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Defense & Plants
The Defense Adaptations
In Plants
Cody Jones
Introduction to Plant Biology 1030/1035
Professor William Tanner
April 23, 2013
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The Defense Adaptations in Plants
As humans we have many attributes that contribute to our well-being and our capability
to defend ourselves and others. If we feel the least bit threatened we have many options to
defend both physically and emotionally. Plants on the other hand don’t have limbs that move or
swat animals away when they are threatened. They’re stuck where they are dealing with the
situation. Which brings me to the question, how can plants defend themselves? And how have
they adapted to survive in their environments?
Over time plants have adapted to defend themselves. Some develop modified branchlets,
modified leaves, epidermal emergences, and others produce poisonous compounds.
Modified branchlets are branches (stems) that have over time hardened and made
a point at the tip. We know these to be called thorns. When you think of a thorn, you think of
roses, right? Well the sad truth is that roses don’t have thorns they have epidermal emergences,
we’ll discuss this later on in the paper. An example of this can be found in Honey Locust trees
which are found in, “the pea family, comprising the genus Gleditsia, which has only about a
dozen species, chiefly American and Asiatic, but one in tropical Africa.” (Taylor, 1936) Another
example can be found in Hawthorns which is in the genus Crataegus, “an enormous genus (over
1000 species) of thorny shrubs and trees of the rose family, found in the North Temperate Zone
but most common in eastern North America.” (Taylor, 1936) The thorns that plants produce
allow wanted animals in to help with seed dispersal if the plant is designed that way, meaning
they bear fruit that incases the seeds and when the right animal comes along they can access the
fruit and dispose of the waste which would include the seeds. The thorns keep out the larger
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animals that may damage the plant by stripping off layers of the epidermal tissues and secondary
phloem (if in a woody plants) exposes the plant to disease and dangerous bacteria.
Another type of defense that plants use is spines, which are modified leaves or stipules.
Some examples of plants that have these are in the Cactaceae genus. “The cactus family is an
immense group of succulent, mostly spiny, desert plants, all but a handful confined to dry or
desert regions of the New World.” (Taylor, 1936) Cacti don’t have leaves, because their
environment would cause the plant to dehydrate faster due to the scarce rainfall and eventually
kill the plant. So they decrease the “leaf” surface so there is less transpiration of water vapor in
the plant (hi[H2O] in the plant and lo[H2O] outside of the cuticle). This means that it has a
better chance of surviving. Their spines also protect the main part of the cactus sometimes
keeping out unwanted animals. Occasionally you will find birds making holes in cacti for their
homes. Another type of plant that has spines is the Thistle. “200 known species (family
Compsoitae) scattered over the North Temperate Zone, only a very few are of any garden
interest, and some are most pernicious weeds. They are prickly herbs with alternate or basal
leaves that are nearly always cut or lobed and horribly spiny-margined.” (Taylor, 1936) What a
perfect type of defense. If an animal comes along wanting to graze on thistles, I don’t think
they’d like it very much; having a mouth full of spines.
A third type of defense would be prickles. What are prickles? Prickles are pointed
epidermal emergences that come from either the cortex or the epidermis. This is what a rose has
instead of thorns (it’s pretty sad to realize that everything you thought you knew about plants
changes after you take a plant biology class). True roses belong to the genus Rosa, “a genus of
shrubs or vines, comprising all the true roses, and typifying the family Rosaceae. According to
the usual concept of them there may be 200 true species of roses, but one authority has credited
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to Eurasia alone over 4000 species.” (Taylor, 1936) Another example of a plant with prickles
would be the raspberries. There are “four groups of raspberries, each with many varieties, are
grown in North America. The European red raspberry, derived from the wild red raspberry of
Europe; the American red raspberry, the black raspberry, or blackcap, also a cultivated native;
and the purple-cane raspberries, hybrids between varieties of the two reds and the black
raspberry. All of these belong to the genus Rubus.” (Taylor, 1936)
All of the defenses that I have talked about so far don’t interfere with the way that they
are pollenated. They keep out unwanted animals and allow the good “influences” in. Nor will
the last defense that I’m going to talk about effect the way it is pollenated, it’s just a defense to
protect from animals.
