HYBRID PRIMULAS.
The oxlip a hybrid naturally produced between Primula veris and vulgaris.
The differences in structure and function between the two parent-species.
Effects of crossing long-styled and short-styled oxlips with one another and
with the two forms of both parent-species.
Character of the offspring from oxlips artificially self-fertilised and cross-
fertilised in a state of nature.
Primula elatior shown to be a distinct species.
Hybrids between other heterostyled species of Primula.
Supplementary note on spontaneously produced hybrids in the genus Verbascum.
The various species of Primula have produced in a state of nature throughout
Europe an extraordinary number of hybrid forms. For instance, Professor Kerner
has found no less than twenty-five such forms in the Alps. (2/1. “Die
Primulaceen-Bastarten” ‘Oesterr. Botanische Zeitschrift’ Jahr 1875 Numbers 3, 4
and 5. See also Godron on hybrid Primulas in ‘Bull. Soc. Bot. de France’ tome 10
1853 page 178. Also in ‘Revue des Sciences Nat.’ 1875 page 331.) The frequent
occurrence of hybrids in this genus no doubt has been favoured by most of the
species being heterostyled, and consequently requiring cross-fertilisation by
insects; yet in some other genera, species which are not heterostyled and which
in some respects appear not well adapted for hybrid-fertilisation, have likewise
been largely hybridised. In certain districts of England, the common oxlip--a
hybrid between the cowslip (P. veris, vel officinalis) and the primrose (P.
vulgaris, vel acaulis)--is frequently found, and it occurs occasionally almost
everywhere. Owing to the frequency of this intermediate hybrid form, and to the
existence of the Bardfield oxlip (P. elatior), which resembles to a certain
extent the common oxlip, the claim of the three forms to rank as distinct
species has been discussed oftener and at greater length than that of almost any
other plant. Linnaeus considered P. veris, vulgaris and elatior to be varieties
of the same species, as do some distinguished botanists at the present day;
whilst others who have carefully studied these plants do not doubt that they are
distinct species. The following observations prove, I think, that the latter
view is correct; and they further show that the common oxlip is a hybrid between
P. veris and vulgaris.
The cowslip differs so conspicuously in general appearance from the primrose,
that nothing need here be said with respect to their external characters. (2/2.
The Reverend W.A. Leighton has pointed out certain differences in the form of
the capsules and seed in ‘Annals and Magazine of Natural History’ 2nd series
volume 2 1848 page 164.) But some less obvious differences deserve notice. As
both species are heterostyled, their complete fertilisation depends on insects.
The cowslip is habitually visited during the day by the larger humble-bees
(namely Bombus muscorum and hortorum), and at night by moths, as I have seen in
the case of Cucullia. The primrose is never visited (and I speak after many
years’ observation) by the larger humble-bees, and only rarely by the smaller
kinds; hence its fertilisation must depend almost exclusively on moths. There is
nothing in the structure of the flowers of the two plants which can determine
the visits of such widely different insects. But they emit a different odour,
and perhaps their nectar may have a different taste. Both the long-styled and
short-styled forms of the primrose, when legitimately and naturally fertilised,
yield on an average many more seeds per capsule than the cowslip, namely, in the
proportion of 100 to 55. When illegitimately fertilised they are likewise more
fertile than the two forms of the cowslip, as shown by the larger proportion of
their flowers which set capsules, and by the larger average number of seeds
which the capsules contain. The difference also between the number of seeds
produced by the long-styled and short-styled flowers of the primrose, when both
are illegitimately fertilised, is greater than that between the number produced
under similar circumstances by the two forms of the cowslip. The long-styled
flowers of the primrose when protected from the access of all insects, except
such minute ones as Thrips, yield a considerable number of capsules containing
on an average 19.2 seeds per capsule; whereas 18 plants of the long-styled
cowslip similarly treated did not yield a single seed.
The primrose, as every one knows, flowers a little earlier in the spring than
the cowslip, and inhabits slightly different stations and districts. The
primrose generally grows on banks or in woods, whilst the cowslip is found in
more open places. The geographical range of the two forms is different. Dr.
Bromfield remarks that “the primrose is absent from all the interior region of
northern Europe, where the cowslip is indigenous.” (2/3. ‘Phytologist’ volume 3
page 694.) In Norway, however, both plants range to the same degree of north
latitude. (2/4. H. Lecoq ‘Geograph. Bot. de l’Europe’ tome 8 1858 pages 141,
144. See also ‘Annals and Magazine of Natural History’ 9 1842 pages 156, 515.
Also Boreau ‘Flore du centre de la France’ 1840 tome 2 page 376. With respect to
the rarity of P. veris in western Scotland, see H.C. Watson ‘Cybele Britannica’
2 page 293.)
