GrowLab corrections

dh321 at excite.com dh321 at excite.com
Sun Jun 3 21:22:26 EST 2001


A couple weeks ago, someone mentioned the GrowLab curriculum. About ten
years ago I reviewed the curriculum for a couple of teaching journals and
was appalled by the numerous factual errors. I compiled a list and sent it
to the publishers, however, I am not aware that they ever passed the list on
to their customers. Here is a revised list of corrections for those who are
using the curriculum. 


Corrections for Growlab: Activities for Growing Minds (1990):
 
Page 28 - The statement "Every seed is either a monocot, having one
cotyledon, or dicot, with two." is incorrect because conifer (gymnosperm)
seeds often have many more than two cotyledons. 

Page 28 - In addition to oxygen, water and proper temperature, most seeds
require proper light conditions to germinate. Many seeds either require
light for germination, or their germination is inhibited by light.

Page 28 - The statement "Seeds have their own source of nutrients (in the
cotyledons or endosperm) to sustain them through early life, so they do not
require additional nutrients." is not true for several reasons. Many tiny
seeds, such as orchids, are so small that they have inadequate energy
reserves and must be germinated on a sterile medium containing 2% sugar and
other essential mineral nutrients. Also, all roots require calcium and boron
in the external solution for proper functioning. Seeds may have low reserves
of one or more mineral nutrients if the parent plant was deficient. As soon
as a root emerges, it is capable of absorbing mineral nutrients.

Page 29 - The statement "When a seed is exposed to proper conditions for
germination, water is taken in through the seed coat." is often not true as
the sidebar on seeds with impermeable seed coats indicates. 

Page 29 - The statement "The embryo's cells begin to divide and the seed
coat breaks open." is incorrect since seed expansion causing the seed coat
to break open or radicle to emerge is due to cell enlargement or elongation
rather than cell division. Hartmann and Kester's standard text on Plant
Propagation Principles and
Practices (Prentice-Hall) lists three stages in seed germination:
1) water absorption causing seed swelling and seed coat rupture, 
2) digestion and translocation of stored food 
3) cell division in growing points and expansion of seedling structures. 

Page 32 - The statement "When both cotyledons are removed from a seed or
young plant, the plant will lack adequate nutrients to continue growing." is
not completely correct. Removing cotyledons from a young
plant with its primary leaves expanded, such as a bean, will usually slow
growth but will not usually kill the seedling.

Page 39 - "Water transports nutrients, gases, and waste materials ..." is
misleading since, unlike most animals, plants do not have an excetory system
for wastes. Scientists really do not consider plants to have waste
materials, since they respire starch or other energy carbohydrates to CO2
and water.

Page 50, sidebar - Drop "vitamins" from this sentence: "The nutrients in the
fertilizers we use are more analagous to the vitamins and minerals needed by
humans to help our bodies grow, repair, and function properly." Plants
synthesize the vitamins they require, they do not absorb them from
fertilizers as they do mineral nutrients. 

Page 52, 53 - The source of the 387 miles or 13 million roots figures should
be indicated. These numbers are from Dittmer, H.J. (1937). American Journal
of Botany 24:417-420 for a container-grown plant. 

Page 53 - The term "geotropism" is obsolete. The current term is
gravitropism. This is similar to "heliotropism" the obsolete term for
phototropism.

Page 53 - Hydrotropism is a phenomenon which scientists have found is not
common under natural conditions of plants growing in soil. There are lab
experiments with roots grown in humid air that sometimes show some weak
hydrotropism due to a relative humidity gradient but they are very
artificial. In general, roots require water to grow, they do not grow toward
water. It may appear that way in simple classroom experiments because roots
cannot grow without water, however, these experiments have been
misinterpreted as discussed in Hershey, D.R. 1992. Is hydrotropism all wet?
Science Activities 29(2):20-24.

Page 55 - Rhizomes are defined as horizontal underground stems not just
horizontal stems.

Page 55 - Phototropism is not defined as "the way a plant responds to light
by bending toward it." Phototropism is a directional growth response to a
directional light source. Phototropism can
be positive or negative, meaning the plant organ grows toward (+) or away
(-) from the light. Some vine stems, such as English ivy, are negatively
phototropic, which allows them to grow up walls.

Page 55 - "Where light hits a plant, it inhibits auxins" is incorrect. One
widely accepted theory is that light causes auxin migration to the dark side
of the stem. However, this is not always found so there is a great deal of
controversy on the explanation of phototropism. See Firn, R.D. 1990.
Phototropism. Journal of Biological Education 24:153-157 which provides
further information for teachers.

Page 55 - The explanation of apical dominance that "When this growing point
is cut (pinched off), the auxins concentrate in the other, lower areas, and
lateral branches begin to grow" is incorrect. The theory is that auxin
produced by the apical growing point flows down and inhibits the lateral
buds below it. When the
apex is removed, the flow of auxin stops and lateral buds are no longer
inhibited. Again, there is controversy about this theory - it is probably
too simplistic to be the full explanation. Salisbury and Ross discuss the
controversy in greater detail.

