Woon Teck Yap
Section: M1-3, E53-220
Meeting 3 Out-of-class Exercise 2
1
Introduction
Artificial fertilization has long been employed as a method of producing new colour
variations in ornamental plants. It has been observed that whenever members of the same
species were cross-fertilized, particular hybrid forms appeared with astounding
consistency and regularity.
In an attempt to elucidate the mechanism behind this
phenomenon, experiments involving the hybrids and their progeny were undertaken and
are described herein.
Before this, many other researchers such as Kölreuter, Gärtner, Herbert, Lecoq and
Wichura have spent much time trying to determine the mechanism of inheritance behind
hybrid reproduction. In particular, Gärtner has made some very interesting and valuable
observations in Die Bastarderzeugung im Pflanzenreiche.
In addition, Wichura has
recently published some results regarding his research on the hybrids of the Willow. Due
to the fact that research on hybrids is time-consuming, laborious and difficult and that
observing them requires a large span of time (several years or more), no one has yet come
up with a suitable working model or “universal law” to describe the formation and
development of the botanical hybrids. Such a model can be formulated if and only if
detailed experiments have been made on a variety of plant species.
At the moment, there are no published records of any experiments that have been
adequately and satisfactorily carried out to determine the number of different forms of
hybrid progeny and the separate generations in which these progeny occur. In addition,
statistical analyses of the relationships between the various forms of hybrid progeny have
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Woon Teck Yap
Section: M1-3, E53-220
Meeting 3 Out-of-class Exercise 2
not yet been performed. Such detailed theoretical and experimental data is required
before one can answer any questions regarding the history and formation of any living
species on Earth.
This paper describes the results and conclusions of such a detailed experiment on a small
plant group, which was observed carefully and painstakingly over a period of 8 years. It
is left to the reader’s judgement as to whether the experiments were suitably planned,
conducted and executed, with regarding to obtaining the desired results and detailed
analyses.
2
Materials and Methods
2.1
Growth of Pea Plants
All pea plants were either grown in garden beds or pots. Pea plants were maintained in
their natural upright position by means of sticks, branches of trees or string stretched
between them. For each particular experiment, i.e. set of cross-fertilization experiments,
a specific number of potted pea plants were placed during the blooming period in a
greenhouse to serve as controls for the determination of the effects of any possible
disturbance by insects, i.e. unwanted, random cross fertilization by insects.
2.2
Selection of Suitable Pea Plants
All experimental plants described in this paper are of the genus Pisum, possessed
constant differentiating characteristics and produced hybrids and offspring that showed
no marked decrease in fertility in successive generations. All experimental plants were
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Woon Teck Yap
Section: M1-3, E53-220
Meeting 3 Out-of-class Exercise 2
derived from 34 more or less distinct varieties of peas obtained from several seedsmen.
These peas were grown and the resulting pea plants were then subjected to a 2-year trial,
in which the characteristics of the latter were carefully observed. 22 of these pea plants
that exhibited highly similar characteristics were then selected and cultivated for the
entire 2-year period. All pea plants used in subsequent experiments were then derived
from these 22 plants.
2.3
Characteristics of Pea Plants Examined
7 different characteristics that were easy to follow and classify were chosen for careful
observation in the pea plants that were cultivated and are listed below:
1. Surface morphology of ripe seeds: Seeds were either classified are round and smooth
or irregularly angular and deeply wrinkled. Seeds that were classified as round and
smooth were either round or roundish and any depressions that occurred on the
surface were always shallow.
2. Colour of the seed albumen (endosperm): The colour of the endosperm of the ripe
seeds fell into either one of two categories – green or yellow. The endosperm was
classified as green so long as it possessed a more or less green tint while it was
classified as yellow if it was either pale yellow, bright yellow or orange-coloured.
3. Colour of the seed-coat: The colour of the seed-coat fell into either one of two
categories – white or grey, grey-brown, leather-brown (with or without violet
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Woon Teck Yap
Section: M1-3, E53-220
Meeting 3 Out-of-class Exercise 2
spotting). White seeds coats were constantly correlated with white flowers being
observed in the pea plants while grey, grey-brown and leather-brown seed coats
constantly correlated with flowers in which the standards were violet and the wings
were purple. In addition, the axils of the leaves of these latter pea plants possessed a
reddish tint. All grey seed-coats, upon boiling in water, turned dark brown.
