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Species and Varieties, Their Origin by Mutation

H >> Hugo DeVries >> Species and Varieties, Their Origin by Mutation




By this method I brought my strain within two years up to an average of
nearly 90% of the seedlings with a divided primary leaf. Around this
average the real numbers fluctuated between the maximum of 99% and the
minimum of 70% or thereabouts. This condition was reached by the sixth
generation in the year 1894, and has since proved to be the limit, the
group of figures remaining practically the same during all the
succeeding generations.

Such selected plants are very rich in leaves with four, five and six
blades. Excluding the small leaves at the tops of the branches, and
those on the numerous weaker side-branches, these three groups include
the large majority of all the stronger leaves. In summer the range is
wider, and besides many trifoliolate leaves the curiously shaped
seven-bladed ones are not at all rare. In the fall and in the winter the
range of variability is narrowed, and at first sight the plants often
seem to bear only quinquefoliolate leaves.

[350] I have cultivated a new generation of this race nearly every year
since 1894, using always the strictest selection. This has led to a
uniform type, but has not been adequate to produce any further
improvement. Obviously the extreme limit, under the conditions of
climate and soil, has been reached. This extreme type is always
dependent upon repeated selection. No constant variety, in the older
sense, has been obtained, nor was any indication afforded that such a
type might ever be produced. On the contrary, it is manifest that the
new form belongs to the group of ever-sporting varieties. It is never
quite free from the old atavistic type of the trifoliolate leaves, and
invariably, when external conditions become less favorable, this
atavistic form is apt to gain dominion over the more refined varietal
character. Reversions always occur, both partial and individual.

Some instances of these reversions may now be given. They are not of
such a striking character as those of the snapdragon. Intermediate steps
are always occurring, both in the leaves themselves, and in the
percentages of deviating seedlings of the several parent plants.

On normal plants of my variety the quinquefoliolate leaves usually
compose the majority, when there are no weak lateral branches, or when
they are left out of consideration. Next [351] to these come the fours
and the sixes, while the trifoliolate and seven-bladed types are nearly
equal in number. But out of a lot of plants, grown from seed of the same
parent, it is often possible to choose some in which one extreme
prevails, and others with a preponderating number of leaves with the
other extreme number of leaflets. If seed from these extremes are saved
separately, one strain, that with numerous seven-bladed leaves will
remain true to the type, but the other will diverge more or less,
producing leaves with a varying number of subdivisions.

Very few generations of such opposite selection are required to reduce
the race to an utterly poor one. In three years I was able to nearly
obliterate the type of my variety. I chose the seedlings with an
undivided primary leaf, cultivated them and counted their offspring
separately after the sowing. I found some parents with only 2-3% of
seedlings with divided primary leaves. And by a repeated selection in
this retrograde direction I succeeded in getting a great number of
plants, which during the whole summer made only very few leaves with
more than three blades. But an absolute reversion could no more be
reached in this direction than in the normal one. Any sowing without
selection would be [352] liable to reduce the strain to an average
condition.

The production of varietal and of atavistic leaves is dependent to a
high degree on external conditions. It agrees with the general rule,
that favorable circumstances strengthen the varietal peculiarities,
while unfavorable conditions increase the number of the parts with the
atavistic attribute. These influences may be seen to have their effect
on the single individuals, as well as on the generations growing from
their seed. I cannot cite here all the experimental material, but a
single illustrative example may be given. I divided a strong individual
into two parts, planted one in rich soil and the other in poor sand, and
had both pollinated by bees with the pollen of some normal individuals
of my variety growing between them. The seeds of both were saved and
sown separately, and the two lots of offspring cultivated close to each
other under the same external conditions. In the beginning no difference
was seen, but as soon as the young plants had unfolded three or four
leaves, the progeny of the better nourished half of the parent plant
showed a manifest advance. This difference increased rapidly and was
easily seen in the beds, even before the flowering period.

This experience probably gives an explanation [353] why the
quinquefoliolate variety is so seldom met with in the wild state. For
even if it did occur more often, the plants would hardly find
circumstances favorable enough for the full development of their
varietal character. They must often be so poor in anomalous leaves as to
be overlooked, or to be taken for instances of the commonly occurring
quadrifoliolate leaves and therefore as not indicating the true variety.

