THE SHAPES OF FLOWERS.
§ 232. Following an order like that of preceding chapters, let us first
note a few typical facts respecting the forms of clusters of flowers,
apart from the forms of the flowers themselves. Two kindred kinds of
_Leguminosæ_ serve to show how the members of clusters are distributed
in an all-sided manner or in a two-sided manner, according as the
circumstances are alike on all sides or alike on only two sides. In
_Hippocrepis_, represented in Fig. 226, the flowers growing at the
end of a vertical stem, are arranged round it in radial symmetry.
Contrariwise in _Melilotus_, Fig. 227, where the axillary stem bearing
the flowers is so placed in relation to the main stem, that its outer
and inner faces are differently conditioned, the flowers are all on the
outer face: the cluster is bilaterally symmetrical, since it may be cut
into approximately equal and similar groups by a vertical plane passing
through the main axis.
[Illustration: Figs. 226, 227.]
Plants of this same tribe furnish clusters of intermediate characters
having intermediate conditions. Among these, as among the clusters
which other types present, may be found some in which conformity
to the general law is not obvious. The discussion of these apparent
anomalies would carry us too much out of our course. A clue to the
explanation of them will, I believe, be found in the explanation
presently to be given of certain kindred anomalies in the forms of
individual flowers.
§ 233. The radially-symmetrical form is common to all individual
flowers that have vertical axes. In plants which are practically if
not literally uniaxial, and bear their flowers at the ends of upright
stalks, so that the faces open horizontally, the petals are disposed in
an all-sided way. Crocuses, Tulips, and Poppies are familiar examples
of this structure occurring under these conditions. A Ranunculus
flower, Fig. 228, will serve as a typical one. Similarly, flowers which
have peduncles flexible enough to let them hang directly downwards, and
are not laterally incommoded, are also radial; as in the _Fuchsia_,
Fig. 229, as in _Cyclamen_, _Hyacinth_, &c. These relations of form
to position are, I believe, uniform. Though some flowers carried at
the ends of upright or downright stems have oblique shapes, it is only
when they have inclined axes or are not equally conditioned all round.
No solitary flower having an axis habitually vertical, presents a
bilateral form. This is as we should expect; since flowers which open
out their faces horizontally, whether facing upwards or downwards, are,
on the average, similarly affected on all sides.
[Illustration: Figs. 228, 229.]
[Illustration: Figs. 230, 231.]
At first it seems that flowers thus placed should alone be radial;
but further consideration discloses conditions under which this
type of symmetry may exist in flowers otherwise placed. Remembering
that the radial form is the primitive form--that, morphologically
speaking, it results from the contraction into a whorl, of parts that
are originally arranged in the same spiral succession as the leaves;
we must expect it to continue wherever there are no forces tending
to change it. What now must be the forces tending to change it? They
must be forces which do not simply affect differently the different
parts of an individual flower. They must be forces which affect in
like contrasted ways the homologous parts of other individual flowers,
both on the same plant and on surrounding plants of the same species.
A permanent modification can be expected only in cases where, by
inheritance, the effects of the modifying causes accumulate. That
they may accumulate the flowers must keep themselves so related to
the environment, that the homologous parts may, generation after
generation, be subjected to like differentiating forces. Hence, among
a plant’s flowers which maintain no uniformity in the relations of
their parts to surrounding influences, the radial form will continue.
Let us glance at the several causes which entail this variability.
