Chapter VIII: Fertilisation

Having now considered some of the many wonderful arrangements by which the pollen of plants is dispersed, we will endeavour by tracing the course of the pollen-grains after they reach the stigma, to learn what is meant by the term “fertilisation of the ovules.” These are the minute specks contained in the ovary which are to become seeds, and by means of which the plant will eventually reproduce itself.

To the naked eye the yellow pollen we see on the anthers of flowers appears as small grains; but, when magnified, these grains are seen to be singularly beautiful, each little sphere having on its surface a chequered network and delicately sculptured patterns.

The forms, too, are as varied as the ornamentation.

Some plants have triangular grains, some oval-shaped and others many-sided.

1 _Morina._
2 _Cobea._
3 _Convolvulus._
4 _Dianthus._
5 _Pinus._
6 _Albucca._
7 _Buphthalmum._
]

I have given a few examples, and would specially call attention to the pollen-grains of the Pinus tribe (fir-trees), to which I alluded in the last chapter. These are remarkably buoyant, owing to the two little bladders with which they are furnished.

Now we are going to watch this yellow dust performing its appointed office in the central organ of a flower. In order to do so we will take a white garden lily, and remove the petals, sepals, and stamens, leaving only the pistil, which, as shown in the drawing, consists of three parts, the club-like stigma, a very long style, and its base the ovary, which contains three cavities. In these last we see a number of small, colourless spore-like bodies termed ovules (from _ovum_, an egg), each consisting of an outer coat, and a mass of cells in the centre called the _nucellus_.

An opening exists at one end of each ovule called the micropyle (meaning a little gate or entrance), and this opening leads down into the middle of the nucellus, where lies what we may call the life-principle, but what is known in botany as the embryo-sac.

We need the aid of a microscope to enable us to see how the pollen exerts its influence upon the ovules.

If we place a drop of very weak sugar and water upon a slip of grass, and sprinkle over it some pollen grains of the common white lily, then allowing the slide to remain for a few hours in a dark place, it will be fit for our purpose.

When placed in the microscope we shall observe that many of the grains will have thrown out long thread-like tubes, and this is just what happens when pollen falls upon the viscid stigma of the lily. Referring to the section of a lily pistil we see that a pollen grain has rested on the stigma, and, excited into growth by the sweetish fluid which holds it there, it sends down a slender tube through the centre of the pistil, which is lined with a very delicate loose tissue of cells filled with starch, oils and food-materials. The pollen-tube is stimulated and fed by this nourishment stored up in the conducting tissue, and on it goes until, passing through the micropyle, it enters the embryo-sac of one of the ovules, adheres to it, and renders it fertile.

Only one grain is shown in the drawing for the sake of clearness, but of course each ovule is sought out and fertilised by a pollen-tube. With infinite variation this process takes place in every flower, so that even the commonest weed affords evidence of the marvellous provisions made by an All-Wise Creator for the preservation of species.

The time occupied by the passage of the pollen-tube varies considerably. In the fir tribe it takes nearly twelve months, in the hazel-nut and orchis it requires several weeks, whilst in many other plants the whole process is completed in a few hours.

One of the first results of fertilisation is a rapid withering of the style and flower; the great end of the flowering period has been attained, and so without further expense of energy the bright petals die away.

At the same time other external changes take place, which are obvious to every observer of nature. The lower end of the pistil, known as the ovary, begins its second growth, and in a short time swells into a large structure, the shape of which varies much in different species of plants. Finally, the ovary changes colour and develops other characteristics quite different from its former conditions. These characters have reference to the distribution of its seeds, and in our chapter on fruits we shall learn something about the interesting botanical significance of the various hard and soft fruits, and see how they all arise from fertilisation.

Take, for example, the flower of an apple immediately after fertilisation is effected. The petals fall off, the styles shrivel up and the ovary rapidly enlarges; the tube of the calyx becomes fleshy, and finally the well-formed apple is produced. The change, however, does not end here; in this stage of development the little apple is bitter and is charged with a vegetable acid. As the fruit grows on, however, this acid changes into sweet juice varying in flavour according to the species of apple.

Now let us examine the interior of the ovary and see what changes have arisen as a consequence of fertilisation.

The egg cell which has received the pollen grain becomes filled with an embryo, whilst the thin delicate coat of the ovule develops into strong seed-coats.