The cowslip and primrose, when intercrossed, behave like distinct species, for
they are far from being mutually fertile. Gartner crossed 27 flowers of P.
vulgaris with pollen of P. veris, and obtained 16 capsules; but these did not
contain any good seed. (2/5. ‘Bastarderzeugung’ 1849 page 721.) He also crossed
21 flowers of P. veris with pollen of P. vulgaris; and now he got only five
capsules, containing seed in a still less perfect condition. Gartner knew
nothing about heterostylism; and his complete failure may perhaps be accounted
for by his having crossed together the same forms of the cowslip and primrose;
for such crosses would have been of an illegitimate as well as of a hybrid
nature, and this would have increased their sterility. My trials were rather
more fortunate. Twenty-one flowers, consisting of both forms of the cowslip and
primrose, were intercrossed legitimately, and yielded seven capsules (i.e. 33
per cent), containing on an average 42 seeds; some of these seeds, however, were
so poor that they probably would not have germinated. Twenty-one flowers on the
same cowslip and primrose plants were also intercrossed illegitimately, and they
likewise yielded seven capsules (or 33 per cent), but these contained on an
average only 13 good and bad seeds. I should, however, state that some of the
above flowers of the primrose were fertilised with pollen from the polyanthus,
which is certainly a variety of the cowslip, as may be inferred from the perfect
fertility inter se of the crossed offspring from these two plants. (2/6. Mr.
Scott has discussed the nature of the polyanthus (‘Proceedings of the Linnean
Society’ 8 Botany 1864 page 103), and arrives at a different conclusion; but I
do not think that his experiments were sufficiently numerous. The degree of
infertility of a cross is liable to much fluctuation. Pollen from the cowslip at
first appears rather more efficient on the primrose than that of the polyanthus;
for 12 flowers of both forms of the primrose, fertilised legitimately and
illegitimately with pollen of the cowslip gave five capsules, containing on an
average 32.4 seeds; whilst 18 flowers similarly fertilised by polyanthus-pollen
yielded only five capsules, containing only 22.6 seeds. On the other hand, the
seeds produced by the polyanthus-pollen were much the finest of the whole lot,
and were the only ones which germinated.) To show how sterile these hybrid
unions were I may remind the reader that 90 per cent of the flowers of the
primrose fertilised legitimately with primrose-pollen yielded capsules,
containing on an average 66 seeds; and that 54 per cent of the flowers
fertilised illegitimately yielded capsules containing on an average 35.5 seeds
per capsule. The primrose, especially the short-styled form, when fertilised by
the cowslip, is less sterile, as Gartner likewise observed, than is the cowslip
when fertilised by the primrose. The above experiments also show that a cross
between the same forms of the primrose and cowslip is much more sterile than
that between different forms of these two species.
The seeds from the several foregoing crosses were sown, but none germinated
except those from the short-styled primrose fertilised with pollen of the
polyanthus; and these seeds were the finest of the whole lot. I thus raised six
plants, and compared them with a group of wild oxlips which I had transplanted
into my garden. One of these wild oxlips produced slightly larger flowers than
the others, and this one was identical in every character (in foliage, flower-
peduncle, and flowers) with my six plants, excepting that the flowers of the
latter were tinged of a dingy red colour, from being descended from the
polyanthus.
We thus see that the cowslip and primrose cannot be crossed either way except
with considerable difficulty, that they differ conspicuously in external
appearance, that they differ in various physiological characters, that they
inhabit slightly different stations and range differently. Hence those botanists
who rank these plants as varieties ought to be able to prove that they are not
as well fixed in character as are most species; and the evidence in favour of
such instability of character appears at first sight very strong. It rests,
first, on statements made by several competent observers that they have raised
cowslips, primroses, and oxlips from seeds of the same plant; and, secondly, on
the frequent occurrence in a state of nature of plants presenting every
intermediate gradation between the cowslip and primrose.
The first statement, however, is of little value; for, heterostylism not being
formerly understood, the seed-bearing plants were in no instance protected from
the visits of insects (2/7. One author states in the ‘Phytologist’ volume 3 page
703 that he covered with bell-glasses some cowslips, primroses, etc., on which
he experimented. He specifies all the details of his experiment, but does not
say that he artificially fertilised his plants; yet he obtained an abundance of
seed, which is simply impossible. Hence there must have been some strange error
in these experiments, which may be passed over as valueless.); and there would
be almost as much risk of an isolated cowslip, or of several cowslips if
consisting of the same form, being crossed by a neighbouring primrose and
producing oxlips, as of one sex of a dioecious plant, under similar
circumstances, being crossed by the opposite sex of an allied and neighbouring
species. Mr. H.C. Watson, a critical and most careful observer, made many
experiments by sowing the seeds of cowslips and of various kinds of oxlips, and
arrived at the following conclusion, namely, “that seeds of a cowslip can
produce cowslips and oxlips, and that seeds of an oxlip can produce cowslips,
oxlips, and primroses.” (2/8. ‘Phytologist’ 2 pages 217, 852; 3 page 43.) This
conclusion harmonises perfectly with the view that in all cases, when such
results have been obtained, the unprotected cowslips have been crossed by
primroses, and the unprotected oxlips by either cowslips or primroses; for in
this latter case we might expect, by the aid of reversion, which notoriously
comes into powerful action with hybrids, that the two parent-forms in appearance
pure, as well as many intermediate gradations, would be occasionally produced.
