Chapter VII: Pollination

We now come to the consideration of the real function of the flower of a plant. In whatever form it is developed, whether as a gay and fragrant blossom, in a dull foul-smelling structure like the arum, or as a green inconspicuous little floret like the grass, its main office is to reproduce itself by the formation of seed. We will first glance at some of the wonderful agencies that actively help in this work.

There are at least three distinct processes necessary for the complete formation of a perfect seed, and we must, I fear, persuade ourselves to learn some of the long words by which botanists speak of these processes. They are known as pollination, fertilisation, and the growth of the ovule. There is so much to be said about the first subject, that I must leave the two latter for a succeeding chapter.

Before seed can be formed it is necessary that the powder contained in the anthers, which is called pollen, should be transferred from those anthers to the stigma or upper part of the pistil, and this transference is called pollination. If we examine a tulip or, better still, a buttercup, we find the anthers and stigmas so near together that the transfer of the dust-like pollen to the sticky-looking stigmas can easily take place. This would be called an instance of self-pollination, but although cases of this kind do occur in nature, they are not at all common. As a rule, in order to ensure what is called cross-pollination, the transfer of the pollen of one flower to the stigma of another, many wonderful and interesting arrangements exist even in some of our commonest flowers.

Cross-pollination must be the case in such plants as dog’s mercury, because we find in a colony of these plants—so frequently seen by the roadside—that some plants have flowers with stamens only, and others containing only pistils. Again, in the hazel we may see how impossible it is for self-pollination to take place, as, if we examine the pistils, we find that they consist of scales bearing stamens and pollen only, whilst somewhere close by, on the same stem, hangs the pretty little red flower which possesses the pistil and forked stigma. If seed is to be formed in either of these flowers and in many others similarly arranged, then the pollen of one flower must be transferred to the stigma of the other.

There are interesting facts to be learned about the common primrose. When we examine a little bunch of these flowers we find quite half of them are what children call pin-eyed, meaning that the stigma, which is at the end of a long pistil, is like the head of a pin in the throat of the primrose.

Looking at the sketch, we see at once that self-pollination is hindered by the fact that the anthers in this flower being at the bottom of the tube, the pollen they contain must be transferred by some direct agency before it can come in contact with any stigma. Now let us examine the other flowers in our primrose nosegay; we find the stamens in these are placed in the mouth of the tube, and the pistil is quite short and low down in position. At first sight it appears as if the pollen would fall directly upon the pistil, since the stamens are above that organ, but this is not exactly what happens; the pollen of this particular form of flower is shed before the stigma is mature, so that when it has reached maturity the pollen is all gone.

The arrangement of nature is as follows. An insect attracted by the sweet-smelling bank of primroses will visit the flowers, thrusting its proboscis down a pin-eyed flower until in so doing its head has been dusted with the pollen of the stamens; then withdrawing from that flower the insect visits another near by, possibly one with a short pistil; the pollen on its head is now rubbed off and falls upon the stigma below and pollinates it, for that is the term used when this act takes place.

The pretty maiden-pink will help us still more clearly to understand how cross-pollination is promoted in flowers containing both stamens and pistils. Select a flower that has just opened, the petals of which are spreading and fringed, whilst from the centre of the flower a cluster of stamens projects with the pollen mature and easily shaken out of the anther lobes; the pistil is concealed in the long tube, and in this stage there is no sign of stigma. In a short time, however, if we examine the flower again, we shall find the stamens have shrivelled up, and in their place a forked stigma appears, as shown in the sketch. Here again it is obvious that the fact of the stamens ripening first and expending their energy before the pistil is ripe must mean, that in order to secure seed the pollen from some younger flower must be transferred, probably also by insect agency. It will give fresh interest to our garden rambles if we remember that the bees and flies we see hovering over the flowers are not only collecting honey or feasting upon it, but are also performing a very important office for the benefit of the plants they are visiting.

We may now proceed to notice the various agencies for the conveyance of pollen between flowers.

These agencies are water, wind, insects and birds.

In an earlier chapter I gave an account of the _Vallisneria spiralis_, which will serve as a type of a water-pollinated flower.

Those pollinated by wind are, as I have said in a previous chapter, called anemophilous (_anemos_, wind, and _philos_, loving). They are usually of small size and inconspicuous character, with very little or no scent, and devoid of colour; these are characteristics that are not always associated in the same species; thus in the hazel, which is a wind-pollinated flower, we find a bright yellow catkin (so well known to children as lambs’ tails) and a small but bright red pistil.

Let us notice, however, how wonderfully these plants are adapted for this method of pollination; the stamens are usually hanging, and the pollen, produced in great quantities, is easily set free by the slightest breath of wind. The stigma of the hazel, of different grasses and of sedges are both forked and plumed, so that pollen grains floating in the air are readily intercepted.

The firs and pines are excellent examples of wind-pollinated trees. I remember once possessing a ripe male cone of the _Araucaria imbricata_, and ascertaining that it contained as much as a wine-glassful of pollen. Speaking about this fact to the gardener at the Pinetum at Dropmore, I was shown how this fertilising dust from the great Araucaria (which was planted there in 1830) was carried by the wind for an amazing distance to a female tree on the other side of the garden, pollinating its cones so that they produced fertile seeds. In some of the Canadian pine forests, the trees shed forth such quantities of pollen in the flowering season that the ground becomes perfectly yellow. The early settlers, being unable to account for the strange phenomenon in any other way, ascribed it to showers of sulphur descending from the clouds. Even in our own country the foliage and undergrowth in the neighbourhood of fir woods is often thickly coated with the yellow dust falling from the male catkins of the trees; the structure of the pollen grains is such that they float very buoyantly, each grain being provided with two air bladders. I may mention in passing that this apparently wasted pollen affords a rich feast to endless species of bees and flies, and is in many cases stored up by them as food for their young grubs. The various adaptations for wind pollination will perhaps be better understood if we glance at the attractions which flowers offer to birds and insects.