When flowers are borne on many branches, which have all inclinations
from the vertical to the horizontal--as are the flowers of the Apple,
the Plum, the Hawthorn--they are placed in countless different
attitudes. Consequently, any spontaneous variation in shape which
might be advantageous were the attitude constant, is not likely to
be advantageous; and any functionally-produced modification in one
flower, is likely to be neutralized in offspring by some opposite
functionally-produced modification in another flower. It is quite
comprehensible, therefore, that irregularly-branched plants should
thus preserve their laterally-borne flowers from undergoing permanent
deviations from their primitive radial symmetry. Fig. 230, representing
a blossoming twig of the Blackthorn, illustrates this. Again, upright
panicles, such as those of the Saxifrage exemplified in Fig. 231,
and irregular terminal groups of flowers otherwise named, furnish
conditions under which there is similarly an absence of determinate
relations between the parts of the flowers and the incident forces;
and hence an absence of bilateralness. This inconstancy of relative
position is produced in various other ways--by extreme flexibility of
the stems, as in the Blue-bell; by the tendency of the peduncles to
curl to a greater or less extent in diverse directions, as in _Pyrola_;
by special twistings of the peduncles, differing in degree in different
individuals, as in _Convolvulus_; by unusual laxity of the petals,
as in _Lythrum_. Elsewhere the like general result arises from a
progressive change of attitude, as in _Myosotis_, the stem of which as
it unfolds causes each flower to undergo a transition from an upward
position of the mouth to a lateral position; or as in most _Cruciferæ_,
where the like effect follows from an altered direction of the peduncle.
There are, however, certain seemingly-anomalous cases where radial
symmetry is maintained by laterally-placed flowers, which keep
their parts in relative positions that are tolerably constant. The
explanation of these exceptions is not manifest. It is only when
we take into account certain incident actions liable to be left
unremembered, that we find a probable solution. It will be most
convenient to postpone the consideration of these cases until we have
reached the general rule to which they are exceptions.
§ 234. Transitions varying in degree from the radial towards the
bilateral, are common in flowers that are borne at the ends of branches
or axes which are inclined in tolerably constant ways. We may see this
in sundry garden flowers such as _Petunia_, or such as _Isoloma_ and
_Achimenes_, shown in Figs. 232 and 233. If these plants be examined,
it will be perceived that the mode of growth makes the flower unfold
in a partially one-sided position; that its parts of attachment have
rigidity sufficient to prevent this attitude from being very much
interfered with; and that though the individual flowers vary somewhat
in their attitudes, they do not vary to the extent of neutralizing the
differentiating conditions--there remains an average divergence from a
horizontal unfolding of the flower, to account for its divergence from
radial symmetry.
[Illustration: Figs. 232, 233.]
We pass insensibly from forms like these, to forms having bilateral
symmetry strongly pronounced. Some such forms occur among flowers that
grow at the ends of upright stems; as in _Pinguicula_, and in the
Violet tribe. But this happens only where, in successive generations,
the flower unfolds its parts sideways in constant relative positions.
And in the immense majority of flowers having well-marked two-sided
forms, the habitual exposure of the different parts to different
sets of forces, is effectually secured by the mode of placing. As
illustrations, I may name the genera--_Orchis_, _Utricularia_,
_Salvia_, _Salix_, _Delphinium_, _Mentha_, _Teucrium_, _Ajuga_,
_Ballota_, _Galeopsis_, _Lamium_, _Stachys_, _Nepeta_, _Marrubium_,
_Calamintha_, _Melittis_, _Prunella_, _Scutellaria_, _Bartsia_,
_Euphrasia_, _Rhinanthus_, _Melampyrum_, _Pedicularis_, _Linaria_,
_Digitalis_, _Orobanche_, _Fumaria_, _&c._; to which may be added all
the Grasses and all the _Papilionaceæ_. In most of these cases the
flowers, being sessile on the sides of upright stems, are kept in quite
fixed attitudes; and in the other cases the peduncles are very short,
or else stiff enough to secure general uniformity in the positions. A
few of the more marked types are shown in Figs. 234 to 241.
[Illustration: Figs. 234–241.]
[Illustration: Figs. 242–246.]
[Illustration: Figs. 247, 248.]
Very instructive evidences here meet us. Sometimes within the limits
of one genus we find radial flowers, bilateral flowers, and flowers of
intermediate characters. The genus
_Begonia_ may be instanced. In _B. rigida_ the flowers, various in
their attitudes, are in their more conspicuous characters radial:
though there is a certain bilateralness in the calyx, the five petals
are symmetrically disposed all round. _B. Wageneriana_ furnishes
two forms of flowers. On the same individual plant may be found
radial flowers like Fig. 242, and others, like Fig. 243, which are
merging into the bilateral. More decided is the bilateralness in _B.
albo-coccinea_, Fig. 244; and still more in _B. nitida_, Fig. 245.