In the beginning of my discussion I have asserted the existence of two
different races of "four-leaved" clovers, a poor one and a rich one, and
have insisted on a sharp distinction between them. This distinction
partly depends on experiments with clover, but in great part on tests
with other plants. The previously mentioned circumstance, that clover
cannot be pollinated on a sufficiently large scale otherwise than by
insects, prevents trials in more than one direction at the same time and
in the same garden. For this reason I have chosen another species of
clover to be able to give proof or disproof of the assertion quoted.

This species is the Italian, or crimson clover, which is sometimes also
called scarlet clover (_Trifolium incarnatum_). It is commonly used in
Europe as a crop on less fertile soils than are required by the red
clover. It is annual [354] and erect and more or less hairy, and has
stouter leaves than other kinds of clover. It has oblong or cylindrical
heads with bright crimson flowers, and may be considered as one of the
most showy types. As an annual it has some manifest advantages over the
perennial species, especially in giving its harvest of hay at other
seasons of the year.

I found some stray quaternate leaves of this plant some years ago, and
tried to win from them, through culture and selection, a race that would
be as rich in these anomalies as the red clover. But the utmost care and
the most rigid selection, and all the attention I could afford, failed
to produce any result. It is now ten years since I commenced this
experiment, and more than once I have been willing to give it up. Last
year (1903) I cultivated some hundreds of selected plants, but though
they yielded a few more instances of the desired anomaly than in the
beginning, no trace of a truly rich race could be discovered. The
experimental evidence of this failure shows at least that stray
"four-leaves" may occur, which do not indicate the existence of a true
"four-" or "five-leaved" variety.

This conception seems destined to become of great value in the
appreciation of anomalies, as they are usually found, either in the wild
state [355] or in gardens. And before describing the details of my
unsuccessful pedigree-culture, it may be as well to give some more
instances of what occurs in nature.

Stray anomalies are of course rare, but not so rare that they might not
be found in large numbers when perseveringly sought for. Pitcher-like
leaves may be found on many trees and shrubs and herbs, but ordinarily
one or only two of them are seen in the course of many years on the same
plant, or in the same strain. In some few instances they occur annually
or nearly so, as in some individuals of the European lime-tree (_Tilia
parvifolia_) and of the common magnolia (_Magnolia obovata_). Many of
our older cultivated plants are very rich in anomalies of all kinds, and
_Cyclamen_, _Fuchsia_, _Pelargonium_ and some others are notorious
sources of teratologic phenomena. Deviations in flowers may often be
seen, consisting of changes in the normal number of the several organs,
or alterations in their shape and color. Leaves may have two tips,
instead of one, the mid-vein being split near the apex, and the fissure
extending more or less towards the base. Rays of the umbels of
umbelliferous plants may grow together and become united in groups of
two or more, and in the same way the fruits of [356] the composites may
be united into groups. Many other instances could easily be given.