Nevertheless the two following statements offer considerable difficulty. The
Reverend Professor Henslow raised from seeds of a cowslip growing in his garden,
various kinds of oxlips and one perfect primrose; but a statement in the same
paper perhaps throws light on this anomalous result. (2/9. Loudon’s ‘Magazine of
Natural History’ 3 1830 page 409.) Professor Henslow had previously transplanted
into his garden a cowslip, which completely changed its appearance during the
following year, and now resembled an oxlip. Next year again it changed its
character, and produced, in addition to the ordinary umbels, a few single-
flowered scapes, bearing flowers somewhat smaller and more deeply coloured than
those of the common primrose. From what I have myself observed with oxlips, I
cannot doubt that this plant was an oxlip in a highly variable condition, almost
like that of the famous Cytisus adami. This presumed oxlip was propagated by
offsets, which were planted in different parts of the garden; and if Professor
Henslow took by mistake seeds from one of these plants, especially if it had
been crossed by a primrose, the result would be quite intelligible. Another case
is still more difficult to understand: Dr. Herbert raised, from the seeds of a
highly cultivated red cowslip, cowslips, oxlips of various kinds, and a
primrose. (2/10. ‘Transactions of the Horticultural Society’ 4 page 19.) This
case, if accurately recorded, which I much doubt, is explicable only on the
improbable assumption that the red cowslip was not of pure parentage. With
species and varieties of many kinds, when intercrossed, one is sometimes
strongly prepotent over the other; and instances are known of a variety crossed
by another, producing offspring which in certain characters, as in colour,
hairiness, etc., have proved identical with the pollen-bearing parent, and quite
dissimilar to the mother-plant (2/11. I have given instances in my work ‘On the
Variation of Animals and Plants under Domestication’ chapter 15 2nd edition
volume 2 page 69.); but I do not know of any instance of the offspring of a
cross perfectly resembling, in a considerable number of important characters,
the father alone. It is, therefore, very improbable that a pure cowslip crossed
by a primrose should ever produce a primrose in appearance pure. Although the
facts given by Dr. Herbert and Professor Henslow are difficult to explain, yet
until it can be shown that a cowslip or a primrose, carefully protected from
insects, will give birth to at least oxlips, the cases hitherto recorded have
little weight in leading us to admit that the cowslip and primrose are varieties
of one and the same species.
Negative evidence is of little value; but the following facts may be worth
giving:--Some cowslips which had been transplanted from the fields into a
shrubbery were again transplanted into highly manured land. In the following
year they were protected from insects, artificially fertilised, and the seed
thus procured was sown in a hotbed. The young plants were afterwards planted
out, some in very rich soil, some in stiff poor clay, some in old peat, and some
in pots in the greenhouse; so that these plants, 765 in number, as well as their
parents, were subjected to diversified and unnatural treatment; but not one of
them presented the least variation except in size--those in the peat attaining
almost gigantic dimensions, and those in the clay being much dwarfed.
I do not, of course, doubt that cowslips exposed during SEVERAL successive
generations to changed conditions would vary, and that this might occasionally
occur in a state of nature. Moreover, from the law of analogical variation, the
varieties of any one species of Primula would probably in some cases resemble
other species of the genus. For instance I raised a red primrose from seed from
a protected plant, and the flowers, though still resembling those of the
primrose, were borne during one season in umbels on a long foot-stalk like that
of a cowslip.
With regard to the second class of facts in support of the cowslip and primrose
being ranked as mere varieties, namely, the well-ascertained existence in a
state of nature of numerous linking forms (2/12. See an excellent article on
this subject by Mr. H.C. Watson in the ‘Phytologist’ volume 3 page 43.):--If it
can be shown that the common wild oxlip, which is intermediate in character
between the cowslip and primrose, resembles in sterility and other essential
respects a hybrid plant, and if it can further be shown that the oxlip, though
in a high degree sterile, can be fertilised by either parent-species, thus
giving rise to still finer gradational links, then the presence of such linking
forms in a state of nature ceases to be an argument of any weight in favour of
the cowslip and primrose being varieties, and becomes, in fact, an argument on
the other side. The hybrid origin of a plant in a state of nature can be
recognised by four tests: first, by its occurrence only where both presumed
parent-species exist or have recently existed; and this holds good, as far as I
can discover, with the oxlip; but the P. elatior of Jacq., which, as we shall
presently see, constitutes a distinct species, must not be confounded with the
common oxlip. Secondly, by the supposed hybrid plant being nearly intermediate
in character between the two parent-species, and especially by its resembling
hybrids artificially made between the same two species. Now the oxlip is
intermediate in character, and resembles in every respect, except in the colour
of the corolla, hybrids artificially produced between the primrose and the
polyanthus, which latter is a variety of the cowslip. Thirdly, by the supposed
hybrids being more or less sterile when crossed inter se: but to try this fairly
two distinct plants of the same parentage, and not two flowers on the same
plant, should be crossed; for many pure species are more or less sterile with
pollen from the same individual plant; and in the case of hybrids from
heterostyled species the opposite forms should be crossed. Fourthly and lastly,
by the supposed hybrids being much more fertile when crossed with either pure
parent-species than when crossed inter se, but still not as fully fertile as the
parent-species.
