READING PLANTS
Summary of Major Points for Class Notes
The title of this tour defines its purpose: to teach students
how one can examine a plant and "read" something about
its natural history. Thus students learn that extreme environments
such as desert and pond habitats have given rise to correspondingly
extreme life forms. Because the Desert Garden, the Lily Pond
area, and the Jungle Garden are adjacent, we have an excellent
opportunity to discuss many aspects of plant adaptation as well
as to inform students on related topics, such as plant geography,
environmental quality, and basic elements of plant form and plant
identification.
GENERAL PLANT KNOWLEDGE. A basic understanding of plant
structure and physiology is necessary to teaching the Reading
Plants course. This following text summarizes information and
definitions you will cover as part of your docent training.
The Plant. It is not so easy to define "plant"
as one might believe. We do not include the baffling cases, such
as the Euglena, which both zoologists and botanists have
claimed in the past. And, we no longer include Fungi or Bacteria,
which are now each placed in their own kingdoms. What we say
at The Huntington is: plants are living beings with cellulosic
cell walls. They lack nervous systems, and are basically non-motile.
Most plants are photosynthetic. All plants reproduce through
production of spores; in higher plants the spores are simply one
stage in production of seed. Some modern biologists exclude
all of the algae, but our definition is not that rigid.
Vegetative Features. By vegetative, we refer to parts
of the plant that are not reproductive. Vegetation, specifically,
refers to leaves and the small branches to which they are attached.
The Reading Plants course concentrates on vegetative features;
adaptations (modifications) in stems and leaves constitute
practically the lion's share of what will be discussed with the
students.
Stem. A stem is the growing axis of a plant. Unlike animals,
which seem to grow all over, plants grow only from definite points
(buds) or regions (layers). The primary growth of a plant is
produced from growing tips; growing tips that are non-active and
protected are called buds. Buds can be found at the ends of
branches, but there is usually a lateral bud produced at the base
of each leaf, in the axil between leaf and stem. Primary growth
lengthens stems and produces new stems, new leaves, and new buds.
Leaves are produced by the growing tip (also called the apical meristem). They first show up as small flaps of tissue (leaf primordia), but grow from that stage to become the shape and size that is preset by the plant's genetics. Typically a leaf has a blade and a stalk (the petiole). Some leaves also produce a piece of leafblade-like material at the juncture with; this is a stipule.
There is hardly any end to the shapes and textures of leaves from
one type of plant to another. Many leaves look nothing like the
typical blade and petiole construction, but one can always pick
out a leaf because it occurs along a stem at the same place where
a growing bud is formed. The bud is in the axil between leaf
and stem and is called an axillary bud. Examples of some
different types of leaf will be discussed in class. Modifications
in leaf structure are frequently related to an advantage confered
to the plant in its native habitat. Obvious examples are in the
Desert Garden, where some leaves are succulent and provide moisture
storage. In cacti, spines grow in the place of green leaves
(i.e. the spines are modified leaves.)
Buds: An axillary bud (tucked in the leaf axil) may be
capable of growing into a side branch. With its own growing
tip, the side branch now produces leaves and lateral buds. Lateral
branches can also have modified forms. Leafless, sharp-tipped
branches can be thorns.
Secondary Growth: In addition to the formation of thorns by branches and spines by leaves, the bark of the stem itself can grow faster in one spot than elsewhere and produce a prickle - such as the prickles of roses, Chorisias, or Caesalpinia cacalaco (which is along the main path in the Desert Garden).
Formation of bark prickles is really a kind of bark growth. Production of bark is related to the type of plant growth called secondary growth. Annual rings on trees are secondary growth, bark production is a similar seasonal growth. Both bark and annual rings (of wood tissue) grow from layers of cells parallel to the surface of the stem. These growing layers are called lateral meristems, the layer that produces annual rings is called the vascular cambium.
