 
                   sun angle don't vary all that badly over the course of the 
                   year.  In this situation, there is no point to incurring 
                   the cost of dumping your leaves, and anyone who does is 
                   going to be a less effective competitor in a vegetation 
                   filled with other plants efficiently photosynthesizing and 
                   growing all year.  So the humid tropics have almost 
                   exclusively broadleaf evergreen vegetation, and broadleaf 
                   evergreens and some needleleaf evergreens dominate the 
                   humid subtropics.
               ii. Other plants are evergreen, even in climates with rough 
                   seasons, but they have to come up with some way of coping 
                   with the hazards of variations in photosynthetic efficiency 
                   while respiring all year round.
                   a. Many plants have small leaf surfaces as a way of 
                      retarding water loss during the rough season.  The 
                      downside of this is their photosynthesis is also reduced 
                      in efficiency because there's less surface to 
                      photosynthesize with.  This is less of a cost when 
                      everyone else in your climate is also dealing with the 
                      rough season by reducing leaf size.  
                      1. We can see this effect in local oak trees, which have 
                         small leaves maybe up to 4 or 5 cm long; if you've 
                         traveled Down South, you may have noticed the oak 
                         trees there have humongous leaves, like 10-20 cm 
                         long!  
                      2. Conifer trees are the experts in leaf surface area 
                         reduction:  They've adopted the needleleaf form.  
                         They are needleleaf evergreens.
                   b. Leafless evergreens take this photosynthetic-surface 
                      idea to an extreme:  They have no leaves or just some 
                      vestigial leaves and they do the bulk of their 
                      photosynthesis in their stems, which stay green all year 
                      (a local desert example is a plant called Mormon tea, or 
                      Ephedra nevadensis). Note the green stems.

                      

                   c. Then there are succulent evergreens: They can conduct   
                      photosynthesis all year round in leaves and stems, 
                      whenever conditions are right, becaue they are protected 
                      from dehydration by tough, waxy surfaces and water 
                      storage tissues (e.g. jade plants, cacti). Of course, 
                      all that water might attract thirsty herbivores, so many 
                      succulents are fairly bristling with spines and thorns 
                      to discourage that.
           d. Other plants are deciduous:  Leaves are shed seasonally, 
              originally to avoid dehydration (by respiring and transpiring 
              their water out of their leaf surfaces when little water is 
              available in their root zones). 
                i. Dry season deciduousness is the older form of 
                   deciduousness: Leaves are dropped to avoid respiration 
                   water loss and photosynthetic water need during drought. 
                   The deciduous habit probably evolved in tropical wet and 
                   dry climates and was then found adaptive to cold season 
                   conditions, after successful deciduous plants migrated 
                   poleward. 
               ii. Icy winter deciduousness: Plants cannot absorb ice through 
                   roots, so leaves are dropped to avoid dehydration due to 
                   photosynthesis and respiration/transpiration. 
              iii. Low sun angle deciduousness: Even if the ground is not 
                   frozen, low sun angle and short winter days make 
                   photosynthesis too inefficient to support leaf respiration 
                   24/7, so leaves are shed to avoid "starving." 
               iv. Now, California has a rough SUMMER season, but our native 
                   vegetations are dominated by EVERGREEN plants.  This is 
                   because deciduousness originally evolved in a winter 
                   drought climate, and plants are primed to recognize the 
                   onset of winter.  It would make sense to dump your leaves 
                   in the summer here, which is a lot like the winter in the 
                   tropical wet and dry climate, but plants can't reason and 
                   switch their behavior. So, winter deciduous plants are at a 
                   disadvantage here, because they dump their leaves during 
                   the only season that photosynthesis is possible in this 
                   subtropical climate:  You only see them in washes (e.g., 
                   sycamores and alders) and near springs (e.g., a few 
                   deciduous oak species) where they can get water during the 
                   summer.  The rest of the California vegetation is 
                   evergreen, and they get through the rough summer by other 
                   "xerophytic" adaptations ("xero" means "dry"):  Small 
                   leaves, tough leathery leaves with few stomata 
                   ("sclerophyllous" leaves, meaning "tough" leaves), oils and 
                   resins in their leaves to retard water loss, allelopathic 
                   chemicals to protect their water supplies from competitors.  


Enough of the life form classification for plants!  You need to be familiar 
with the major categories (i.e., trees; shrubs; lianas; herbs, including 
forbs, grasses, and ferns; bryophytes in general; and epiphytes).  You should 
also remember how lichens illustrate symbiosis and that very ancient symbioses 
between various bacteria and cyanobacteria gave rise to the eukaryotic cell.  
Rememer that algae, fungi, plants, and animals are based on eukaryotic cells, 
so those ancient symbioses are important to understanding how all of us came 
to be.  Also, familiarize yourself with common modifications to the basic 
categories of the life form system: size, coverage, and periodicity.  This 
classification of individual plant types will be important later, when I use 
them to describe some of the common vegetation landscapes of the world. 

Document and © maintained by Dr. Rodrigue
First placed on web: 10/30/00
Last revised: 06/28/07