Page 65 - The experiment with a seed placed between a dry paper towel and a
wet paper towel does not demonstrate hydrotropism. A tropism means there has
to be a bending response. This experiment only shows that roots require
water to grow. To demonstrate hydrotropism, you would need to have a layer
of moist soil next to a layer of dry soil. If a root growing in the moist
soil grew into the dry soil and then bent back into the moist soil, that
would be hydrotropism. Also see comments for page 53.

Page 74 - "Using light energy, green plants can convert water and carbon
dioxide into the energy-rich simple sugar, glucose." is misleading since
many plants which photosynthesize are not green, e.g. coleus, purpleleaf
plum, 'Crimson King' maple. Say "plants". 

It would be desirable to modify the sentence keeping in mind the following
statements from Salisbury, F.B. and C.W. Ross 1985. Plant Physiology, 3rd
ed. Wadsworth, Belmont, CA. "In each of the three major pathways by which
CO2 is fixed, the principal leaf storage products accumulating in light are
usually sucrose and starch" ... "For all the mention made of glucose in
biology books, in plants, the majority of it is bound in polymers and other
compounds." 

Page 76 - "Many plants also change glucose into fats and proteins ..." This
is one of the few times when it should read "all" instead of many. The main
message is that all organic compounds in a plant are formed from  carbon
compounds that were products of photosynthesis plus mineral nutrients
absorbed from the soil.

Page 76 - "For example, plant stems grow toward a source of light" is
misleading as mentioned for page 55 because some stems grow away from the
light.

Page 77 - The leaf cross section is labeled incorrectly. Chloroplasts are
not visible. The cells labeled "chloroplasts" are the palisade layer cells. 

Page 77 - "A network of leaf veins carries water, nutrients, gases, ..." .
Omit "gases" since they mainly travel in and out through stomata, not in
liquid-filled veins.

Page 80 - "Don't forget to multiply by two." Scientists do not multiply by
two; they measure the area of leaves, one side only as the standard method.

Page 83, sidebar - Glucose cannot possibly be lost by transpiration, it does
not evaporate. Glucose is not the primary endproduct or even an intermediate
in photosynthesis, rather starch or sucrose is the major endproduct as
mentioned above under page 55. 

Page 86 - "Oxygen bubbles" in drawing are not pure oxygen as discussed in
Journal of Biological Education 24:66-67 (1990). 

Page 89 - "What do you imagine the world would be like if plant stems didn't
grow toward light?"  Not all stems do grow toward light as discussed above
under page 55.

Page 91 - "Leaves' green color indicates that they are absorbing energy from
all parts of the spectrum (red and blue in particular), with the exception
of green." This is a common misconception. Plants do absorb green light
(roughly 60%), they simply reflect and transmit a higher percentage of green
light (roughly 20% reflected) than other colors (only about 5-10% of other
colors reflected). Also, the human eye is much more sensitive to green light
than to red or blue. Another common misconception is that green light cannot
power photosynthesis. It can.

Page 91 - Should make clear that light striking a leaf has three possible
fates - absorption, reflection, and transmission (passes through leaf).

Page 91 - Greenhouse growers do not always control daylength for poinsettia.
Rather, poinsettia naturally flowers in December because the natural days
are short enough.

Page 91 - Again, phototropism misdefined.

Page 92 - Footcandles is an illumination unit, it measures light as
perceived by the human eye. This is much different than how plants respond
to light. Plant scientists measure light very differently, as numbers of
photons striking a unit area per unit time, in units of micromoles per
square meter per second. This is termed  PAR (photosynthetically active
radiation).

Page 95 - "What to expect" Again, a misconception since plants do absorb
green light and can use green light in photosynthesis (see page 91
discussion above).

Page 95, sidebar - This discussion does not combat the common misconception
that a short day is less than 12 hours and a long day is above 12 hours. To
avoid confusion, photoperiodism should be defined in terms of the critical
daylength for each species. A short day plant flowers when daylength is less
than the critical photoperiod. A long day plant flowers when daylength is
more than the critical photoperiod. The critical photoperiod for a long day
plant may be 9 hours, thus daylengths of 10 hours would cause flowering for
that long day plant. Also, the critical photoperiod of a short day plant
might be 12 hours, so a 10 hour
day would cause flowering in that short day plant. 

Page 96 - General directions for the use of a camera light meter to measure
footcandles will be inaccurate since it depends on the camera type. Many
different descriptions are in the literature.

Page 102 - Should indicate the scale on the soil particle drawing. Sand is 2
mm to 0.05 mm, silt 0.05 to 0.002 mm and clay less than 0.002 mm diameter.