Not all plants make secondary growth. Entire groups of plants,
for example the relatives of grasses, lilies, and palms, only
undergo tip growth. This is particularly impressive when one
considers the size of bamboos (grasses) and the many palms. Generalizations
in science have exceptions, and the exception in this case is
the yucca group; the gigantic Yucca filifera plants in
the Desert Garden, though related to agaves and lilies, actually
produce seasonal growth rings.
Flowers: It is not critical (in teaching the Reading Plants course) that one know much about flowers and their structure, but that knowledge contributes to a complete understanding of plant parts and function.
Botanically, a flower is nothing more than a short branch with very special appendages (leaves). The parts of a flower almost without exception are always produced in the same sequence: 1. sepals are the outermost parts, most obvious when in the bud stage.
2 petals are usually the most conspicuous parts of the flower. Like the sepals, the petals are almost certainly modified leaves. The closeness of these two floral parts is obvious in flowers such as lilies, with petals and sepals that are nearly identical in shape and color.
3. stamens are stalked, and have pollen-filled anthers at their tips. Flower pollen is part of the sexual cycle of plants and has the male role, comparable to sperm in animals
4. At the very center of the flower is the pistil, with
its swollen base, the ovary. Inside the ovary are produced
little structures called ovules, each of which manufactures
a single egg. Once the various eggs are fertilized by
the sperm cells from pollen (one pollen grain per egg), the plant
embryo of the new generation begins growth - and the ovule becomes
a seed.
That leaves only the step of getting the sperm cell to the egg.
Pollen is granular and can be carried by the wind, insects, birds,
mammals, or even humans, from the anthers to an appropriate pistil.
Once stuck to the top of the pistil (in a very special place
called the stigma) each pollen grain cracks open and germinates
by growing a long tube that will seek out an egg cell. Once the
tube arrives at the egg cell, the sperm nucleus is delivered to
the egg and fertilization is a fact. It takes two to tango.
Photosynthesis. It is critical for you to understand the basic importance of photosynthesis. The simple steps to study involve:
1. capture of light (sunlight) by the green-colored compound (chlorophyll) in plants.
2. conversion of the energy of sunlight into a form of energy useful to plants (chemical energy). This requires the splitting of water and results in the production of oxygen.
3. fixing of carbon dioxide to become part of a sugar
This activity also has physical impact:
- when stomata (these are the pores in a leaf surface; stoma means "little mouth" and stomata is simply the plural form of the word) are open, carbon dioxide, oxygen, and water vapor are exchanged with the atmosphere
- this loss of water (called transpiration) relates directly to movement and balance of water in the plant
- the distribution and activity of stomata shows adaptations correlated
with the native habitat/climate of the plant
To capture light, a plant cell has to contain the pigment chlorophyll. This pigment (a colored compound) is not floating around free in the cells, but is highly organized and contained in special structures called plastids. Chlorophyll-containing plastids are called chloroplasts.
Chlorophyll captures light energy, in units called photons. Energized electrons are lost from the chlorophyll, and move through various reactions to yield chemical energy. The demand for replacement electrons drives the splitting of water, which yields hydrogen ions, electrons, and free oxygen. These light reactions are therefore the source of oxygen and the chemical energy that fixes carbon to make sugars.
The making of sugars can happen in light or darkness, as long sufficient carbon dioxide is available. These events are called the dark reactions. Sugars are the currency of plant cells. Not only are they building blocks (cellulose for example is an extremely long chain of sugars linked end to end), they are also a ready source of energy. A plant or an animal can break a sugar down bit by bit to yield new compounds and chemical energy. Removing carbon from these sugars requires oxygen, and is called respiration.
The water needed for photosynthesis comes from the soil, enters
the roots, and rises to the cells through a system of veins.
Soil also provides the minerals plants require for their physiological
processes. Water is the stuff of life. It is the universal solvent
with which the sugars and many other compounds will mix. Water
is the source of pressure within individual plant cells that keep
them inflated, rather than limp (think of what happens when plants
begin to dry out - they wilt). Water moves constantly throughout
the plant and is constantly being lost as vapor escapes from the
stomata. In tropical plants from wet areas, transpiration may
be no problem; in plants that grow in water, this is usually no
limitation, but in the desert, development of adaptations that
conserve water has been obviously paramount.
