By G. Mendel.
(_Communicated to the Meeting 9 June, 1869[50]._)
[50] [Published in _Verh. naturf. Ver. Brünn, Abhandlungen_, VIII.
1869, p. 26, which appeared in 1870.]
Although I have already undertaken many experiments in fertilisation
between species of _Hieracium_, I have only succeeded in obtaining the
following 6 hybrids, and only from one to three specimens of them.
_H. Auricula_ ♀ × _H. aurantiacum_ ♂
_H. Auricula_ ♀ × _H. Pilosella_ ♂
_H. Auricula_ ♀ × _H. pratense_ ♂
_H. echioides_[51] ♀ × _H. aurantiacum_ ♂
_H. præaltum_ ♀ × _H. flagellare_ Rchb. ♂
_H. præaltum_ ♀ × _H. aurantiacum_ ♂
[51] The plant used in this experiment is not exactly the typical _H.
echioides_. It appears to belong to the series transitional to _H.
præaltum_, but approaches more nearly to _H. echioides_ and for this
reason was reckoned as belonging to the latter.
The difficulty of obtaining a larger number of hybrids is due to the
minuteness of the flowers and their peculiar structure. On account of
this circumstance it was seldom possible to remove the anthers from
the flowers chosen for fertilisation without either letting pollen
get on to the stigma or injuring the pistil so that it withered away.
As is well known, the anthers are united to form a tube, which closely
embraces the pistil. As soon as the flower opens, the stigma, already
covered with pollen, protrudes. In order to prevent self-fertilisation
the anther-tube must be taken out before the flower opens, and for this
purpose the bud must be slit up with a fine needle. If this operation
is attempted at a time when the pollen is mature, which is the case two
or three days before the flower opens, it is seldom possible to prevent
self-fertilisation; for with every care it is not easily possible to
prevent a few pollen grains getting scattered and communicated to
the stigma. No better result has been obtained hitherto by removing
the anthers at an earlier stage of development. Before the approach
of maturity the tender pistil and stigma are exceedingly sensitive
to injury, and even if they are not actually injured, they generally
wither and dry up after a little time if deprived of their protecting
investments. I hope to obviate this last misfortune by placing the
plants after the operation for two or three days in the damp atmosphere
of a greenhouse. An experiment lately made with _H. Auricula_ treated
in this way gave a good result.
To indicate the object with which these fertilisation experiments were
undertaken, I venture to make some preliminary remarks respecting the
genus _Hieracium_. This genus possesses such an extraordinary profusion
of distinct forms that no other genus of plants can compare with it.
Some of these forms are distinguished by special peculiarities and
may be taken as type-forms of species, while all the rest represent
intermediate and transitional forms by which the type-forms are
connected together. The difficulty in the separation and delimitation
of these forms has demanded the close attention of the experts.
Regarding no other genus has so much been written or have so many and
such fierce controversies arisen, without as yet coming to a definite
conclusion. It is obvious that no general understanding can be arrived
at, so long as the value and significance of the intermediate and
transitional forms is unknown.
Regarding the question whether and to what extent hybridisation plays
a part in the production of this wealth of forms, we find very various
and conflicting views held by leading botanists. While some of them
maintain that this phenomenon has a far-reaching influence, others, for
example, Fries, will have nothing to do with hybrids in _Hieracia_.
Others take up an intermediate position; and while granting that
hybrids are not rarely formed between the species in a wild state,
still maintain that no great importance is to be attached to the fact,
on the ground that they are only of short duration. The [suggested]
causes of this are partly their restricted fertility or complete
sterility; partly also the knowledge, obtained by experiment, that in
hybrids self-fertilisation is always prevented if pollen of one of
the parent-forms reaches the stigma. On these grounds it is regarded
as inconceivable that _Hieracium_ hybrids can constitute and maintain
themselves as fully fertile and constant forms when growing near their
progenitors.
The question of the origin of the numerous and constant intermediate
forms has recently acquired no small interest since a famous
_Hieracium_ specialist has, in the spirit of the Darwinian teaching,
defended the view that these forms are to be regarded as [arising] from
the transmutation of lost or still existing species.
From the nature of the subject it is clear that without an exact
knowledge of the structure and fertility of the hybrids and the
condition of their offspring through several generations no one
can undertake to determine the possible influence exercised by
hybridisation over the multiplicity of intermediate forms in
_Hieracium_. The condition of the _Hieracium_ hybrids in the range
we are concerned with must necessarily be determined by experiments;
for we do not possess a complete theory of hybridisation, and we
may be led into erroneous conclusions if we take rules deduced from
observation of certain other hybrids to be Laws of hybridisation, and
try to apply them to _Hieracium_ without further consideration. If by
the experimental method we can obtain a sufficient insight into the
phenomenon of hybridisation in _Hieracium_, then by the help of the
experience which has been collected respecting the structural relations
of the wild forms, a satisfactory judgment in regard to this question
may become possible.
Thus we may express the object which was sought after in these
experiments. I venture now to relate the very slight results which I
have as yet obtained with reference to this object.
1. Respecting the structure of the hybrids, we have to record the
striking phenomenon that the forms hitherto obtained by similar
fertilisation are not identical. The hybrids _H. præaltum_ ♀ x _H.
aurantiacum_ ♂ and _H. Auricula_ ♀ x _H. aurantiacum_ ♂ are each
represented by two, and _H. Auricula_ ♀ x _H. pratense_ ♂ by three
individuals, while as to the remainder only one of each has been
obtained.
If we compare the individual characters of the hybrids with the
corresponding characters of the two parent types, we find that they
sometimes present intermediate structures, but are sometimes so near
to one of the parent characters that the [corresponding] character
of the other has receded considerably or almost evades observation.
