Another Inaccurate Botany Teaching Article

David Hershey dh321z at yahoo.com
Sat Aug 7 14:30:42 EST 2004


In July, 1999, there was a discussion in this group of
the badly flawed "Supermarket Botany" article in the
American Biology Teacher (ABT). Ross Koning suggested
that such flawed articles could be used in teaching as
bad examples. He recommended that students be assigned
to read the inaccurate article and correct the errors.
In that light, another ABT article (DeGolier 2002)
might prove useful. 

I submitted a letter to ABT pointing out the numerous
errors in DeGolier (2002) but the editor refused to
publish it. I protested and eventually ABT published a
heavily censored version that was all of three
sentences long. I recently discovered that McGraw-Hill
featured DeGolier (2002) in their "POWERWEB : Botany",
but not as a bad example.

http://www.dushkin.com/catalog/007254905x.mhtml?SECTION=TOC


Therefore, I felt compelled to post my list of
corrections for the article. 

Reference  
 
DeGolier, T. (2002). Cold war: Flora's undercover
agents, a campus winter field trip to illustrate that
plants do indeed thermoregulate. American Biology
Teacher, 64, 45-51. 


My letter as originally submitted to American Biology
Teacher follows:


Plants Rarely Thermoregulate

DeGolier's (2002) winter field trips to examine plant
adaptations to survive winter seem very worthwhile.
However, the term thermoregulation is misapplied
because plants rarely thermoregulate as warm-blooded
animals do. In the Plant Kingdom, the term
thermoregulation is applied only to flowers of a few
species (Seymour 1997) such as lotus (Nelumbo
nucifera), split leaf philodendron (Philodendron
selloum) and skunk cabbage (Symplocarpus foetidus).
Flowers in these species can raise their temperature
well above ambient as warm-blooded animals do. 

Perhaps the only plant to thermoregulate to protect
its flowers from cold injury is skunk cabbage, native
throughout the Northeastern U.S. west to Iowa, south
to Georgia and north into southern Canada. It can
raise the internal temperature of its inflorescence 15
to 35 C above ambient air temperatures of -15 to 15 C
and can maintain thermoregulation for two weeks or
more when it blooms in February and March (Knutson
1974, Seymour and Blaylock 1999). Skunk cabbage often
melts the snow covering it. Its inflorescence tissue
is not frost resistant (Knutson 1974) so
thermoregulation does seem to prevent cold damage.
Other functions of flower thermoregulation are to
vaporize floral scents and to attract insect
pollinators. 

DeGolier (2002) contains several incorrect or
incomplete facts as follows: 

1.) The statement that retaining normally deciduous
leaves over winter would be a huge energy cost to the
plant is very hypothetical and does not seem to be a
major reason for leaf abscission. Leaves of deciduous
species are programmed to senesce in the fall. Even if
they did not senesce, deciduous leaves would soon be
killed by the cold temperatures. If the deciduous
leaves did somehow survive during winter, their
metabolic level would be lower than normal because of
the lower temperatures. They would also likely to be
able to photosynthesize to some extent to at least
partially meet their winter energy needs. The fact
that many needleleaf and broadleaf trees are evergreen
indicates that overwintering leaves are not a huge
energy cost. The mean net primary production (grams
per square meter per year) of temperate forests is
actually slightly higher for evergreen than for
deciduous forests (Salisbury and Ross 1985). 

It may seem counterintuitive that evergreen forests
are dominant farther north than deciduous forests.
Ecologists have correlated the dominance of evergreens
in certain areas of both cold and warm climates with
the low mineral nutrient availability in those areas
(Aerts 1995). Evergreen leaves make more efficient use
of mineral nutrients than deciduous leaves. In very
cold biomes, such as the far north taiga or boreal
coniferous forest, mineral nutrient availability is
low because of the low temperatures. Therefore,
deciduous forests typically dominate in areas with
higher mineral nutrient availability, which are south
of the taiga. With no mineral nutrient limitation,
deciduous trees can make new leaves every year and not
have the disadvantage of lower photosynthetic rates of
evergreen leaves that are old or winter-damaged. The
deciduous habit is thought to have evolved in warm
climates as an advantage where there is seasonal
drought (Axelrod 1966). A cold winter might also be
considered a dry season. Another possible advantage of
dropping leaves in cold winter climates is that there
is far less surface area to accumulate loads of ice or
snow sufficient to break branches (Lemon 1961). Ice
storms often cause tremendous damage to deciduous
trees even without their leaves. In the U.S.,
deciduous forests in the Northeast and Midwest
correspond to areas where winter ice storms are
common. 