While in _B. heracleifolia_, Fig. 246, the change reaches its extreme
by the disappearance of the lateral petals. On examining the modes
of growth in these several species, they will be seen to explain
these changes in the manner alleged. Even more conclusive are the
nearly-allied transformations occurring in artificially-produced
varieties of the same species. _Gloxinia_ may be named in illustration.
In Fig. 247 is represented one of the ordinary forms, which shows us
bilateralness of shape along with a mode of growth that renders the
conditions alike on the two sides while different above and below.
But in _G. erecta_, Fig. 248, we have the flower assuming an upright
attitude, and at the same time assuming the radial type. This is not
to be interpreted as a production of radial symmetry out of bilateral
symmetry, under the action of the appropriate conditions. It is rather
to be taken as a case of what is termed “peloria”--a reversion to the
primitive radial type, from which the bilateral modification had been
derived. The significant inference to be drawn from it is, that this
primitive radial type had an upright attitude; and that the derivation
of a bilateral type from it, occurred along with the assumption of an
inclined attitude.
We come now to a group of cases above referred to, in which radial
symmetry continues to co-exist with that constant lateral attitude
ordinarily accompanied by the two-sided form. Two examples will
suffice: one a very large flower, the Hollyhock, and the other a very
small flower, the Agrimony. Why does the radial form here remain
unchanged? and how does its continuance consist with the alleged
general law?
Until quite recently I have been unable to find any probable answers
to these questions. When the difficulty first presented itself, I
could think of no other possible cause for the anomaly, than that the
parts of the Hollyhock-flower, unfolding spirally as they do, might
have different degrees of spiral twist in different flowers, and might
thus not be unfolded in sufficiently-constant positions. But this
seemed a questionable interpretation; and one which did not obviously
apply to the case of the Agrimony. It was only on inquiring what are
the special causes of modifications in the forms of flowers, that a
more feasible explanation suggested itself; and this would probably
never have suggested itself, had not Mr. Darwin’s investigations into
the fertilization of Orchids led me to take into account an unnoticed
agency.
[Illustration: Fig. 249.]
The actions which affect the forms of leaves, affect much less
decidedly the forms of flowers; and the forms of flowers are influenced
by actions which do not influence the forms of leaves. Partly through
the direct action of incident forces and partly through the indirect
action of natural selection, leaves get their parts distributed in
ways that most facilitate their assimilative functions, under the
circumstances in which they are placed; and their several types
of symmetry are thus explicable. But in flowers, the petals and
fructifying organs of which do not contain chlorophyll, the tendency to
grow most where the supply of light is greatest, is less decided, if
not absent; and a shape otherwise determined is hence less liable to
alter in consequence of altered relations to sun and air. Gravity, too,
must be comparatively ineffective in causing modifications: the smaller
sizes of the parts, as well as their modes of attachment, giving them
greater relative rigidity. Not, indeed, that these incident forces of
the inorganic world are here quite inoperative. Fig. 249, representing
a species of _Campanula_, shows that the developments of individual
flowers are somewhat modified by the relations of their parts to
general conditions. But the fact to be observed is, that the extreme
transformations which flowers undergo are not likely to be thus caused:
some further cause must be sought. And if we bear in mind the functions
of flowers, we shall find in their adaptations to these functions,
under conditions that are extremely varied, an adequate cause for the
different types of symmetry, as well as for the exceptions to them.