Page 102 - "vermiculite provides good drainage and a texture lighter ..." 
In soil terminology, "texture" is defined as the percentage by weight of
sand, silt, and clay in a soil. The correct term here is "bulk density",
which is the dry mass of soil per unit volume, e.g. g/liter or pounds/cubic
foot.

Page 118 - "Without pollination and fertilization, fruit and seed production
cannot occur." This is often not true. Parthenocarpy is fruit formation
without pollination and fertilization; it occurs in naval
oranges, oriental persimmon, cultivated banana, etc.

Page 140 - "Auxins stimulate ... the production of leaves ... " Auxins do
stimulate roots but cytokinins stimulate shoots and leaves. 

Page 140 - "trees can be grafted to produce seedless oranges" This is
misleading. To be clear say "reproduce", not "produce", the latter indicates
that the process of grafting causes seedlessness.

Page 154 - The five kingdoms of life are discussed. Name them. e.g.
Prokaryotae, Protista, Fungi, Plantae, and Animalia. 

Page 155 - You say a hierarchy of eight but only list seven taxa. Provide an
example for both plant and animal:

Kingdom:  Plantae                                 Animalia
Phylum:   Spermatophyta (Seed bearing)  Phylum:   Chordata
Class:    Angiospermae (Seeds in ovary)           Mammalia
Subclass: Dicotyledonae (Seeds with 2 cotyledons) 
Order:    Rosales                                 Primates
Family:   Rosaceae (Rose family)                  Hominidae
Genus:    Malus                                   Homo
Species:  pumila                                  sapiens
Cultivar: 'Red Delicious'               Name:     Joe Smith

Include cultivar since this is so important in horticulture.

Page 164, sidebar - say cultivar, not variety. Horticulture uses cultivars.
Variety is a botanical term for wild plants.

Page 180, sidebar - Some of the example plants do not seem to be from
tropical rainforests, e.g. Digitalis purpurea (foxglove) is from the western
Mediterranean, and willows are mainly from cold temperate zones. 

Page 181 - "Some people estimate that 55 acres of rainforest are lost per
hamburger produced!" It may be true that "some people" have made such
estimates, but it is also true that it is a ridiculous estimate. Salisbury
and Ross gives a world tropical rainforest area of 17 million square km or
4.2 billion acres. At 55 acres per hamburger, destruction of all tropical
rainforests would produce only 76 million hamburgers which is less than one
day's consumption on a worldwide basis!

Page 181, sidebar - Give lawn transpiration in a depth (cm) which is easier
to conceptualize since rainfall is commonly expressed in such units. The
27,000 gallons per acre is the same as 2.5 cm or 1 inch on a depth basis.

Page 200, sidebar - Give the scientific name for flixweed (Descurainia
sophia). Also show how the calculation is made for flixweed population
expansion, i.e. , first year 1 plant, second year 730,000 plants, 3rd year
730,000 squared = 533 billion plants. It would also be clearer to
conceptualize if the number of flixweed plants per square meter of earth's
land area was calculated. 

Page 213 - Composting is presented as an environmentally good thing to do,
yet no mention was made that composting produces CO2 which contributes to
the greenhouse effect. In terms of the greenhouse effect, might it not be
better to seal waste organic material in landfills, where it does not
decompose?

Page 248, sidebar - Do not use "acidity and alkalinity" as synonyms for pH.
They have different meanings. The statement, "The term pH stands for the
percentage of hydrogen ions" is wrong. The term pH stands for the negative
logarthm of the hydrogen ion concentration.

Page 248 - "Both acids and bases can be either weak or strong." This is a
true statement, but is misleading since the specific definitions of strong
and weak are not given and do not refer to acid or base concentration. A
strong acid or base is completely dissociated in solution, e.g. HCl
dissociates into H+ and Cl-. A weak acid or base is only partially
dissociated, e.g. acetic acid occurs mainly as undissociated acetic acid in
solution. 

Page 252 - Do not use vinegar (acetic acid) because this is not what plants
are exposed to. Dilute concentrations of sulfuric or nitric acids, of the
levels in acid rain, are no more dangerous than
identical concentrations of acetic acid, otherwise, people would be injured
by acid rain. Using acetic acid makes the experiment totally unrealistic.

Page 256, life in fast lane sidebar - What does the "gee-whiz" figure of 5
tons of CO2 per person per year include? - just breathing or also auto use,
solid waste incineration, composting, domesticated food animals, etc. This
figure has little meaning without specifying the global total of CO2 in the
atmosphere (700 billion
metric tons, according to Salisbury and Ross)

Page 260, sidebar - Besides mechanical scarification, acid scarification is
also widely used and is often the preferred method as it is for honey locust
seed.

Page 279 - Table of CO2 increase is very wrong. CO2 was not less than 50 ppm
in 1860. Before the industrial revolution in the 1890s, the CO2 level was
about 290 ppm.

Page 287 - Graph of day vs. plant height has axis labels reversed. Height
should be on vertical axis, as on page 43.  


David Hershey
dh321 at excite.com







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