328. The end of the flower is attained when the ovules become seeds. A
flower remains for a certain time (longer or shorter according to the
species) in _anthesis_, that is, in the proper state for the fulfilment
of this end. During anthesis, the ovules have to be fertilized by the
pollen; or at least some pollen has to reach the stigma, or in
gymnospermy the ovule itself, and to set up the peculiar growth upon its
moist and permeable tissue, which has for result the production of an
embryo in the ovules. By this the ovules are said to be _fertilized_.
The first step is _pollination_, or, so to say, the sowing of the proper
pollen upon the stigma, where it is to germinate.
Sec. 1. ADAPTATIONS FOR POLLINATION OF THE STIGMA.
329. These various and ever-interesting adaptations and processes are
illustrated in the "Botanical Text Book, Structural Botany," chap. VI.
sect. iv., also in a brief and simple way in "Botany for Young People,
How Plants Behave." So mere outlines only are given here.
330. Sometimes the application of pollen to the stigma is left to
chance, as in dioecious wind-fertilized flowers; sometimes it is
rendered very sure, as in flowers that are fertilized in the bud;
sometimes the pollen is prevented from reaching the stigma of the same
flower, although placed very near to it, but then there are always
arrangements for its transference to the stigma of some other blossom of
the kind. It is among these last that the most exquisite adaptations are
met with.
331. Accordingly, some flowers are particularly adapted to close or
self-fertilization; others to cross fertilization; some for either,
according to circumstances.
_Close Fertilization_ occurs when the pollen reaches and acts upon a
stigma of the very same flower (this is also called self-fertilization),
or, less closely, upon other blossoms of the same cluster or the same
individual plant.
_Cross Fertilization_ occurs when ovules are fertilized by pollen of
other individuals of the same species.
_Hybridization_ occurs when ovules are fertilized by pollen of some
other (necessarily some nearly related) species.
332. =Close Fertilization= would seem to be the natural result in
ordinary hermaphrodite flowers; but it is by no means so in all of them.
More commonly the arrangements are such that it takes place only after
some opportunity for cross fertilization has been afforded. But close
fertilization is inevitable in what are called
_Cleistogamous Flowers_, that is, in those which are fertilized in the
flower-bud, while still unopened. Most flowers of this kind, indeed,
never open at all; but the closed floral coverings are forced off by the
growth of the precociously fertilized pistil. Common examples of this
are found in the earlier blossoms of Specularia perfoliata, in the later
ones of most Violets, especially the stemless species, in our wild Jewel
weeds or Impatiens, in the subterranean shoots of Amphicarpaea. Every
plant which produces these cleistogamous or bud-fertilized flowers bears
also more conspicuous and open flowers, usually of bright colors. The
latter very commonly fail to set seed, but the former are prolific.
333. =Cross Fertilization= is naturally provided for in dioecious
plants (249), is much favored in monoecious plants (249), and hardly
less so in dichogamous and in heterogonous flowers (338). Cross
fertilization depends upon the transportation of pollen; and the two
principal agents of conveyance are winds and insects. Most flowers are
in their whole structure adapted either to the one or to the other.
334. =Wind-fertilizable or Anemophilous= flowers are more commonly
dioecious or monoecious, as in Pines and all coniferous trees, Oaks, and
Birches, and Sedges; yet sometimes hermaphrodite, as in Plantains and
most Grasses; they produce a superabundance of very light pollen,
adapted to be wind-borne; and they offer neither nectar to feed winged
insects, nor fragrance nor bright colors to attract them.
335. =Insect-fertilizable or Entomophilous= flowers are those which are
sought by insects, for pollen or for nectar, or for both. Through their
visits pollen is conveyed from one flower and from one plant to another.