4. Morphology of ripe seed pods: Seed pods were either inflated and not contracted in
any places or more or less wrinkled and deeply constricted in the areas between the
seeds.
5. Colour of unripe seed pods: Unripe seed pods were either light to dark green or
vividly and intensely yellow. For yellow unripe seed pods, the stalks, leaf-veins and
calyx of the corresponding pea plants were also coloured yellow.
6. Position of flowers: Flowers were either classified as axial or terminal. Axial refers
to flowers being distributed along the main stem while terminal means that the
flowers were bunched at the top of the stem and arranged in a false umbel, in which
the upper part of the stem was more or less widened in cross-section.
7. Length of stem: The length of the stem was either categorized as long or short. Long
stems refer to those in which the long axis was around 6 to 7 feet while short stems
refer to those in which the long axis was around 0.75 to 1.5 feet.
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Woon Teck Yap
Section: M1-3, E53-220
Meeting 3 Out-of-class Exercise 2
2.4
Artificial Cross-Fertilization
Artificial cross-fertilization between two distinct pea plants was achieved by the careful
opening of the bud before it fully developed into a flower. The keel of the bud was then
removed and each stamen was carefully extracted by means of forceps so as to prevent its
pollen from falling on the stigma present in the same bud. The stigma was subsequently
dusted over with foreign pollen obtained from a pea plant exhibiting the other constant
differentiating character under investigation.
2.5
Production and Propagation of Hybrids
Each pair of differentiating characters described above (Refer to Section 2.3) were united
into one pea plant by artificial cross-fertilization (Refer to Section 2.4) of two pea plants
which exhibited constant differentiation in one of the two distinct forms of a particular
character.
In total, 7 experiments were carried out in order to unite the 7 different characters under
investigation.
The details of these 7 experiments are listed in Table 1.
Fertilization/cross-fertilization was carried out on only the most vigorous members of a
large number of pea plants or the same variety. In experiments involving reciprocal
crossings, if a particular pea plant served as the seed-bearer in one set of fertilizations,
then it was used to provide pollen in the other set of fertilizations.
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Woon Teck Yap
Section: M1-3, E53-220
Meeting 3 Out-of-class Exercise 2
Table 1: Details of the 7 experiments in which the 2 different forms of a particular
character are united in the same plant by cross-fertilization.
Experiment / Experimental
Set
1
2
3
4
5
6
7
2.6
Number of fertilizations
60
58
35
40
23
34
37
Number of plants
fertilized
15
10
10
10
5
10
10
Production of First Generation from Cross-Fertilization of Hybrids
The hybrids produced by the artificial cross-fertilization of parental pea plants with
constant differentiating characters (Refer to Section 2.5) were then artificially crossfertilized with other hybrids within the same experimental set, as described in Section 2.4.
2.7
Production of Second Generation from First Generation
Seeds from first generation pea plants resulting from the artificial cross-fertilization of
the hybrids (Refer to Section 2.6) were grown and their offspring were meticulously
observed, with respect to the 7 differentiating characters under investigation.
3
Results
3.1
Characteristics of Hybrids – Dominant Forms of Differentiating Characters
For all 7 of the crosses that had been performed (Refer to Sections 2.3 and 2.4), it has
been observed that with respect to a specific form of a particular differentiating character,
the hybrids closely resembled one of the parental pea plants to such an extent that both
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Woon Teck Yap
Section: M1-3, E53-220
Meeting 3 Out-of-class Exercise 2
the hybrid and the parental pea plant were virtually indistinguishable. In fact, the other
form of exhibited by the other parental pea plant was never observed or rarely observed
with certainty. As such, the particular form of an observed differentiating character that
is exhibited by the hybrid and one of its parents is termed dominant while the observed
form exhibited by the other parent is term recessive. A list of the 7 differentiating
characters investigated and their respective dominant forms is presented in Table 2.
Table 2: List of 7 differentiating characters of the pea plants investigated and their
corresponding dominant forms.