CLIMATES, HABITATS, ADAPTATIONS, ETC...Successful growth
of plants depends on the conditions of the environment. In the
Reading Plants course, we discuss three environments represented
by the Desert Garden, Lily Ponds, and Jungle Garden.
DESERT: A region that receives less than 25 cm (approximately
10 inches) of precipitation annually will be either desert or
desert scrub. True desert generally receives less than 12.5 cm
(5 inches) of precipitation each year. The less water available,
the sparser the vegetation. Whittaker (Communities and Ecosystems,
1970) lists 35 biomes (see below), five of these (#17-21)
are different types of deserts.
PONDS AND MARGINS: In contrast to deserts, the wet communities
show a super abundance of water. Whittaker lists ten different
biomes (#22-31) of this nature. In the Lily Ponds we have the
opportunity to explain hydric situations (wet soils), lentic
areas (bodies of water), and even lotic communities (streams).
TROPICAL FOREST: Whittaker lists two biomes of tropical
forest, rain forest and seasonal forest. In the first there is
water available throughout the year. The second has a pronounced
dry season. Tropical wet forests have greenhouse climates - conditions
that we normally consider limiting (lack of precipitation, freezing
weather) are not in force.
Whittaker's Biomes
Forest & Woodland types: 1. Tropical rain forest; 2.
Tropical seasonal forest; 3. Temperate rain forest; 4. Temperate
deciduous forest; 5. Temperate evergreen forest; 6. Taiga (subarctic-subalpine
needle-leaved forest); 7. Elfinwoods; 8. Tropical broadleaf woodlands;
9. Thornwoods; 10. Temperate woodlands; 11. Temperate shrublands.
Grasslands and Treeless zones: 12. Savannas; 13. Temperate
grasslands; 14. Alpine shrublands; 15. Alpine grasslands; 16.
Tundras.
Deserts: 17. Warm semi-desert scrubs; 18. Cool semi-deserts;
19. Arctic-alpine semi-deserts; 20. True deserts; 21. Arctic-alpine
deserts.
Wetlands: 22-26. Hydric communities; 27. Lentic fresh-water
communities (lakes and ponds); 28. Lotic fresh-water communities
(streams); 29. Littoral (coastal) - marine rocky shores; 30. Marine
sandy beaches; 31 Marine mud flats; 32. Coral reefs; 33. Marine
surface pelagic (open ocean); 34 Marine deep pelagic; 35. Continental
shelf benthos (bottom communities).
Handouts detail more information on these biomes. Another manner
of looking at major plant communities is the Holdridge Life Zone
chart. If you look at the sides of the triangle, you will see
various scales - temperature, precipitation, evapo-transpiration
(potential for loss of water from plant foliage), and humidity.
Two parallel horizontal scales represent changes in latitude
and altitude. This summarizes very simply for us the similarity
of changes in vegetation from the base of a tall mountain to its
top as compared to heading toward the nearest polar region from
the base of the same mountain. Alexander von Humbolt, explorer
and biogeographer of the last century, was the first person to
describe this relationship. By studying the Life Zone chart,
one can better understand the relationships between different
zones - which approximate biomes.
ADAPTATIONS: In Reading Plants, we must be careful to
utilize only one meaning of "adapt." The genetic composition
of a population of plants changes over countless generations because
some individuals of a group of plants are particularly successful
in a given habitat and others (of the same type of plant) are
comparatively unsuccessful. The more successful individual plants
leave their imprint on the genetics of the whole population by
contributing a greater share of offspring. The character of the
population from one generation to another shifts to reflect this.
This steady change in the genetic make-up of a population is
evolution, and the result of evolution is a population
that differs from the ancestral population. This shift in character
in response to the environment is adaptation.
LIFE FORMS are convenient catagories in which to place
plants to understand plant adaptations. The traditional system
for consideration of life forms is that of Raunkiaer, which places
plants in five catagories: phanerophytes, chamaephytes, hemicryptophytes,
geophytes, and therophytes.