Colour serves to render flowers attractive to insects, and to make them conspicuous; the bracts, petals, and sepals of flowers are usually of some light or dark colour quite distinct from the green tone of the foliage.

It has been ascertained also that plants which are pollinated by night-flying moths generally have white or light-yellow flowers so as to be easily seen in twilight.

One of the most interesting of these night-pollinated flowers is _Silene nutans_, the Nottingham catchfly. In the daytime the five narrow petals are curled up and look dead and withered, but as night comes on they change their position, and the flower has the expanded shape of an alpine pink. In this open condition it is visited by the moths which, flying from one flower to another, transfer the pollen, and thus accomplish at night what more frequently occurs in the sunlight; at daybreak the petals roll up once more, and one would again suppose the flower to be dead; but no, it will continue to open at nightfall until some moth finally succeeds in pollinating its blossom. A small species of moth[17] visits this catchfly in order to deposit its eggs; these, by means of a very long ovipositor, it places in the ovary, and in that somewhat inflated cavity they produce microscopic caterpillars which find shelter and nutriment in the strange nest. When the caterpillars arrive at maturity they escape by biting a hole in the wall of the capsule, and creeping out, they seek for a suitable place in which to turn to chrysalides.

[17] _Dianthræcia albimacula._

Scentless flowers usually have some equivalent form of attraction, such as honey, brilliant colour pollen in abundance, or the grouping of a number of small florets, in order to secure a conspicuous effect as in the ox-eye daisy, or hedge parsley.

Strong and varied odours are great helps to ensure pollination by insects. The bee-tribe and moths and butterflies are specially attracted by the sweet scents of roses, violets, carnations, and sweet-peas, and the powerful odour emitted by such flowers as the evening primrose, tobacco, and night-flowering rocket as evening comes on tends to guide the nocturnal moths to these and similar flowers. An odour may, of course, be pleasant to an insect which to us would be simply intolerable. The arum of the hedges, and those curious plants, the aristolochias and stapelias, all emit scents of the most fœtid description, as we think, but flies, on the contrary, are attracted by thousands, and hold apparently joyous revels in the blossoms which they are pollinating by their frequent visits.

A little care and patience in watching the visits of insects to different flowers will soon be rewarded by a perception of the tastes and likings of insect life, and we shall gradually learn to expect to see certain insects on the flowers they specially frequent.

I would call attention to the interesting fact that if one agency fails to effect pollination, another is adopted in order to attain the desired end. Thus, when the flowers of the common bartsia first open, they are visited by insects; but, in the later stages of flowering, the pollen is blown out by the wind, and the neighbouring stigmas thus become pollinated. We see in the arrangement of the flower of the St. John’s wort (_Hypericum_) a perfect type of this provision against any possible failure of pollination. The stigma is surrounded by groups of stamens of unequal length; those in the centre nearest to the stigma are as long as the style itself, whilst those on the outside are short, and these shed their pollen first, whilst those in close contact with the stigma shed their contents last. Thus we find that if insects fail to effect cross-pollination by means of the short and early opened stamens, it is secured by means of the longer stamens whose anthers are in close contact with the stigma. Again, when we stand under a sycamore tree, we may see that the green tassel-like flowers are having their pollen dispersed both by wind and bees.

We cannot draw hard-and-fast lines in nature, for although a special end may be kept in view, the various means and adaptations by which it is attained are a continual source of admiration and wonder to the reverent student of nature.

We have already seen that there are all kinds of devices by which the pollen of one flower may be made sure to reach the stigma of another; but, if by any means this crossing fails, if the weather is such that insects are scarce, or other conditions cause failure, then, in the case of many flowers, most curious contrivances are provided to secure seed by self-pollination. Truly this is one of the most beautiful of God’s wonders in floral construction. One of the gems of my own flower garden is a lovely little Japanese toad-lily (_Tricyrtis hirta_). In this flower there are three styles which stand well above the stamens; the points of the styles are bent over as in the plate, and the stigmatic surface grows mature before the anthers shed their pollen; if, however, no insect visits the flowers, pollination is effected in the following way. The styles bend down and place their forked points in direct contact with the open anther-lobes (as shown in drawing), the style assuming almost the form of a semicircle. This is done very deliberately, for it is often fully a week before the act is complete.

_Stigma and Stamen._]

Pollination is effected in tropical countries not only by insects of many kinds, but by the lovely tribes of humming-birds which abound in those regions. Their slender, curved beaks are specially adapted to penetrate the honey-laden flowers with long-tubed blossoms, which could only be pollinated by some such agency.

Those who are within reach of the Natural History Museum at South Kensington may there see a gallery filled with exquisite specimens of humming-birds, arranged in cases, and some of the birds are shown as they appear in life, hovering over tropical flowers, drawing honey from their hanging blossoms, and performing the useful office of transferring the pollen from one flower to another, thus ensuring the fertilisation of the seed.

I might go on multiplying examples of the various methods by which seed is rendered fertile, but perhaps enough has been said to show what hidden force exists in flowers to enable them to attain the end for which they mainly exist, namely, the perpetuation of their species by means of seed.

Specimens to be obtained and compared with the descriptions in this chapter:—Buttercup flower, dog’s mercury, hazel catkins, primrose flowers, male blossoms of pine trees in June, Nottingham catchfly, ox-eye daisy, bartsia, St. John’s wort flowers, and Japanese toad-lily.

CHAPTER VIII

_FERTILISATION_

“The men
Whom Nature’s works can charm, with God Himself
Hold converse.”

+Akenside.+