For the sake of ascertaining the two latter points, I transplanted a group of
wild oxlips into my garden. They consisted of one long-styled and three short-
styled plants, which, except in the corolla of one being slightly larger,
resembled each other closely. The trials which were made, and the results
obtained, are shown in tables 2.14, 2.15, 2.16, 2.17 and 2.18. No less than
twenty different crosses are necessary in order to ascertain fully the fertility
of hybrid heterostyled plants, both inter se and with their two parent-species.
In this instance 256 flowers were crossed in the course of four seasons. I may
mention, as a mere curiosity, that if any one were to raise hybrids between two
trimorphic heterostyled species, he would have to make 90 distinct unions in
order to ascertain their fertility in all ways; and as he would have to try at
least 10 flowers in each case, he would be compelled to fertilise 900 flowers
and count their seeds. This would probably exhaust the patience of the most
patient man.
TABLE 2.14. Crosses inter se between the two forms of the common Oxlip.
Column 1: Illegitimate union.
Short-styled oxlip, by pollen of short-styled oxlip: 20 flowers fertilised, did
not produce one capsule.
Column 2: Legitimate union.
Short-styled oxlip, by pollen of long-styled oxlip: 10 flowers fertilised, did
not produce one capsule.
Column 3: Illegitimate union.
Long-styled oxlip, by its own pollen: 24 flowers fertilised, produced five
capsules, containing 6, 10, 20, 8, and 14 seeds. Average 11.6.
Column 4: Legitimate union.
Long-styled oxlip, by pollen of short-styled oxlip: 10 flowers fertilised, did
not produce one capsule.
TABLE 2.15. Both forms of the Oxlip crossed with Pollen of both forms of the
Cowslip, P. veris.
Column 1: Illegitimate union.
Short-styled oxlip, by pollen of short-styled cowslip: 18 flowers fertilised,
did not produce one capsule.
Column 2: Legitimate union.
Short-styled oxlip, by pollen of long-styled cowslip: 18 flowers fertilised,
produced three capsules, containing 7, 3, and 3 wretched seeds, apparently
incapable of germination.
Column 3: Illegitimate union.
Long-styled oxlip, by pollen of long-styled cowslip: 11 flowers fertilised,
produced one capsule, containing 13 wretched seeds.
Column 4: Legitimate union.
Long-styled oxlip, by pollen of short-styled cowslip: 5 flowers fertilised,
produced two capsules, containing 21 and 28 very fine seeds.
TABLE 2.16. Both forms of the Oxlip crossed with Pollen of both forms of the
Primrose, P. vulgaris.
Column 1: Illegitimate union.
Short-styled oxlip, by pollen of short-styled primrose: 34 flowers fertilised,
produced two capsules, containing 5 and 12 seeds.
Column 2: Legitimate union.
Short-styled oxlip, by pollen of long-styled primrose: 26 flowers fertilised,
produced six capsules, containing 16, 20, 5, 10, 19, and 24 seeds. Average 15.7.
Many of the seeds very poor, some good.
Column 3: Illegitimate union.
Long-styled oxlip, by pollen of long-styled primrose: 11 flowers fertilised,
produced four capsules, containing 10, 7, 5, and 6 wretched seeds. Average 7.0.
Column 4: Legitimate union.
Long-styled oxlip, by pollen of short-styled primrose: 5 flowers fertilised,
produced five capsules, containing 26, 32, 23, 28, and 34 seeds. Average 28.6.
TABLE 2.17. Both forms of the Cowslip crossed with Pollen of both forms of the
Oxlip.
Column 1: Illegitimate union.
Short-styled cowslip, by pollen of short-styled oxlip: 8 flowers fertilised, did
not produce one capsule.
Column 2: Legitimate union.
Long-styled cowslip, by pollen of short-styled oxlip: 8 flowers fertilised,
produced one capsule, containing 26 seeds.
Column 3: Illegitimate union.
Long-styled cowslip, by pollen of long-styled oxlip: 8 flowers fertilised,
produced three capsules, containing 5, 6 and 14 seeds. Average 8.3.
Column 4: Legitimate union.
Short-styled cowslip, by pollen of long-styled oxlip: 8 flowers fertilised,
produced 8 capsules, containing 58, 38, 31, 44, 23, 26, 37, and 66 seeds.
Average 40.4.
TABLE 2.18. Both forms of the Primrose crossed with Pollen of both forms of the
Oxlip.
Column 1: Illegitimate union.
Short-styled primrose, by pollen of short-styled oxlip: 8 flowers fertilised,
did not produce one capsule.
Column 2: Legitimate union.
Long-styled primrose, by pollen of short-styled oxlip: 8 flowers fertilised,
produced two capsules, containing 5 and 2 seeds.
Column 3: Illegitimate union.
Long-styled primrose, by pollen of long-styled oxlip: 8 flowers fertilised,
produced 8 capsules, containing 15, 7, 12, 20, 22, 7, 16, and 13 seeds. Average
14.0.
Column 4: Legitimate union.
Short-styled primrose, by pollen of long-styled oxlip: 8 flowers fertilised,
produced 4 capsules, containing 52, 52, 42, and 49 seeds, some good and some
bad. Average 48.7.