Insects are attracted to such blossoms by their bright colors, or their
fragrance, or by the nectar (the material of honey) there provided for
them. While supplying their own needs, they carry pollen from anthers to
stigmas and from plant to plant, thus bringing about a certain amount of
cross fertilization. Willows and some other dioecious flowers are so
fertilized, chiefly by bees. But most insect-visited flowers have the
stamens and pistils associated either in the same or in contiguous
blossoms. Even when in the same blossom, anthers and stigmas are very
commonly so situated that under insect-visitation, some pollen is more
likely to be deposited upon other than upon own stigmas, so giving a
chance for cross as well as for close fertilization. On the other hand,
numerous flowers, of very various kinds, have their parts so arranged
that they must almost necessarily be cross-fertilized or be barren, and
are therefore dependent upon the aid of insects. This aid is secured by
different exquisite adaptations and contrivances, which would need a
volume for full illustration. Indeed, there is a good number of volumes
devoted to this subject.[1]
336. Some of the adaptations which favor or ensure cross fertilization
are peculiar to the particular kind of blossom. Orchids, Milkweeds,
Kalmia, Iris, and papilionaceous flowers each have their own special
contrivances, quite different for each.
337. Irregular flowers (253) and especially irregular corollas are
usually adaptations to insect-visitation. So are all _Nectaries_,
whether hollow spurs, sacs, or other concavities in which nectar is
secreted, and all _nectariferous glands_.
338. Moreover, there are two arrangements for cross fertilization common
to hermaphrodite flowers in various different families of plants, which
have received special names, _Dichogamy_ and _Heterogony_.
339. =Dichogamy= is the commoner case. Flowers are _dichogamous_ when
the anthers discharge their pollen either before or after the stigmas of
that flower are in a condition to receive it. Such flowers are
_Proterandrous_, when the anthers are earlier than the stigmas, as in
Gentians, Campanula, Epilobium, etc.
_Proterogynous_, when the stigmas are mature and moistened for the
reception of pollen, before the anthers of that blossom are ready to
supply it, and are withered before that pollen can be supplied.
Plantains or Ribworts (mostly wind-fertilized) are strikingly
proterogynous: so is Amorpha, our Papaws, Scrophularia, and in a less
degree the blossom of Pears, Hawthorns, and Horse-chestnut.
340. In Sabbatia, the large-flowered species of Epilobium, and
strikingly in Clerodendron, the dichogamy is supplemented and perfected
by movements of the stamens and style, one or both, adjusted to make
sure of cross fertilization.
341. =Heterogony.= This is the case in which hermaphrodite and fertile
flowers of two sorts are produced on different individuals of the same
species; one sort having higher anthers and lower stigmas, the other
having higher stigmas and lower anthers. Thus reciprocally disposed, a
visiting insect carries pollen from the high anthers of the one to the
high stigma of the other, and from the low anthers of the one to the low
stigma of the other. These plants are practically as if dioecious,
with the advantage that both kinds are fruitful. Houstonia and
Mitchella, or Partridge-berry, are excellent and familiar examples.
These are cases of
_Heterogone Dimorphism_, the relative lengths being only short and long
reciprocally.
_Heterogone Trimorphism_, in which there is a mid-length as well as a
long and a short set of stamens and style; occurs in Lythrum Salicaria
and some species of Oxalis.
342. There must be some essential advantage in cross fertilization or
cross breeding. Otherwise all these various, elaborate, and exquisitely
adjusted adaptations would be aimless. Doubtless the advantage is the
same as that which is realized in all the higher animals by the
distinction of sexes.
Sec. 2. ACTION OF POLLEN, AND FORMATION OF THE EMBRYO.
343. =Pollen-growth.= A grain of pollen may be justly likened to one of
the simple bodies (_spores_) which answer for seeds in Cryptogamous
plants. Like one of these, it is capable of germination. When deposited
upon the moist surface of the stigma (or in some cases even when at a
certain distance) it grows from some point, its living inner coat
breaking through the inert outer coat, and protruding in the form of a
delicate tube. This as it lengthens penetrates the loose tissue of the
stigma and of a loose conducting tissue in the style, feeds upon the
nourishing liquid matter there provided, reaches the cavity of the
ovary, enters the orifice of an ovule, and attaches its extremity to a
sac, or the lining of a definite cavity, in the ovule, called the
_Embryo-Sac_.
344. =Origination of the Embryo.= A globule of living matter in the
embryo-sac is formed, and is in some way placed in close proximity to
the apex of the pollen tube; it probably absorbs the contents of the
latter; it then sets up a special growth, and the _Embryo_ (8-10) or
rudimentary plantlet in the seed is the result.
FOOTNOTES:
[1] Beginning with one by C. C. Sprengel in 1793, and again in our day
with Darwin, "On the Various Contrivances by which Orchids are
fertilized by Insects," and in succeeding works.