Flowers are parts in which fertilization is effected; and the active
agents of this fertilization are insects--bees, moths, butterflies,
&c. Mr. Darwin has shown in many cases, that the forms and positions
of the essential organs of fructification, are such as to facilitate
the actions of insects in transferring pollen from the anthers of one
flower to the pistil of another--an arrangement produced by natural
selection. And here we shall find reason for concluding, that the forms
and positions of those subsidiary parts which give their shapes to
flowers, similarly arise by the survival of individuals which have the
subsidiary parts so adjusted as to aid this fertilizing process--the
deviations from radial symmetry being among such adjustments. The
reasoning is as follows. So long as the axis of a flower is vertical
and the conditions are similar all round, a bee or butterfly alighting
on it, will be as likely to come from one side as from another; and
hence, hindrance rather than facilitation would result if the several
sides of the flower did not afford it equally free access. In like
manner, flowers which are distributed over a plant in such ways that
their discs open out on planes of all directions and inclinations, will
have no tendency to lose their radial symmetry; since, on the average,
no part of the periphery is differently related to insect-agency
from any other part. But flowers so fixed as to open out sideways in
tolerably-constant attitudes, have their petals differently related
to insect-agency. A bee or butterfly coming to a laterally-growing
flower, does not settle on it in one way as readily as in another; but
almost of necessity settles with the axis of its body inclined upwards
towards the stem of the plant. Hence the side-petals of a flower so
fixed, habitually stand to the alighting insect in relations different
from those in which the upper and lower petals stand; and the upper
and lower petals differ from one another in their relations to it. If,
then, there so arises an habitual attitude of the insect towards the
petals, there is likely to be some arrangement of the petals that will
be most convenient to the insect--will most facilitate its entrance
into the flower. Thus we see in many cases, that a long undermost
petal or lip, by enabling the insect to settle in such way as to bring
its head opposite to the opening of the tube, aids its fertilizing
agency. But whatever be the special modifications of the corolla which
facilitate the actions of the particular insects concerned, all of them
will conduce to bilateral symmetry; since they will be alike for the
two sides but unlike for the top and bottom. And now we are prepared
for understanding the exceptions. Flowers growing sideways can become
thus adapted by survival of the fittest, only if they are of such sizes
and structures that insect-agency can affect them in the way described.
But in the plants named above, this condition is not fulfilled. A
Hollyhock-flower is so open, as well as so large, that its petals
are not in any appreciable degree differently related to the insects
which visit it. On the other hand, the flower of the Agrimony is so
small, that unless visited by insects of a corresponding size which
settle as bees and butterflies settle, its parts will not be affected
in the alleged manner. That all anomalies of this kind can at once be
satisfactorily explained, is scarcely to be expected: the circumstances
of each case have to be studied. But it seems not improbable that they
are due to causes of the kind indicated.[36]
§ 235. We have already glanced at clusters of flowers for the purpose
of considering their shapes as clusters. We must now return to them to
observe the modifications undergone by their component flowers. Among
these occur illustrations of great significance.
An example of transition from the radial to the bilateral form in
clustered flowers of the same species, is furnished by the cultivated
_Geraniums_, called by florists _Pelargoniums_. Some of these, bearing
somewhat small terminal clusters of flowers, which are closely packed
together with their faces almost upwards, have radially-symmetrical
flowers. But among other varieties having terminal clusters of which
the members are mutually thrust on one side by crowding, the flowers
depart very considerably from the radial shape towards the bilateral
shape. A like result occurs under like conditions in Rhododendrons
and Azaleas. The _Verbena_, too, furnishes an illustration of radial
flowers rendered slightly two-sided by the slight two-sidedness
of their relations to other flowers in the cluster. And among the
_Cruciferæ_ a kindred case occurs in the cultivated Candytuft.
Evidence of a somewhat different kind is offered us by clustered
flowers in which the peripheral members of the clusters differ from
the central members; and this evidence is especially significant
where we find allied species that do not exhibit the deviation, at
the same time that they do not fulfil the conditions under which it
may be expected. Thus, in _Scabiosa succisa_, Fig. 250, which bears
its numerous small flowers in a hemispherical knob, the component
flowers, similarly circumstanced, are all equal and all radial; but in
_Scabiosa arvensis_, Fig. 251, in which the numerous small flowers form
a flattened disk only the confined central ones are radial: round the
edge the flowers are much larger and conspicuously bilateral.
[Illustration: Figs. 250, 251.]
[Illustration: Fig. 252.]