If we select some of these anomalies for breeding-experiments, our
results will not agree throughout, but will tend to group themselves
under two heads. In some cases the isolation of the deviating
individuals will at once show the existence of a distinct variety, which
is capable of producing the anomaly in any desired number of instances;
only dependent on a favorable treatment and a judicious selection. In
other cases no treatment and no selection are adequate to give a similar
result, and the anomaly remains refractory despite all our endeavors to
breed it. The cockscomb and the peloric fox-glove are widely known
instances of permanent anomalies, and others will be dealt with in
future lectures. On the other hand I have often tried in vain to win an
anomalous race from an accidental deviation, or to isolate a teratologic
variety out of more common aberrations. Two illustrative examples may be
quoted. In our next lecture we shall deal with a curious phenomenon in
poppies, consisting in the change of the stamens into pistils and giving
rise to a bright crown of secondary capsules around the central one.
Similar anomalies may be occasionally met with in other species of the
same genus. But they are rare, and may show [357] the conversion of only
a single stamen in the described manner. I observed this anomaly in a
poppy called _Papaver commutatum_, and subjected it during several years
to a rigid selection of the richest individuals. No amelioration was to
be gained and the culture had to be given up. In the same way I found on
the bulbous buttercup (_Ranunculus bulbosus_) a strain varying largely
in the number of the petals, amounting often to 6-8, and in some flowers
even yet to higher figures. During five succeeding years I cultivated
five generations, often in large numbers, selecting always those which
had the highest number of petals, throwing out the remainder and saving
the seed only from the very best plants. I got a strain of selected
plants with an average number of nine petals in every flower, and found
among 4,000 flowers four having 20 petals or more, coming up even to 31
in one instance. But such rare instances had no influence whatever on
the selection, since they were not indicative of individual qualities,
but occurred quite accidentally on flowers of plants having only the
average number of petals. Now double flowers are widely known to occur
in other species of the buttercups, both in the cultivated varieties and
in some wild forms. For this reason it might be expected that through a
continuous selection of [358] the individuals with the largest numbers a
tendency to become double would be evolved. Such, however, was not the
case. No propensity to vary in any definite direction could be observed.
Quite on the contrary, an average condition was quickly reached, and
then remained constant, strongly counteracting all selection.

Such experiences clearly show that the same anomaly may occur in
different species, and no doubt in strains of the same species from
different localities, according to at least two different standards. The
one is to be called the poor, and the other the rich variety. The first
always produces relatively few instances of the deviation, the last is
apt to give as many of them as desired. The first is only half-way a
variety, and therefore would deserve the name of a half-race; the second
is not yet a full constant variety, but always fluctuates to and fro
between the varietal and the specific mark, ever-sporting in both
directions. It holds a middle position between a half-race and a
variety, and therefore might be called a "middle-race." But the term
ever-sporting variety seems more adequate to convey a right idea of the
nature of this curious type of inheritance.

From this discussion it will be seen that the behavior of the crimson
clover is not to be considered [359] as an exception, but as a widely
occurring type of phenomenon, occurring perhaps in all sorts of
teratologic deviations, and in wide ranges of species and genera. Hence
it may be considered worth while to give some more details of this
extended experiment.

Ten years ago (1894-5) I bought and sowed about a pound of seed of the
crimson clover. Among many thousands of normal seedlings I found two
with three and one with four cotyledons. Trusting to the empirical rules
of correlation, I transplanted these three individuals in order to
isolate them in the flowering period.

One of them produced during the ensuing summer one four-bladed and one
five-bladed leaf. The seeds were saved separately and sown the following
spring and the expected result could soon be seen. Among some 250
individual plants I counted 22 with one or two deviations, and 10 with
from three to nine four- or five-bladed leaves. Proportions nearly
similar have been observed repeatedly. Better nourished individuals have
produced more deviating leaves on one plant, partly owing to the larger
number of stems and branches, and poor or average specimens have mostly
been without any aberration or with only one or two abnormal leaves. No
further improvement could be attained. Quadrifoliolate leaves were
always rare, never [360] attaining a number that would put its stamp on
a whole bed. I have endeavored to get some six- and seven-bladed crimson
clover leaves, but in vain; selection, culture of many hundreds of
individuals, manure, and the best possible treatment has not been
adequate to produce them. Of course I am quite convinced that a
repetition of my experiment on a far larger scale would yield the
desired types, but then only in such rare instances that they would have
no influence whatever on the average, or on the improvement of the race.
The eighth generation in the year 1903 has not been noticeably better
than the second and third generations after the first selection.

In comparing this statement with the results gained in the experiment
with the red clover, the difference is at once striking. In one case a
rich variety was isolated, and, by better treatment and sharp methods of
selection, was brought up in a few years to its highest pitch of
development. In the other case a very weak race was shown to exist, and
no amount of work and perseverance was adequate to improve it to any
noticeable degree.