The last defense would be a poisonous compound, either in the roots, foliage, the flower,
or the fruit (if it bears fruit), or all parts of the plant may be poisonous. The examples that I will
be using for this are Lily -of-the Valley (Convallaria majalis) the Lupine (Lupinus), and Lobelia
(Lobelia spp.). Lily-of-the Valley contains poison in all of its parts; from the roots to the flowers
and fruit and everything in between. It belongs to, “the genus Convallaria of the lily family,
found wild in Eurasia and in the higher mountains from Virginia to South Carolina. The only
species, C. majalis, is cultivated throughout the temperate world for its fine, persistent, but not
evergreen foliage, but especially for its delicately scented, white, nodding flowers.” (Taylor,
1936) The other example is the Lupine. “It’s a genus of many species belonging to the pea
family. It is found in North America, South America, and on the shores of the Mediterranean
Sea. All are hardy, and can be divided into three groups: the tree, herbaceous perennial and
annual lupines.” (Taylor, 1936) The poison in lupines varies from species to species of where
the poison is located, but the most common place is in the pods and in the seeds. The third
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example is Lobelia. Now in Lobelia all parts of this plant are poisonous. They are, “showyflowered perennial or annual herbs of the family Lobeliaceae, comprising over 250 species,
some of the tropical sorts being trees.” (Taylor, 1936) When an animal tries to eat any of these
plants they will feel a discomfort, may become ill, or even die. Even though these are a plant’s
defenses and they’re supposed to ward off animals they can be very helpful for humans if used
correctly. Two of the examples that I used above have helped humans and have caused fatalities.
Lily-of-the Valley, “contains a heart stimulant similar to digitalis, it’s used to control irregular
heartbeats.” (James E. Bidlack, 2011) And Lobelia contains, “lobeline sulfate obtained from
dried leaves; drug used in preparations to aid in cessation of smoking and in treatment of
respiratory disorders.” (James E. Bidlack, 2011) Lupine also has large quantities of a poison
compound, but hasn’t caused any fatalities in humans that I know of.
So far we’ve discussed how plants protect themselves from animals. They, through time
and cell division modify their branches into thorns. They modify their leaves or stipules into
spines. And they obtain epidermal emergences either from the cortex (just underneath the
epidermis) or on top of the epidermis. And they produce a poisonous compound that can be
dispersed when the plant is being chewed on or just by touching the leaf the way poison ivy
defends itself.
Do these types of defenses work? Is Mother Nature thinning out the weak and stupid
from the group? I have some statistics (present) where there have been human fatalities caused
by plant consumption and poisoning due to plants.
I got this information from the Europe PMC Funders Group online. “Deaths from plant
poisoning are rare. Of 24,950 cases of plant poisoning reported to the Swiss Toxicological
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Information Center between 1966 and 1994 significant poisoning occurred in 152 cases. Five
deaths were noted, due to poisoning with meadow saffron, hemlock water dropwort, English
yew, and daffodil.” (Michael Eddleston, 2003) “A recent study in the U.S. showed that 30
fatalities from plant poisoning over 18 years (1983-2000) seven were caused by various Cicuta
spp. and five by jimson weed or thorn apple.” (Michael Eddleston, 2003)
Some of these deaths were caused by accidental consumption by children (younger),
because of their cognitive brain function. They’re too young to understand the dangers of the
world when they are growing and exploring their world.
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Bibliography
James E. Bidlack, S. H. (2011). Stern's Introductory Plant Biology 12th edition. New York: McGraw-Hill.
Michael Eddleston, H. P. (2003). Acute Plant Poisoning and Antitoxin Antibodies. Retrieved April 22,
2013, from Europe PMC Funders Group: www.ncbi.nlm.nih.gov
Taylor, N. (1936). The Garden Dictionary. New York: Houghton Mifflin Company.
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