But the most remarkable and most conclusive proofs of these relations
between forms and positions, are those given by the clustered flowers
called _Umbelliferæ_. In some cases, as where the component flowers
have all plenty of room, or where the surface of the umbel is more or
less globular, the modifications are not conspicuous; but where, as
in _Viburnum_, _Chærophyllum_, _Anthriscus_, _Torilis_, _Caucalis_,
_Daucus_, _Tordylium_, &c., we have flowers clustered in such ways as
to be differently conditioned, we find a number of modifications that
are marked and varied in proportion as the differences of conditions
are marked and varied. In _Chærophyllum_, where the flowers of each
umbellule are closely placed so as to form a flat surface, but
where the umbellules are wide apart and form a dispersed umbel, the
umbellules do not differ from one another; though among the flowers
of each umbellule there are decided differences: the central flowers
being small and radial, while the peripheral ones are large and
bilateral. But in other genera, where not only the flowers of each
umbellule but also the umbellules themselves, are closely clustered
into a flat surface, the umbellules themselves become contrasted;
and many remarkable secondary modifications arise. In an umbel of
_Heracleum_, for instance, there are to be noted the facts;--first,
that the external umbellules are larger than the internal ones; second,
that in each umbellule the central flowers are less developed than the
peripheral ones; third, that this greater development of the peripheral
flowers is most marked in the outer umbellules; fourth, that it is
most marked on the outer sides of the outer umbellules; fifth, that
while the interior flowers of each umbellule are radial, the exterior
ones are bilateral; sixth, that this bilateralness is most marked in
the peripheral flowers of the peripheral umbellules; seventh, that the
flowers on the outer sides of these peripheral umbellules are those in
which the bilateralness reaches a maximum; and eighth, that where the
outer umbellules touch one another, the flowers, being unsymmetrically
placed, are unsymmetrically bilateral.[37] The like modifications are
displayed, though not in so clearly-traceable a way, in an umbel of
_Tordylium_, Fig. 252. Considering how obviously these various forms
are related to the various conditions, we should be scarcely able, even
in the absence of all other facts, to resist the conclusion that the
differences in the conditions are the causes of the differences in the
forms.
[Illustration: Fig. 253.]
Composite flowers furnish evidence so nearly allied to that which
clustered flowers furnish, that we may fitly glance at them under
the same head. Such a common type of this order as the Sun-flower,
exemplifies the extremely marked difference which arises in many
of these plants between the closely-packed internal florets, each
similarly circumstanced on all sides, and the external florets, not
similarly circumstanced on all sides. In Fig. 253, representing the
inner and outer florets of a Daisy, the contrast is marked between the
small radial corolla of the one and the larger bilateral corolla of the
other. In many cases, however, this contrast is less marked: the inner
florets also having their outward-growing prolongations--a difference
possibly related to some difference in the habits of the insects that
fertilize them. Nevertheless, these composite flowers which have
inner florets with strap-shaped corollas outwardly directed, equally
conform to the general principle; both in the radial arrangement of
the assemblage of florets, and in the bilateral shape of each floret;
which has its parts alike on the two sides of a line passing from the
centre of the assemblage to the circumference. Certain other members
of this order fulfil the law somewhat differently. In _Centaurea_,
for instance, the inner florets are small and vertical in direction,
while the outer florets are large and lateral in direction. And here
may be remarked, in passing, a clear indication of the effect which
great flexibility of the petals has in preventing a flower from
losing its original radiate form; for while in _C. cyanus_, the large
outward-growing florets, having short, stiff divisions of the corolla,
are decidedly bilateral, in _C. scabiosa_, where the divisions of the
corolla are long and flexible, the radial form is scarcely at all
modified. On bearing in mind the probable relations of the forms to
insect-agency, the meaning of this difference will not be difficult to
understand.[38]
§ 236. In extremely-varied ways there are thus re-illustrated among
flowers, the general laws of form which leaves and branches and
entire plants disclose to us. Composed as each cluster of flowers
is of individuals that are originally similar; and composed as each
flower is of homologous foliar organs; we see both that the like
flowers become unlike and the like parts of each flower become unlike,
where the positions involve unlike incidence of forces. The symmetry
remains radial where the conditions are equal all round; shows
deviation towards two-sidedness where there is slight two-sidedness
of conditions; becomes decidedly bilateral where the conditions are
decidedly bilateral; and passes into an unsymmetrical form where the
relations to the environment are unsymmetrical.