I wish to point out that the decision of what is to be expected from
deviating specimens may become manifest within one or two generations.
Even the generation grown from the seeds of [361] the first observed
aberrant-individuals, if gathered after sufficient isolation during the
period of blossoming, may show which type of inheritance is present,
whether it is an unpromising half-race, or a richly endowed sporting
variety. I have kept such strains repeatedly after the first isolation,
and a special case, that of cotyledoneous aberrations, will be dealt
with later. The first generation always gave a final decision, provided
that a suitable method of cultivation for the species under observation
was found at the beginning. This however, is a condition, which it is
not at all easy to comply with, when new sorts are introduced into a
garden. Especially so when they had been collected in the wild state.
Often one or two years, sometimes more, are necessary to find the proper
method of sowing, manuring, transplanting and, other cultural methods
satisfactory to the plants. Many wild species require more care and more
manure in gardens than the finest garden flowers. And a large number are
known to be dependent on very particular conditions of soil.

One of the most curious features of anomalies, which has been learned
from accumulated instances, is the fact that they obey definite laws as
to their occurrence on the different parts of the plant. Obviously such
laws are [362] not apparent as long as each plant produces only one or
two, or, at most, a few instances of the same deviation. On the
contrary, any existing regularity must betray itself, as soon as a
larger number of instances is produced. A rule of periodicity becomes
most clearly manifest in such cases.

This rule is shown by no other race in a more undoubted and evident
manner than by the "five-leaved" clover. Evidently the several degrees
of deviation, going from three to seven leaflets, may be regarded as
responses to different degrees of variation, and their distribution over
the stems and branches, or over the whole plant, may be considered as
the manifestation of the ever-changing internal tendency to vary.

Considered from this point of view, my plants always showed a definite
periodicity in this distribution, which is the same for the whole plant.
Each of them, and each of the larger branches, begin with atavistic
leaves or with slight deviations. These are succeeded by greater
deviations, but only the strongest axes show as many as seven leaflets
on a stalk. This ordinarily does not occur before the height of
development is reached, and often only towards its close. Then the
deviation diminishes rapidly, returning often to atavistic leaves at the
summit of the stem or branch. I give the numbers of the [363] leaves of
a branch, in their order from the base to the top. They were as follows:

3. 4. 5. 6. 7. 5. 5. 4.

But this is a selected case, and such regular examples of the expected
periodicity are rarely found. Often one or more of the various steps are
lacking, or even leaves with smaller numbers may be interspersed among
those with larger numbers of leaflets. But while the regularity of the
periodicity is in some degree diminished by such occurrences, yet the
rule always holds good, when taken broadly. It may be expressed by
stating that the bases and apices have on the average fewer leaflets on
each leaf than the middle parts of the stem and branches, and that the
number of leaflets gradually increases from the base toward a maximum,
which is reached in organs on the middle or upper part of the axis, and
then diminishes from this toward the apex.

This periodicity is not limited to the stems and branches, considered
singly, but also holds good in a comparison made between the branches of
a single stem, in regard to their relative places on that stem. So it is
also for the whole plant. The first stems, produced by the subterranean
axis, ordinarily show only a low maximum deviation: the next succeeding
being [364] more divergent and the last ones returning to less
differentiated forms.

It is evident that on a given stem the group of deviating leaves will be
extended upward and downward, with the increase of the number of these
organs. This shows that a stem, or even a plant, promises a higher
degree of differentiation if it commences with its aberration earlier.
Hence it becomes possible to discern the most promising individuals in
early youth, and this conclusion leads to a very easy and reliable
method of selection, which may be expressed simply as follows: the
seedlings which commence earliest with the production of four- and
five-foliolate leaves are the best and should be selected for the
continuance of the race. And it is easily seen that this rule agrees
with that given above, and which was followed in my pedigree-culture.

Furthermore it is seen that there is a complete agreement between the
law of periodicity and the responses of the deviations to nourishment
and other conditions of life. Weak plants only produce low degrees of
deviation, the stronger the individual becomes, the higher it reaches in
the scale of differentiation, and the more often it develops leaves with
five or more blades. Whether weakness or strength are derived from outer
causes, or from the internal [365] succession of the periods of life, is
evidently of no consequence, and in this way the law of periodicity may
be regarded as a special instance of the more general law of response to
external conditions.