We see in Tables 2/14 to 2/18 the number of capsules and of seeds produced, by
crossing both forms of the oxlip in a legitimate and illegitimate manner with
one another, and with the two forms of the primrose and cowslip. I may premise
that the pollen of two of the short-styled oxlips consisted of nothing but
minute aborted whitish cells; but in the third short-styled plant about one-
fifth of the grains appeared in a sound condition. Hence it is not surprising
that neither the short-styled nor the long-styled oxlip produced a single seed
when fertilised with this pollen. Nor did the pure cowslips or primroses when
illegitimately fertilised with it; but when thus legitimately fertilised they
yielded a few good seeds. The female organs of the short-styled oxlips, though
greatly deteriorated in power, were in a rather better condition than the male
organs; for though the short-styled oxlips yielded no seed when fertilised by
the long-styled oxlips, and hardly any when illegitimately fertilised by pure
cowslips or primroses, yet when legitimately fertilised by these latter species,
especially by the long-styled primrose, they yielded a moderate supply of good
seed.
The long-styled oxlip was more fertile than the three short-styled oxlips, and
about half its pollen-grains appeared sound. It bore no seed when legitimately
fertilised by the short-styled oxlips; but this no doubt was due to the badness
of the pollen of the latter; for when illegitimately fertilised (Table 2.14) by
its own pollen it produced some good seeds, though much fewer than self-
fertilised cowslips or primroses would have produced. The long-styled oxlip
likewise yielded a very low average of seed, as may be seen in the third
compartment of Tables 2.15 to 2.18, when illegitimately fertilised by, and when
illegitimately fertilising, pure cowslips and primroses. The four corresponding
legitimate unions, however, were moderately fertile, and one (namely that
between a short-styled cowslip and the long-styled oxlip in Table 2.17) was
nearly as fertile as if both parents had been pure. A short-styled primrose
legitimately fertilised by the long-styled oxlip (Table 2.18) also yielded a
moderately good average, namely 48.7 seeds; but if this short-styled primrose
had been fertilised by a long-styled primrose it would have yielded an average
of 65 seeds. If we take the ten legitimate unions together, and the ten
illegitimate unions together, we shall find that 29 per cent of the flowers
fertilised in a legitimate manner yielded capsules, these containing on an
average 27.4 good and bad seeds; whilst only 15 per cent of the flowers
fertilised in an illegitimate manner yielded capsules, these containing on an
average only 11.0 good and bad seeds.
In a previous part of this chapter it was shown that illegitimate crosses
between the long-styled form of the primrose and the long-styled cowslip, and
between the short-styled primrose and short-styled cowslip, are more sterile
than legitimate crosses between these two species; and we now see that the same
rule holds good almost invariably with their hybrid offspring, whether these are
crossed inter se, or with either parent-species; so that in this particular
case, but not as we shall presently see in other cases, the same rule prevails
with the pure unions between the two forms of the same heterostyled species,
with crosses between two distinct heterostyled species, and with their hybrid
offspring.
Seeds from the long-styled oxlip fertilised by its own pollen were sown, and
three long-styled plants raised. The first of these was identical in every
character with its parent. The second bore rather smaller flowers, of a paler
colour, almost like those of the primrose; the scapes were at first single-
flowered, but later in the season a tall thick scape, bearing many flowers, like
that of the parent oxlip, was thrown up. The third plant likewise produced at
first only single-flowered scapes, with the flowers rather small and of a darker
yellow; but it perished early. The second plant also died in September; and the
first plant, though all three grew under very favourable conditions, looked very
sickly. Hence we may infer that seedlings from self-fertilised oxlips would
hardly be able to exist in a state of nature. I was surprised to find that all
the pollen-grains in the first of these seedling oxlips appeared sound; and in
the second only a moderate number were bad. These two plants, however, had not
the power of producing a proper number of seeds; for though left uncovered and
surrounded by pure primroses and cowslips, the capsules were estimated to
include an average of only from fifteen to twenty seeds.
From having many experiments in hand, I did not sow the seed obtained by
crossing both forms of the primrose and cowslip with both forms of the oxlip,
which I now regret; but I ascertained an interesting point, namely, the
character of the offspring from oxlips growing in a state of nature near both
primroses and cowslips. The oxlips were the same plants which, after their seeds
had been collected, were transplanted and experimented on. From the seeds thus
obtained eight plants were raised, which, when they flowered, might have been
mistaken for pure primroses; but on close comparison the eye in the centre of
the corolla was seen to be of a darker yellow, and the peduncles more elongated.
As the season advanced, one of these plants threw up two naked scapes, 7 inches
in height, which bore umbels of flowers of the same character as before. This
fact led me to examine the other plants after they had flowered and were dug up;
and I found that the flower-peduncles of all sprung from an extremely short
common scape, of which no trace can be found in the pure primrose. Hence these
plants are beautifully intermediate between the oxlip and the primrose,
inclining rather towards the latter; and we may safely conclude that the parent
oxlips had been fertilised by the surrounding primroses.
From the various facts now given, there can be no doubt that the common oxlip is
a hybrid between the cowslip (P. veris, Brit. Fl.) and the primrose (P.
vulgaris, Brit. Fl.), as has been surmised by several botanists. It is probable
that oxlips may be produced either from the cowslip or the primrose as the seed-
bearer, but oftenest from the latter, as I judge from the nature of the stations
in which oxlips are generally found (2/13. See also on this head Hardwicke’s
‘Science Gossip’ 1867 pages 114, 137.), and from the primrose when crossed by
the cowslip being more fertile than, conversely, the cowslip by the primrose.