DESERT LIFE FORMS that are most prominent are:
Annuals (Therophytes) - 73% of the plants in deserts are annuals. In this respect, our Desert Garden is truly unrepresentative of a desert. What we have on display is a selection of succulents from desert regions
Succulents can fall into any of the first four of Raunkiaer's catagories. Most are either phanerophytes or chamaephytes. Few are really geophytes.
Stem succulents have modified, deciduous, or reduced (vestigial) leaves. Generally, the photosynthetic function is a property of the green, fleshy stems.
Leaf succulents are plants that have tough, fleshy leaves. Of the leaf succulents that are monocots, most seem to be rosette plants (Agaves, yuccas, aloes).
Scrub is a term for shrubs encountered in dry regions.
These plants normally have small leaves (hence they are also
described as microphyllous) that are deciduous during the dryest
time of the year. Often they also have green stems.
STRATEGIES FOR DESERT PLANTS have evolved that make them better adapted to their environments. These strategies may involve adaptations in shape, anatomy, physiology, etc. In deserts, the most obvious strategies have appeared in response to stresses:
Low available water - Many plants, especially in our garden, show water storage capability and concommitant structures that serve to protect the fleshy storage tissues. In cacti the leaves are commonly modified as spines and in the euphorbs, the stipules are spiny; certainly these needles serve some protective role in an environment where animals experience low available water. Armature and protection are found in many other plant groups that occur in dry regions. Sometimes, as in the agaves, the fleshy organ (leaf) may be very fibrous or have a very tough protective surface. In many of these instances, the leaf margins are also lined with sharp spines. Some desert plants are known to be particularly high in chemical compounds, which we assume deter herbivores.
High light intensity - There is a limit as to the amount of light a plant's photosynthetic system can absorb without being destroyed. Plants are known to burn when moved suddenly from a low light to high light situation. Some plants from deserts have coverings of hairs or waxes that one could interpret as useful in reducing the amount of light received. However, it is difficult to separate this potential problem from that of high temperature.
Temperature extremes are characteristic of desert regions.
Additionally, plants often must tolerate wide fluctuations in
temperature from day to night, or from one season to another.
Extremely high temperatures can cook or dessicate plant tissue.
Adaptations that could function to reduce light reception, might
also serve to reduce temperature.
CONVERGENCE: Similar habitats (and on a larger scale,
the same biome) can occur in various places in the world. These
different locations representing the same type of biome will have
a similarity in plant cover, due to similar stratagies having
evolved by plants that inhabit the region. At the top of the Desert
Garden you will see cacti from American deserts across the path
from the euphorbs of African deserts. These stem succulents are
an excellent example of adaptation having arrived at a similar
solution to the same problem, even though in one instance the
plants were in America and in the other, in Africa. This convergence
in form and strategy is termed convergent evolution.
AQUATIC PLANTS: In the Lily Ponds, LILY PONDS, you will
see aquatic plants and plants of wet soils. Here we study plant
adaptations that reflect abundant water in the habitat.
LIFE FORMS can be of all sorts. Raunkier did not even consider
the water plants in his definitions. Plants of water (not including
margins of streams and lakes or seashores) are typically discussed
as submerged, emergent, or floating.
Submerged plants commonly have thin-textured, feathery
leaves that are near the surface of the water. These plants often
exchange gases and nutrients straight into the leaf tissues from
the surrounding water - without the use of stomata. If roots
are present, their main function may be anchoring. Elodea is
always present in our ponds; it perfectly fits the mold just described.
In the streambed of the waterfall are strands of algae that also
conform to this description (though algae can't even make roots).
Some plants are emergent. They are rooted in the bottom
and send leaves and sometimes stems above the water's surface.
Generally, these are plants of shallow water. In the ponds you
will see lotus, which is an erect emergent, and water lilies,
which have floating leaves. Water lilies are not floating plants,
because the stems are firmly rooted in the bottom of the pools.