The validity of this law of periodicity is of course not limited to our
"five-leaved" clover. Quite on the contrary it is universal in
eversporting varieties. Moreover it may be ascertained and studied in
connection with the most widely different morphologic abnormalities, and
therefore affords easily accessible material for statistical inquiry. I
will now give some further instances, but wish to insist first upon the
necessity of an inquiry on a far larger scale, as the evidence as yet is
very scanty.

The great celandine (_Chelidonium majus_) has a very curious double
variety. Its flowers are simpler and much more variable than in ordinary
garden-varieties. The process of doubling consists mainly in a change of
stamens into petals. This change is dependent on the season. On each
stem the earliest flowers are single. These are succeeded by blossoms
with one or two converted stamens, and towards the summer this number
increases gradually, attaining 10-11 and in some instances even more
altered filaments. Each year the same succession may be seen repeating
itself on the stems of [366] the old roots. Double tuberous begonias are
ordinarily absolutely sterile throughout the summer, but towards autumn
the new flowers become less and less altered, producing some normal
stamens and pistils among the majority of metamorphosed organs. From
these flowers the seeds are saved. Sometimes similar flowers occur at
the beginning of the flowering-period. Double garden-camomiles
(_Chrysanthemum inodorum plenissimum_) and many other double varieties
of garden-plants among the great family of the composites are very
sensitive to external agencies, and their flower-heads are fuller the
more favorable the external conditions. Towards the autumn many of them
produce fewer and fewer converted heads and often only these are fertile
and yield seeds.

Ascidia afford another instance of this periodicity, though ordinarily
they are by far too rare to show any regularity in their distribution.
However, it is easy to observe that on lime-trees they prefer the lower
parts of each twig, while on magnolias the terminal leaves of the
branches are often pitcher-bearing. Ascidia of the white clover have
been found in numbers, in my own experiment-garden, but always in the
springtime. The thickleaved saxifrage (_Saxifraga crassifolia_) is often
very productive of ascidia, especially in [367] the latter part of the
season, and as these organs may be developed to very different degrees,
they afford fine material for the study of the law of periodicity. On a
garden-cytisus (_Cytisus candicans attleyanus_) I once had the good
fortune to observe a branch with ascidia, which ordinarily are very rare
in this species. It had produced seven ascidia in all, each formed by
the conversion of one leaflet on the trifoliolate leaves. The first six
leaves were destitute of this malformation and were quite normal. Then
followed a group of five leaves, constituting the maximum of the period.
The first bore one small pitcher-like blade, the second and third, each
one highly modified organ, the fourth, two ascidia, and the last, one
leaflet with slightly connate margins. The whole upper part of the
branch was normal, with the exception of the seventeenth leaf, which
showed a slight change in the same direction. All in all, the tendency
to produce ascidia increased from the beginning to the tenth leaf, and
decreased from this upward.

The European Venus' looking-glass was observed in my garden to produce
some quaternate and some quinate flowers on the same specimens. The
quinate were placed at the end of the branches, those with four petals
and sepals lower down. The peloric fox-glove shows the [368] highest
degree of metamorphy in the terminal flowers of the stem itself, the
weaker branches having but little tendency towards the formation of the
anomaly. The European pine or _Pinus sylvestris_ ordinarily has two
needles in each sheath, but trifoliolate sheaths occur on the stems and
stronger branches, where they prefer, as a rule, the upper parts of the
single annual shoots. _Camellia japonica_ is often striped in the fall
and during the winter, but when flowering in the spring it returns to
the monochromatic type.

Peloric flowers are terminal in some cases, but occur in the lower parts
of the flower-spikes in others. Some varieties of gladiolus commence on
each spike with more or less double flowers, which, higher up, are
replaced by single ones. A wide range of bulbs and perennial
garden-plants develop their varietal characters only partly when grown
from seed and flowering for the first time. The annual
garden-forget-me-not of the Azores (_Myosotis azorica_) has a variety
with curiously enlarged flowers, often producing 20 or more
corolla-segments in one flower. But this number gradually diminishes as
the season advances. It would be quite superfluous to give further proof
of the general validity of the law of periodicity in ever-sporting
varieties.

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