The hybrids themselves are also rather more fertile when crossed with the
primrose than with the cowslip. Whichever may be the seed-bearing plant, the
cross is probably between different forms of the two species; for we have seen
that legitimate hybrid unions are more fertile than illegitimate hybrid unions.
Moreover a friend in Surrey found that 29 oxlips which grew in the neighbourhood
of his house consisted of 13 long-styled and 16 short-styled plants; now, if the
parent-plants had been illegitimately united, either the long- or short-styled
form would have greatly preponderated, as we shall hereafter see good reason to
believe. The case of the oxlip is interesting; for hardly any other instance is
known of a hybrid spontaneously arising in such large numbers over so wide an
extent of country. The common oxlip (not the P. elatior of Jacq.) is found
almost everywhere throughout England, where both cowslips and primroses grow. In
some districts, as I have seen near Hartfield in Sussex and in parts of Surrey,
specimens may be found on the borders of almost every field and small wood. In
other districts the oxlip is comparatively rare: near my own residence I have
found, during the last twenty-five years, not more than five or six plants or
groups of plants. It is difficult to conjecture what is the cause of this
difference in their number. It is almost necessary that a plant, or several
plants belonging to the same form, of one parent-species, should grow near the
opposite form of the other parent-species; and it is further necessary that both
species should be frequented by the same kind of insect, no doubt a moth. The
cause of the rare appearance of the oxlip in certain districts may be the rarity
of some moth, which in other districts habitually visits both the primrose and
cowslip.
Finally, as the cowslip and primrose differ in the various characters above
specified,--as they are in a high degree sterile when intercrossed,--as there is
no trustworthy evidence that either species, when uncrossed, has ever given
birth to the other species or to any intermediate form,--and as the intermediate
forms which are often found in a state of nature have been shown to be more or
less sterile hybrids of the first or second generation,--we must for the future
look at the cowslip and primrose as good and true species.
Primula elatior, Jacq., or the Bardfield Oxlip, is found in England only in two
or three of the eastern counties. On the Continent it has a somewhat different
range from that of the cowslip and primrose; and it inhabits some districts
where neither of these species live. (2/14. For England, see Hewett C. Watson
‘Cybele Britannica’ volume 2 1849 page 292. For the Continent, see Lecoq
‘Geograph. Botanique de l’Europe’ tome 8 1858 page 142. For the Alps see ‘Annals
and Magazine of Natural History’ volume 9 1842 pages 156 and 515.) In general
appearance it differs so much from the common oxlip, that no one accustomed to
see both forms in the living state could afterwards confound them; but there is
scarcely more than a single character by which they can be distinctly defined,
namely, their linear-oblong capsules equalling the calyx in length. (2/15.
Babington ‘Manual of British Botany’ 1851 page 258.) The capsules when mature
differ conspicuously, owing to their length, from those of the cowslip and
primrose. With respect to the fertility of the two forms when these are united
in the four possible methods, they behave like the other heterostyled species of
the genus, but differ somewhat (see Tables 1.8 and 1.12.) in the smaller
proportion of the illegitimately fertilised flowers which set capsules. That P.
elatior is not a hybrid is certain, for when the two forms were legitimately
united they yielded the large average of 47.1 seeds, and when illegitimately
united 35.5 per capsule; whereas, of the four possible unions (Table 2.14)
between the two forms of the common oxlip which we know to be a hybrid, one
alone yielded any seed; and in this case the average number was only 11.6 per
capsule. Moreover I could not detect a single bad pollen-grain in the anthers of
the short-styled P. elatior; whilst in two short-styled plants of the common
oxlip all the grains were bad, as were a large majority in a third plant. As the
common oxlip is a hybrid between the primrose and cowslip, it is not surprising
that eight long-styled flowers of the primrose, fertilised by pollen from the
long-styled common oxlip, produced eight capsules (Table 1.18), containing,
however, only a low average of seeds; whilst the same number of flowers of the
primrose, similarly fertilised by the long-styled Bardfield oxlip, produced only
a single capsule; this latter plant being an altogether distinct species from
the primrose. Plants of P. elatior have been propagated by seed in a garden for
twenty-five years, and have kept all this time quite constant, excepting that in
some cases the flowers varied a little in size and tint. (2/16. See Mr. H.
Doubleday in the ‘Gardener’s Chronicle’ 1867 page 435, also Mr. W. Marshall
ibid. page 462.) Nevertheless, according to Mr. H.C. Watson and Dr. Bromfield
(2/17. ‘Phytologist’ volume 1 page 1001 and volume 3 page 695.), plants may be
occasionally found in a state of nature, in which most of the characters by
which this species can be distinguished from P. veris and vulgaris fail; but
such intermediate forms are probably due to hybridisation; for Kerner states, in
the paper before referred to, that hybrids sometimes, though rarely, arise in
the Alps between P. elatior and veris.