The round, floating leaf blades are well attached to the stems
by their long, flexible petioles. The emergent plants have an
amphibious lifestyle. The seedlings, and in the mature plant
the roots and growing stem tips are below water, but produce plant
tissue (leaves and flowers) that lives above the water.
Free-floating plants are not rooted in the soil. They
usually have large feathery root systems that absorb nutrients
in a perfectly hydroponic way. Lightweight tissues (often very
airy and spongy) are incorporated into plant parts so that the
plant will float. In the upper lily pond at the base of the waterfall
are water hyacinths. These plants have swollen petioles filled
with large, spongy cells that make the plant bouyant. Also in
that pond are the water ferns which are mainly leaf blade - on
the upper surface of the blade are hairs that help prevent wetting
of the upper leaf surfaces.
JUNGLE GARDEN. In this garden one talks about forests, especially the characteristics of tropical wet forests, in which water is sufficient to allow development of a dense and highly structured vegetation. In tropical wet forest, even the limitations imposed by a cold season are eliminated. The sky, it seems, is the limit.
The warm temperatures and high rainfall, however, cause heavy
leaching so that tropical soils are nutrient poor. Nutrients
that are present are normally tied up in the bodies of the trees
and other plants and the animals that cover the area. Detritus
(fallen leaves, branches, dead animals, etc.) is quickly decomposed
and reutilized by plants to make more living tissue.
LIFE FORMS in a tropical wet forest are overwhelmingly (96%) phanerophytes (plants that on a year-around basis can keep their growing tips at least 25 cm above ground level). The dominant plants are tall trees. Unlike temperate forests that may have few tree species, the tropical forest is normally highly diverse with many different kinds of trees making up the canopy.
In a very complex forest, below the canopy is another layer of smaller trees. Lower yet are a shrub and finally a ground layer. Weaving throughout the forest is a system of woody vines, lianas, that manage to reach light gaps and areas in the canopy. Growing along the branches of trees one encounters epiphytes, that capture water from rainfall and runoff.
Encumbered with clamboring vines and festooned with well-attached epiphytes, the tropical forest interactions suggest that light availability is a limiting factor for plant growth. The limitations inherent in capturing water available only above the ground's surface must be secondary to the opportunities for light reception in the canopy.
This is obvious also in the large size of many tropical leaves.
With adequate available water, loss by transpiration is not so
critical that foliage must be especially tough or reduced in size.
The philodendrons and banana plants in the Jungle Garden give
notice that big is alright.
CONSERVATION: The Jungle Garden is a good time to discuss
the importance of forest conservation and preservation of species
diversity.
REPRESENTATION: None of the gardens which the students visit should be construed as being faithful to the habitats we are discussing. The Desert Garden does not feel like a desert; plantings are many times denser than the vegetation of dry
regions. As we reorganize this garden, at least visitors will see plants grouped geographically (as in the Baja California bed). The Lily Ponds are constructed and water is circulated by a pump. Plants are from many different regions of the world. The aspect is something like a stream and pond habitat, but we cannot represent the many other wetland habitats, such as sand dunes, marshes, rivers, lake, and oceanic. The Jungle Garden is somewhat true in appearance to a tropical forest, but the species we use to plant the garden are from many different lands and habitats. We are interested in making a garden.
None of the gardens the students will see represent evolved assemblages
of plants. These plants manage to tolerate the conditions in
our gardens. In the sense that you and I can adapt to a new work-place
or adapt ourselves to living in a city with a different climate,
one could say these plants have adapted themselves to grow at
the Huntington. Evolutionarily however these plants are adapted
only to grow in their native habitats. This is the sense of the
word adaptation which is important in the Reading Plants
course.
TAKE HOME LESSONS. When the students return to their schools, we hope they will retain:
1. an abiding interest in plants.
2. the knowledge that plants take Earth, Wind, Fire, and Water, which are the physical elements of the world, and build the basis of an organic, living world
3. the realization that each different climate and biome imposes its own particular limitations and pressures on plant growth and evolution.
4. a sense that each community is a system of interdependent plants and animals.
5. the desire to understand at least fragments of the complex biological world that surrounds them.