So, for instance, we see in one of the two forms of _H. Auricula_ ♀ x
_H. aurantiacum_ ♂ pure yellow disc-florets; only the petals of the
marginal florets are on the outside tinged with red to a scarcely
noticeable degree: in the other on the contrary the colour of these
florets comes very near to _H. aurantiacum_, only in the centre of the
disc the orange red passes into a deep golden-yellow. This difference
is noteworthy, for the flower-colour in _Hieracium_ has the value of a
constant character. Other similar cases are to be found in the leaves,
the peduncles, &c.
If the hybrids are compared with the parent types as regards the sum
total of their characters, then the two forms of _H. præaltum_ ♀ x _H.
aurantiacum_ ♂ constitute approximately intermediate forms which do
not agree in certain characters. On the contrary in _H. Auricula_ ♀
x _H. aurantiacum_ ♂ and in _H. Auricula_ ♀ x _H. pratense_ ♂ we see
the forms widely divergent, so that one of them is nearer to the one
and the other to the other parental type, while in the case of the
last-named hybrid there is still a third which is almost precisely
intermediate between them.
The conviction is then forced on us that we have here only single terms
in an unknown series which may be formed by the direct action of the
pollen of one species on the egg-cells of another.
2. With a single exception the hybrids in question form seeds capable
of germination. _H. echioides_ ♀ x _H. aurantiacum_ ♂ may be described
as fully fertile; _H. præaltum_ ♀ x _H. flagellare_ ♂ as fertile; _H.
præaltum_ ♀ x _H. aurantiacum_ ♂ and _H. Auricula_ ♀ x _H. pratense_ ♂
as partially fertile; _H. Auricula_ ♀ x _H. Pilosella_ ♂ as slightly
fertile, and _H. Auricula_ ♀ x _H. aurantiacum_ ♂ as unfertile. Of the
two forms of the last named hybrid, the red-flowered one was completely
sterile, but from the yellow-flowered one a single well-formed seed
was obtained. Moreover it must not pass unmentioned that among the
seedlings of the partially fertile hybrid _H. præaltum_ ♀ x _H.
aurantiacum_ ♂ there was one plant which possessed full fertility.
[3.] As yet the offspring produced by self-fertilisation of the hybrids
have not varied, but agree in their characters both with each other and
with the hybrid plant from which they were derived.
From _H. præaltum_ ♀ x _H. flagellare_ ♂ two generations have flowered;
from _H. echioides_ ♀ x _H. aurantiacum_ ♂, _H. præaltum_ ♀ x _H.
aurantiacum_ ♂, _H. Auricula_ ♀ x _H. Pilosella_ ♂ one generation in
each case has flowered.
4. The fact must be declared that in the case of the fully fertile
hybrid _H. echioides_ ♀ x _H. aurantiacum_ ♂ the pollen of the parent
types was not able to prevent self-fertilisation, though it was applied
in great quantity to the stigmas protruding through the anther-tubes
when the flowers opened.
From two flower-heads treated in this way seedlings were produced
resembling this hybrid plant. A very similar experiment, carried
out this summer with the partially fertile _H. præaltum_ ♀ x _H.
aurantiacum_ ♂ led to the conclusion that those flower-heads in which
pollen of the parent type or of some other species had been applied
to the stigmas, developed a notably larger number of seeds than those
which had been left to self-fertilisation alone. The explanation of
this result must only be sought in the circumstance that as a large
part of the pollen-grains of the hybrid, examined microscopically,
show a defective structure, a number of egg-cells capable of
fertilisation do not become fertilised by their own pollen in the
ordinary course of self-fertilisation.
It not rarely happens that in fully fertile species in the wild state
the formation of the pollen fails, and in many anthers not a single
good grain is developed. If in these cases seeds are nevertheless
formed, such fertilisation must have been effected by foreign pollen.
In this way hybrids may easily arise by reason of the fact that many
forms of insects, notably the industrial Hymenoptera, visit the flowers
of _Hieracia_ with great zeal and are responsible for the pollen
which easily sticks to their hairy bodies reaching the stigmas of
neighbouring plants.
From the few facts that I am able to contribute it will be evident
the work scarcely extends beyond its first inception. I must express
some scruple in describing in this place an account of experiments
just begun. But the conviction that the prosecution of the proposed
experiments will demand a whole series of years, and the uncertainty
whether it will be granted to me to bring the same to a conclusion have
determined me to make the present communication. By the kindness of Dr
Nägeli, the Munich Director, who was good enough to send me species
which were wanting, especially from the Alps, I am in a position to
include a larger number of forms in my experiments. I venture to hope
even next year to be able to contribute something more by way of
extension and confirmation of the present account.
If finally we compare the described result, still very uncertain,
with those obtained by crosses made between forms of _Pisum_, which
I had the honour of communicating in the year 1865, we find a very
real distinction. In _Pisum_ the hybrids, obtained from the immediate
crossing of two forms, have in all cases the same type, but their
posterity, on the contrary, are variable and follow a definite law in
their variations. In _Hieracium_ according to the present experiments
the exactly opposite phenomenon seems to be exhibited. Already in
describing the _Pisum_ experiments it was remarked that there are also
hybrids whose posterity do not vary, and that, for example, according
to Wichura the hybrids of _Salix_ reproduce themselves like pure
species. In _Hieracium_ we may take it we have a similar case. Whether
from this circumstance we may venture to draw the conclusion that the
polymorphism of the genera _Salix_ and _Hieracium_ is connected with
the special condition of their hybrids is still an open question, which
may well be raised but not as yet answered.