Finally, although we may freely admit that Primula veris, vulgaris, and elatior,
as well as all the other species of the genus, are descended from a common
primordial form, yet from the facts above given, we must conclude that these
three forms are now as fixed in character as are many others which are
universally ranked as true species. Consequently they have as good a right to
receive distinct specific names as have, for instance, the ass, quagga, and
zebra.
Mr. Scott has arrived at some interesting results by crossing other heterostyled
species of Primula. (2/18. ‘Journal of the Linnean Society Botany’ volume 8 1864
page 93 to end.) I have already alluded to his statement, that in four instances
(not to mention others) a species when crossed with a distinct one yielded a
larger number of seeds than the same species fertilised illegitimately with its
own-form pollen, though taken from a distinct plant. It has long been known from
the researches of Kolreuter and Gartner, that two species when crossed
reciprocally sometimes differ as widely as is possible in their fertility: thus
A when crossed with the pollen of B will yield a large number of seeds, whilst B
may be crossed repeatedly with pollen of A, and will never yield a single seed.
Now Mr. Scott shows in several cases that the same law holds good when two
heterostyled species of Primula are intercrossed, or when one is crossed with a
homostyled species. But the results are much more complicated than with ordinary
plants, as two heterostyled dimorphic species can be intercrossed in eight
different ways. I will give one instance from Mr. Scott. The long-styled P.
hirsuta fertilised legitimately and illegitimately with pollen from the two
forms of P. auricula, and reciprocally the long-styled P. auricula fertilised
legitimately and illegitimately with pollen from the two forms of P. hirsuta,
did not produce a single seed. Nor did the short-styled P. hirsuta when
fertilised legitimately and illegitimately with the pollen of the two forms of
P. auricula. On the other hand, the short-styled P. auricula fertilised with
pollen from the long-styled P. hirsuta yielded capsules containing on an average
no less than 56 seeds; and the short-styled P. auricula by pollen of the short-
styled P. hirsuta yielded capsules containing on an average 42 seeds per
capsule. So that out of the eight possible unions between the two forms of these
two species, six were utterly barren, and two fairly fertile. We have seen also
the same sort of extraordinary irregularity in the results of my twenty
different crosses (Tables 2.14 to 2.18), between the two forms of the oxlip,
primrose, and cowslip. Mr. Scott remarks, with respect to the results of his
trials, that they are very surprising, as they show us that “the sexual forms of
a species manifest in their respective powers for conjunction with those of
another species, physiological peculiarities which might well entitle them, by
the criterion of fertility, to specific distinction.”
Finally, although P. veris and vulgaris, when crossed legitimately, and
especially when their hybrid offspring are crossed in this manner with both
parent-species, were decidedly more fertile, than when crossed in an
illegitimate manner, and although the legitimate cross effected by Mr. Scott
between P. auricula and hirsuta was more fertile, in the ratio of 56 to 42, than
the illegitimate cross, nevertheless it is very doubtful, from the extreme
irregularity of the results in the various other hybrid crosses made by Mr.
Scott, whether it can be predicted that two heterostyled species are generally
more fertile if crossed legitimately (i.e. when opposite forms are united) than
when crossed illegitimately.
SUPPLEMENTARY NOTE ON SOME WILD HYBRID VERBASCUMS.
In an early part of this chapter I remarked that few other instances could be
given of a hybrid spontaneously arising in such large numbers, and over so wide
an extent of country, as that of the common oxlip; but perhaps the number of
well-ascertained cases of naturally produced hybrid willows is equally great.
(2/19. Max Wichura ‘Die Bastardbefruchtung etc. der Weiden’ 1865.) Numerous
spontaneous hybrids between several species of Cistus, found near Narbonne, have
been carefully described by M. Timbal-Lagrave (2/20. ‘Mem. de l’Acad. des
Sciences de Toulouse’ 5e serie tome 5 page 28.), and many hybrids between an
Aceras and Orchis have been observed by Dr. Weddell. (2/21. ‘Annales des Sc.
Nat.’ 3e serie Bot. tome 18 page 6.) In the genus Verbascum, hybrids are
supposed to have often originated in a state of nature (2/22. See for instance
the ‘English Flora’ by Sir J.E. Smith 1824 volume 1 page 307.); some of these
undoubtedly are hybrids, and several hybrids have originated in gardens; but
most of these cases require, as Gartner remarks, verification. (2/23. See
Gartner ‘Bastarderzeugung’ 1849 page 590.) Hence the following case is worth
recording, more especially as the two species in question, V. thapsus and
lychnitis, are perfectly fertile when insects are excluded, showing that the
stigma of each flower receives its own pollen. Moreover the flowers offer only
pollen to insects, and have not been rendered attractive to them by secreting
nectar.
