The Elements of Botany Part 17

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.

-- 1. ADAPTATIONS FOR POLLINATION OF THE STIGMA.

329. These various and ever-interesting adaptations and processes are ill.u.s.trated 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 dicious 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 circ.u.mstances.

_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 cl.u.s.ter 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 dicious plants (249), is much favored in moncious plants (249), and hardly less so in dichogamous and in heterogonous flowers (338). Cross fertilization depends upon the transportation of pollen; and the two princ.i.p.al 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 dicious or moncious, as in Pines and all coniferous trees, Oaks, and Birches, and Sedges; yet sometimes hermaphrodite, as in Plantains and most Gra.s.ses; 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 dicious flowers are so fertilized, chiefly by bees. But most insect-visited flowers have the stamens and pistils a.s.sociated 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 ill.u.s.tration. 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 dicious, with the advantage that both kinds are fruitful. Houstonia and Mitch.e.l.la, 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 s.e.xes.

-- 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.

Section XIV. THE FRUIT.

345. =Its Nature.= The ovary matures into the Fruit. In the strictest sense the fruit is the seed-vessel, technically named the PERICARP. But practically it may include other parts organically connected with the pericarp. Especially the calyx, or a part of it, is often incorporated with the ovary, so as to be undistinguishably a portion of the pericarp, and it even forms along with the receptacle the whole bulk of such edible fruits as apples and pears. The receptacle is an obvious part in blackberries, and is the whole edible portion in the strawberry.

346. Also a cl.u.s.ter of distinct carpels may, in ripening, be consolidated or compacted, so as practically to be taken for one fruit.

Such are raspberries, blackberries, the Magnolia fruit, etc. Moreover, the ripened product of many flowers may be compacted or grown together so as to form a single compound fruit.

347. =Its kinds= have therefore to be distinguished. Also various names of common use in descriptive botany have to be mentioned and defined.

348. In respect to composition, accordingly, fruits may be cla.s.sified into

_Simple_, those which result from the ripening of a single pistil, and consist only of the matured ovary, either by itself, as in a cherry, or with calyx-tube completely incorporated with it, as in a gooseberry or cranberry.

_Aggregate_, when a cl.u.s.ter of carpels of the same flower are crowded into a ma.s.s; as in raspberries and blackberries.

[Ill.u.s.tration: Fig. 366. Forming fruit (capsule) of Gaultheria, with calyx thickening around its base. 367. Section of same mature, the berry-like calyx nearly enclosing the capsule.]

[Ill.u.s.tration: Fig. 368. Section of a part of a strawberry. Compare with Fig. 360.]

[Ill.u.s.tration: Fig. 369. Similar section of part of a blackberry. 370.

One of its component simple fruits (drupe) in section, showing the pulp, stone, and contained seed; more enlarged. Compare with Fig. 375.]

_Accessory_ or _Anthocarpous_, when the surroundings or supports of the pistil make up a part of the ma.s.s; as does the loose calyx changed into a fleshy and berry-like envelope of our Wintergreen (Gaultheria, Fig.

366, 367) and Buffalo-berry, which are otherwise simple fruits. In an aggregate fruit such as the strawberry the great ma.s.s is receptacle (Fig. 360, 368); and in the blackberry (Fig. 369) the juicy receptacle forms the central part of the savory ma.s.s.

_Multiple_ or _Collective_, when formed from several flowers consolidated into one ma.s.s, of which the common receptacle or axis of inflorescence, the floral envelopes, and even the bracts, etc., make a part. A mulberry (Fig. 408, which superficially much resembles a blackberry) is of this multiple sort. A pine-apple is another example.