2.) Tulips and daffodils do not die back in winter.
They die back in summer. Tulip and daffodil leaves
often begin to emerge aboveground during winter in
USDA Cold Hardiness Zone 6 and possibly farther north
if the soil is not frozen solid. Their leaves are cold
hardy and survive subfreezing temperatures. 

3.) Not all ferns will "frost off" in most areas of
the U.S. Some native ferns are cold hardy evergreens
including the Christmas fern (Polystrichum
acrostichoides), which is hardy to USDA hardiness zone
4. USDA hardiness zone 4 includes the southern half of
Minnesota. 

4.) Certain trees, such as some oaks (Quercus spp.)
and some beeches (Fagus), often retain some dead
leaves during winter. These leaves are termed
marcescent, defined as withering without falling off.
There seems to be no evidence that marcescent leaves
provide additional cold protection to buds. Marcescent
leaves have been considered a juvenile characteristic
(Leopold and Kriedemann 1975). Marcescent leaves are
found mainly in the lower, more juvenile, parts of
larger trees. Hormone levels are probably responsible
because gibberellic acid applied to mature plants can
cause them to revert to juvenile characteristics,
hormones trigger abscission, and hormones can delay
leaf senescence (Taiz and Zeiger 1991). Some juvenile
characteristics, such as thorns and spiny needles,
discourage herbivory. It has also been hypothesized
that marcescent leaves are a defense against large
herbivores, such as deer (Svendsen 2001). However,
even in trees that normally drop all their leaves in
the fall, leaf abscission is sometimes prevented by
cold temperatures that kill the leaves before the
abscission layer has completely formed. 

5.) Marcescent leaves do not remain "on oak buds". The
oak bud is not subpetiolar or under the petiole. An
example of a true subpetiolar bud is sycamore
(Plantanus spp.). There seems to be a common
misconception that buds on marcescent trees are
subpetiolar and that bud growth in the spring simply
pushs marcescent leaves off the tree. However, Hoshaw
and Guard (1949) found that in pin oak (Quercus
palustris) the petiole base remained alive during
winter even though the rest of the leaf was dead. In
West Lafayette, IN, they found anatomical changes in
the petiole base starting in late February that
resulted in rapid dropping of marcescent leaves at the
end of March. Thus, the abscission zone completes
development in late winter or early spring rather than
in the fall. 

6.) There was not a clear answer for the question
"Does the bark of a tree have any insulating value?"
Evidence that bark is often a poor insulator in many
deciduous trees is the widespread occurence of damage
termed cup shakes, frost canker and frost cracks
(Harris 1983, Pirone et al. 1988). These all occur
when branches or trunks are rapidly thawed or frozen.
Cup shakes is a separation of wood along an annual
ring which occurs when a frozen trunk is quickly
warmed causing the outer layers to thaw first, expand,
and separate from the inner, still-frozen wood. Cup
shakes are not visible externally but weaken the tree
and lower lumber quality. Frost cankers and frost
cracks occur when a warmed trunk or branch freezes
quickly, as when the sun suddenly goes behind an
opaque object. In frost canker, the cambium or bark is
injured or killed. Frost cracks occur when the outer
trunk layer freezes and therefore shrinks faster than
the inner layers. This results in a long longitudinal
crack in the bark and wood that often extends all the
way to the center of the trunk. A loud noise often
accompanies the formation of a frost crack. The frost
cracks reopen in subsequent winters when the
temperature is low enough, so they cannot heal shut.
To prevent frost canker and frost cracks, young tree
trunks are often wrapped with burlap or painted white
to prevent them from being thawed by the sun. 

7.) Not all conifers are evergreen. Larches (Larix
spp.), bald cypress (Taxodium distichum), dawn redwood
(Metasequoia glyptostroboides), and golden larch
(Pseudolarix kaempferi) are deciduous, needleleaf
conifer trees. The deciduous larches have similar
aboveground production rates as sympatric evergreen
conifers and because they are deciduous, they can
survive in far northern areas too harsh for evergreens
(Gower and Richards 1990). Larches seem to be an
exception to the generalization that evergreens are
favored in areas with low mineral nutrient
availability. 

8.) Many coniferous needles do not have stomata just
on the underside. Pine needles have stomata on all
surfaces (Mauseth 1995). Some pine species are even
round in cross section. 