Experiment
1
2
3
4
5
6
7
3.2
Differentiating Character
Dominant Form
Surface morphology of ripe seeds: Round Round and smooth, with and
and smooth or angular and wrinkled
without shallow depressions
Colour of seed albumen (endosperm):
Yellow
Green or yellow
Colour of seed coat: White with white
Grey/grey-brown/leatherflowers or grey/grey-brown/leather-brown
brown with violet-red
with purple flowers
blossoms and reddish spots
in leaf axils
Morphology of ripe seed pods: Inflated or
Simple inflated
wrinkled
Colour of unripe seed pods: Green or
Green
yellow with similarly-coloured stems,
leaf-veins and calyx
Position of flowers: Axial or terminal
Axial
Length of stem: Long (6 – 7 feet) or short
Long
(0.75 – 1.5 feet)
Characteristics of the First Generation from Cross-Fertilization of Hybrids
The first generation of pea plants obtained from the cross-fertilization of hybrids as
described in Section 2.6 were subjected to careful observation, with respect to the 7
differentiating characters under investigation. It was noted that in this generation, there
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Woon Teck Yap
Section: M1-3, E53-220
Meeting 3 Out-of-class Exercise 2
appeared both kinds of pea plants i.e. those with the dominant form of the differentiating
character under investigation and those with the corresponding recessive form. It was
also fairly obvious that the number of pea plants exhibiting the dominant form of a
particular character to that exhibiting the corresponding recessive form was a 3:1 ratio.
In addition, transitional forms were not observed in any experiments, confirming the
hypothesis that one form is clearly dominant to the other.
The results of these
observations are summarized in Table 3.
Table 3: Summary of results obtained from the cross-fertilization of hybrids within the
same experimental set, with respect to the differentiating character under investigation,
the number of hybrids observed, the specific plant part observed and the total number of
such observation, the dominant and recessive forms of the specific character observed
and their respective numbers and ratios.
Expt
Differentiating
Character
1
Seed surface
morphology
Colour of
albumen
Colour of seedcoats
2
3
Number
Plant
of
Part /
Hybrids Number
253
Seed /
7324
258
Seed /
8023
929
Seed / -
1181
Pod / -
580
Pod / -
6
Pod
morphology
Colour of
unripe pods
Flower position
858
7
Length of stem
1064
Flower /
Stem / -
4
5
Dominat
Form /
Number
Round /
5474
Yellow /
6022
Greybrown /
705
Inflated /
882
Green /
428
Axial /
651
Long /
787
Recessive
Form /
Number
Angular /
1850
Green /
2001
White / 224
Ratio
Dominant:
Recessive
2.96:1
Constricted
/ 299
Yellow /
152
Terminal /
207
Short / 277
2.95:1
3.01:1
3.15:1
2.82:1
3.14:1
2.84:1
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Woon Teck Yap
Section: M1-3, E53-220
Meeting 3 Out-of-class Exercise 2
The average ratio of dominant forms to recessive forms was found to be 2:98:1, being
closely approximated by a 3:1 ratio.
3.3
Characteristics of the Second Generation Grown from the Seeds of First
Generation Pea Plants
It was observed that those pea plants from the first generation that exhibited the recessive
forms of the differentiating characters produced offspring that consistently exhibited the
recessive forms. In addition, it was noticed that for the pea plants which exhibited the
dominant forms in the first generation, ⅓ of their offspring showed constant transmission
of the dominant forms. The remaining ⅔ of their offspring gave rise to progeny that
displayed the dominant and recessive forms of the differentiating characters in the
familiar 3:1 ratio. Table 4 shows the results obtained following the procedure described
in Section 2.7 for the pea plants exhibiting the dominant forms of the 7 characters.
Table 4:
Expt
Type of Seed/Plant
Used
1
2
Round seed
Yellow albumen
seed
Seed with greybrown seed coat
Plant with inflated
pods
Plant with green
pods
Plant with axial
flowers
Tall plant
3
4
5
6
7
Number of
Plants/Seeds
Used
565
519
Number of
Plants with Both
Forms
372
353
Number of
Plants with only
Dominant Form
193
166
Ratio
1.93:1
2:13:1
100
64
36
1.78:1
100
71
29
2.45:1
100
60
40
1.5:1
100
67
33
2.03:1
100
72
28
2.57:1
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Woon Teck Yap
Section: M1-3, E53-220
Meeting 3 Out-of-class Exercise 2
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