I transplanted a young wild plant into my garden for experimental purposes, and
when it flowered it plainly differed from the two species just mentioned and
from a third which grows in this neighbourhood. I thought that it was a strange
variety of V. thapsus. It attained the height (by measurement) of 8 feet! It was
covered with a net, and ten flowers were fertilised with pollen from the same
plant; later in the season, when uncovered, the flowers were freely visited by
pollen-collecting bees; nevertheless, although many capsules were produced, not
one contained a single seed. During the following year this same plant was left
uncovered near plants of V. thapsus and lychnitis; but again it did not produce
a single seed. Four flowers, however, which were repeatedly fertilised with
pollen of V. lychnitis, whilst the plant was temporarily kept under a net,
produced four capsules, which contained five, one, two, and two seeds; at the
same time three flowers were fertilised with pollen of V. thapsus, and these
produced two, two, and three seeds. To show how unproductive these seven
capsules were, I may state that a fine capsule from a plant of V. thapsus
growing close by contained above 700 seeds. These facts led me to search the
moderately-sized field whence my plant had been removed, and I found in it many
plants of V. thapsus and lychnitis as well as thirty-three plants intermediate
in character between these two species. These thirty-three plants differed much
from one another. In the branching of the stem they more closely resembled V.
lychnitis than V. thapsus, but in height the latter species. In the shape of
their leaves they often closely approached V. lychnitis, but some had leaves
extremely woolly on the upper surface and decurrent like those of V. thapsus;
yet the degree of woolliness and of decurrency did not always go together. In
the petals being flat and remaining open, and in the manner in which the anthers
of the longer stamens were attached to the filaments, these plants all took more
after V. lychnitis than V. thapsus. In the yellow colour of the corolla they all
resembled the latter species. On the whole, these plants appeared to take rather
more after V. lychnitis than V. thapsus. On the supposition that they were
hybrids, it is not an anomalous circumstance that they should all have produced
yellow flowers; for Gartner crossed white and yellow-flowered varieties of
Verbascum, and the offspring thus produced never bore flowers of an intermediate
tint, but either pure white or pure yellow flowers, generally of the latter
colour. (2/24. ‘Bastardzeugung’ page 307.)
My observations were made in the autumn; so that I was able to collect some
half-matured capsules from twenty of the thirty-three intermediate plants, and
likewise capsules of the pure V. lychnitis and thapsus growing in the same
field. All the latter were filled with perfect but immature seeds, whilst the
capsules of the twenty intermediate plants did not contain one single perfect
seed. These plants, consequently, were absolutely barren. From this fact,--from
the one plant which was transplanted into my garden yielding when artificially
fertilised with pollen from V. lychnitis and thapsus some seeds, though
extremely few in number,--from the circumstance of the two pure species growing
in the same field,--and from the intermediate character of the sterile plants,
there can be no doubt that they were hybrids. Judging from the position in which
they were chiefly found, I am inclined to believe they were descended from V.
thapsus as the seed-bearer, and V. lychnitis as the pollen-bearer.
It is known that many species of Verbascum, when the stem is jarred or struck by
a stick, cast off their flowers. (2/25. This was first observed by Correa de
Serra: see Sir J.E. Smith’s ‘English Flora’ 1824 volume 1 page 311; also ‘Life
of Sir J.E. Smith’ volume 2 page 210. I was guided to these references by the
Reverend W.A. Leighton, who observed this same phenomenon with V. virgatum.)
This occurs with V. thapsus, as I have repeatedly observed. The corolla first
separates from its attachment, and then the sepals spontaneously bend inwards so
as to clasp the ovarium, pushing off the corolla by their movement, in the
course of two or three minutes. Nothing of this kind takes place with young
barely expanded flowers. With Verbascum lychnitis and, as I believe, V.
phoeniceum the corolla is not cast off, however often and severely the stem may
be struck. In this curious property the above-described hybrids took after V.
thapsus; for I observed, to my surprise, that when I pulled off the flower-buds
round the flowers which I wished to mark with a thread, the slight jar
invariably caused the corollas to fall off.
These hybrids are interesting under several points of view. First, from the
number found in various parts of the same moderately-sized field. That they owed
their origin to insects flying from flower to flower, whilst collecting pollen,
there can be no doubt. Although insects thus rob the flowers of a most precious
substance, yet they do great good; for, as I have elsewhere shown, the seedlings
of V. thapsus raised from flowers fertilised with pollen from another plant, are
more vigorous than those raised from self-fertilised flowers. (2/26. ‘The
Effects of Cross and Self-fertilisation’ 1876 page 89.) But in this particular
instance the insects did great harm, as they led to the production of utterly
barren plants. Secondly, these hybrids are remarkable from differing much from
one another in many of their characters; for hybrids of the first generation, if
raised from uncultivated plants, are generally uniform in character. That these
hybrids belonged to the first generation we may safely conclude, from the
absolute sterility of all those observed by me in a state of nature and of the
one plant in my garden, excepting when artificially and repeatedly fertilised
with pure pollen, and then the number of seeds produced was extremely small. As
these hybrids varied so much, an almost perfectly graduated series of forms,
connecting together the two widely distinct parent-species, could easily have
been selected. This case, like that of the common oxlip, shows that botanists
ought to be cautious in inferring the specific identity of two forms from the
presence of intermediate gradations; nor would it be easy in the many cases in
which hybrids are moderately fertile to detect a slight degree of sterility in
such plants growing in a state of nature and liable to be fertilised by either
parent-species. Thirdly and lastly, these hybrids offer an excellent
illustration of a statement made by that admirable observer Gartner, namely,
that although plants which can be crossed with ease generally produce fairly
fertile offspring, yet well-pronounced exceptions to this rule occur; and here
we have two species of Verbascum which evidently cross with the greatest ease,
but produce hybrids which are excessively sterile.