The embryo is the first germ of the young plant that is to be. It is a tiny speck indeed in its beginning, but deeply interesting to us when we realise that, because it possesses life, it will grow on and on, and result, according to its species, either in a plant but a few inches in height, or in a grand forest-tree which may give shelter to man and animals for hundreds of years.

The naked eye can scarcely trace any indications of form in the embryo, but when dissected and examined with a lens it is seen to consist of a tiny plant, root, stem and leaves (cotyledons).

The size of the embryo in comparison with the other part of the seed is a point which should be observed.

As the embryo develops it absorbs the special nutrient or reserve tissue that exists in all ovules; a bean embryo, for example, rapidly absorbs all the nucellus of the ovule, so that at length the seed-coat contains nothing but the embryo, the two cotyledons of which are thick and filled with stores of food for the first growth of the seed.

I would advise students to plant a few broad beans in a little damp cocoa fibre, and carefully watch their growth. It is advisable to dissect these beans successively at different stages, so as to watch the development of the radicle (root) and plumule (young leaf-bud). Place the seed in what position we may, the radicle will always find its way down into the earth, while the plumule obeys its vegetable instinct, and rises into the air. The embryo of the castor-oil bean and that of the cocoa-nut do not, however, use up all the nutritive matter in the ovule as the broad bean does, so that when the seed is ripe we find inside it, not only the embryo, but also a quantity of cheesy matter known as _albumen_, and seeds of this kind are hence called _albuminous_, whilst peas, beans and hazel-nuts are classed as _ex-albuminous_ (without albumen).

An interesting development consequent upon fertilisation is a growth which occurs in some plants from the base of the ovule. The pretty red coverings of the seeds of the spindle-tree, and the bright berry-like structure on the seeds of the yew-tree are examples of this growth, which is known botanically as an aril (from _arillus_, a wrapper). In the willows this aril is a very lovely covering of silky hairs, these serve to float the seeds on the atmosphere at every puff of wind.

The pretty lace-like covering on the nutmeg is another example of an aril, better known to us in the form of the fragrant spice called mace.

The style, which in most plants dies as soon as the ovules are fertilised, is in other cases persistent, as in the hedge-climber called travellers’ joy. The white, feathery-looking seeds owe their special character to the persisting styles, which, after fertilisation, grow into the bunches of fluffy seeds, which hang in profusion on hedges in the country.

I will conclude this chapter with a reference to a change of quite a different character. Not unfrequently, fertilisation results in the suppression of certain chambers in the ovary, and in the consequent failure of the development of the ovules.

A cross-section of a young oak ovary shows a three-chambered structure, each cavity containing two ovules, so that the ovary in this stage contains six ovules in three chambers. Soon after the act of fertilisation, one of the fertilised ovules takes the lead in growth, starves the other five ovules, and, as it grows, pushes the partitions of the other chambers aside, and gradually fills up the whole interior, converting it into a one-celled structure. This change happens also in the birch; its two-chambered ovary becomes one; and in the lime, though at first it has a many-chambered ovary, yet in the ripened fruit there is rarely more than one to be found.

In a few plants, changes of quite an opposite character take place. In the ovary of the datura,[18] for instance, we find two cells; after fertilisation, two false or spurious partitions are developed, dividing the original two-celled structure into four parts, and as a consequence we get a four-chambered fruit. The same change takes place in some of the pea family.

[18] Thorn-apple.

Specimens to be observed:—Examine pollen grains, with lens or microscope, dissect white lily, flower-pollen on glass slide. Observe changes in growing apple, plant broad beans, castor-oil seeds, and maize; examine spindle-tree berries (_euonymus_), yew-tree berries, willow seeds, nutmeg, and mace; travellers’ joy (clematis), section of oak ovary in the pistillate flower. Examine birch catkins and lime-tree flowers. Datura seed-vessel.

CHAPTER IX

_FRUIT_

“Here, as I steal along the sunny wall,
Where Autumn basks, with fruit empurpled deep,
My pleasing theme continual prompts my thought;
Presents the downy peach; the shining plum;
The ruddy, fragrant nectarine; and dark,
Beneath his ample leaf, the luscious fig.
The vine, too, here her curling tendrils shoots,
Hangs out her clusters, glowing to the south,
And scarcely wishes for a warmer sky.”

+James Thomson.+