9.) Is there any evidence that V-shaped needles drain
water away so "stomata don't drown"? The surface
tension of water, the waxy cuticle, and the air that
would likely be trapped by surface water in the
depression above the sunken stomata would all seem to
make it difficult for water to enter sunken stomata,
particularly on the underside of the leaf. Given that
leaves evaporate large quantities of water, temporary
waterlogging of needles seems unlikely to be a serious
problem. Darwin and Acton (1895) had a teaching
experiment to demonstrate waterlogging of broad
leaves, but not conifer needles. They reported that
when a frozen ivy leaf was thawed underwater it became
waterlogged because the intercellular ice melted, was
absorbed back into the cells and pulled water into the
leaf through the stomata. Darwin and Acton's (1895)
experiment seems a bit extreme because it is unlikely
that evergreen needles would be completely submerged
in water as they thaw. However, conifer needles that
prevent freezing by supercooling would have no
intercellular ice (Taiz and Zeiger 1991) so that
particular waterlogging mechanism would not work
unless temperatures were too low for supercooling. 

Some additional items worth mentioning on a winter
field trip include the following: 

A.) Any examination of plant cold hardiness would
benefit from a discussion of the USDA Plant Hardiness
Zone Map (Cathey 1990) or the Arnold Arboretum Plant
Hardiness Zone Map (Wyman 1990). The USDA Plant Cold
Hardiness Zone Map is widely available on the
internet, in horticulture and gardening books and in
plant catalogs. 

B.) Another important aspect of plant cold hardiness
is that roots are typically less cold hardy than the
shoots. People often learn this lesson the hard way
when cold hardy plants grown in aboveground containers
die because the roots are killed by low temperatures. 

C.)Flower buds are typically less hardy than
vegetative buds. This leads to substantial economic
losses nearly every year when cold temperatures kill
peach or other fruit crop flower buds in some
orchards. Some spring flowering shrubs and trees, such
as forsythia (Forsythia spp.) and flowering dogwood
(Cornus florida), may have their flower buds killed or
damaged in harsh winters. 

David R. Hershey 
dh321z at yahoo.com 

References

Aerts, R. (1995). The advantages of being evergreen.
Trends in Ecology and Evolution, 10, 402-406. 

Axelrod, D.I. (1966). Origin of deciduous and
evergreen habits in temperate forests. Evolution, 20,
1-15. 

Cathey, H.M. (1990). USDA Plant Hardiness Zone Map.
Washington, DC: USDA Agricultural Research Service
Misc. Publication 1475. 

Darwin, F. and Acton, E.H. (1895). Practical
Physiology of Plants. London: Cambridge University
Press. 

DeGolier, T. (2002). Cold war: Flora's undercover
agents, a campus winter field trip to illustrate that
plants do indeed thermoregulate. American Biology
Teacher, 64, 45-51. 

Gower, S.T and Richards, J.H. (1990). Larches:
Deciduous conifers in an evergreen world. BioScience,
40, 818-826. 

Harris, R.W. (1983). Arboriculture: Care of Trees,
Shrubs and Vines. Englewood Cliffs, NJ: Prentice-Hall.


Hoshaw, R.W. and Guard, A.T. (1949). Abscission of
marcescent leaves of Quercus palustris and Q.
coccinea. Botanical Gazette, 110, 587-593. 

Knutson, R.M. (1974). Heat production and temperature
regulation in Eastern skunk cabbage. Science, 186,
746-747. 

Lemon, P.C. (1961). Forest ecology of ice storms.
Bulletin of the Torrey Botanical Club, 88, 21-29. 

Leopold, A.C. and Kriedemann, P.E. (1975). Plant
Growth and Development. New York: McGraw-Hill. 

Mauseth, J.D. (1995). Botany: An Introduction to Plant
Biology. Philadelphia: Saunders College. 

Pirone, P.P., Hertman, J.R., Sall, M.A. and Pirone,
T.P. (1988). Tree Maintenance. New York: Oxford
University Press. 

Salisbury, F.B. and Ross, C.W. (1985). Plant
Physiology. Belmont, CA: Wadsworth. 

Seymour, R.S. (1997). Plants that warm themselves.
Scientific American, 276(3), 104-109. 

Seymour, R.S. and Blaylock, A.J. (1999). Switching off
the heater: Influence of ambient temperature on
themoregulation by Eastern skunk cabbage, Symplocarpus
foetidus. Journal of Experimental Botany, 50,
1525-1532. 

Svendsen, C.R. (2001). Effects of marcescent leaves on
winter browsing by large herbivores in northern
temperate deciduous forests. Alces, 37(2) 475-482. 

Taiz, L. and Zeiger, E. (1991). Plant Physiology. New
York: Benjamin/Cummings. 

Wyman, D. (1990). Trees for American Gardens. New
York: Macmillan. 







		
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