Pollinator

“ Microscopic examination of the stigma at the time of pollination also shows that the pollen from another flower usually germinates before the pollen which has fallen from the anthers of the same flower. This latter fact alone in most cases renders it unlikely for a flower to produce seeds by its own pollen. Darwin worked for years on the pollination of many insect-visited flowers, and discovered in almost every case that showy, sweet-scented, or otherwise attractive flowers were adapted or fitted to be cross-pollinated by insects. ”

“ When ripe the wind blows the pollen as it scatters from the dangling stamens, and some of the grains reach the stigmas of the female flowers; here they germinate, each pollen grain sending a delicate pollen-tube down the style into the ovary of the flower. This process of application of the pollen grains to the stigma is termed pollination, and depends on the wind.
The female inflorescences are also spikes (Fig. 32, A) , but they bear only one to five flowers, and stand off from the axils of the foliage leaves. ”

“ Then the pollen clings to the stigma, and down it goes through its tiny tube, to the little nest of ovules, or seeds, where it mates and at once causes the seeds to become alive. Then they grow and grow until their time comes to burst forth and develop new plants themselves.
Now, it is also important that the little visitors who come for the nectar, or honey, should fly from flower to flower and not crawl, because in doing so the pollen would drop off and never reach the mother flower, who is anxiously awaiting this important substance, so that the little seeds, or babies, may begin to grow. ”

“ All the pollen and honey must be brought into the hive by them. Have you ever seen the little baskets which working bees have for carrying pollen? Perhaps you do not know what pollen is. Well, some day look right down in the centre of a flower and you will find some fine yellow powder. That is pollen, or bee bread, and the bees are very fond of it. On the hind leg of the worker is a nice smooth place, and on each side of it are stiff, curved hairs which cover it over. Into this little cage the bees push the pollen, then fly swiftly away toward the hive. ”

“ In this class the stamens and style unite into a column, and at the top of the column the pollen masses are situated; these are covered by the anther-cap, and in a cavity is the stigma with its viscid surface to receive the pollen grains.
So diverse and intricate are the forms of the flowers, and especially labellums, that there is little doubt that insect aid is necessary in their natural habitats to bring about pollination. It has been proved by the operations carried out in cross-fertilisation in gardens that no class of plants can be so readily crossed under artificial conditions. ”

“ The bee that creeps down into the flower for a sip of nectar releases a tiny spring, like a mouse entering a trap. The released anther flies up and dusts its pollen on the hairy body of the insect. Now, if you take this pollen and put it under a good microscope, you will see that each grain is in reality a cluster of four tiny balls resembling oranges. Indeed, in passing it may be observed that each species of plant seems to possess some special whim in the shape of its pollen, with its own peculiar devices of exterior decoration and structural form. ”

“ If he would explain why certain flowers are not colored at all, and how it has come about that other flowers are colored, some in chromatic harmony, and others not, he must look to the flowers themselves, and to their servants the insects. Every one knows that the pollen must find its way from the anther to the stigma, else the flower, lacking impregnation, can set no seed. For many trees, and for grasses generally, the office of transporting the pollen is performed by the wind. ”

“ Flies and some other insects are agents in cross-pollination. Humming birds are also active agents in [Pg 44] some flowers. Snails are said in rare instances to carry pollen. Man and the domesticated animals undoubtedly frequently pollinate flowers by brushing past them through the fields.
Pollination by the Wind.—Not all flowers are dependent upon insects or other animals for cross-pollination. Many of the earliest of spring flowers appear almost before the insects do. Such flowers are dependent upon the wind for carrying pollen from the stamens of one flower to the pistil of another. ”

“ When the bee, thus provided, flies to another flower, or to the same flower a second time, and is pushed by its comrades into the bucket and then crawls out by the passage, the pollen-mass necessarily comes first into contact with the viscid stigma, and adheres to it, and the flower is fertilised. Now at last we see the full use of every part of the flower, of the water-secreting horns of the bucket half-full of water, which prevents the bees from flying away, and forces them to crawl out through the spout, and rub against the properly placed viscid pollen-masses and the viscid stigma. ”

“ Five thousand of the strongest and most active will be sent to the lime-trees, while three thousand juniors sally forth to the clover. Those who yesterday were gathering nectar will to-day give a rest to their tongues and the glands of their sac, and will bring back red pollen from the mignonette or yellow pollen from the tall lilies; for you will never find a bee gathering or mixing up pollens of a different color or species, and indeed it is one of the special cares of the hive to keep the different-hued pollens apart in separate store-rooms. ”

“ For seventy years this astonishing fact puzzled the world, and was at last solved by the great Darwin, who showed that nearly all flowers shun their own pollen, and are so constructed, by thousands [208] of singular devices, that the insect shall bring to each the pollen of another flower of the same species, and thus effect what is known as cross-fertilization.
We must then look at all flowers as expressions of welcome to some insect—day-flowering blossoms mostly to bees and butterflies, and night-bloomers to moths. ”

“ There is no prospect here of any seed being set unless the pollen of another flower is secured. Now what are the chances in favor of this? They are many: In the first place the blossom is unusually large and showy, from its size and shape alone almost certain to arrest the attention of the passing bee; next, the color is not only conspicuous, but it is also one which has been found to be especially attractive to bees; blue and purple flowers being particularly sought by these insects. ”

“ The smallness of the flowers, the absence of honey, the dryness and lightness of the pollen, the size of the stigma and the abundance of hairs found on many stigmas are all adaptations for wind-pollination. It is also an advantage that the flowers are generally pollinated before the foliage-leaves are developed, thus preventing the pollen being entangled by the leaves. ”

“ When the stigma is ready, an unopened anther from the desired flower is crushed on the finger nail or a knife blade, and the pollen is rubbed on the stigma by means of a tiny brush, the point of a knife blade, or a sliver of wood. The flower is again covered with the bag, which is allowed to remain for several days until all danger of other pollination is past. Care must be taken completely to cover the stigmatic surface with pollen, if possible. The seeds produced by a crossed flower produce hybrids, or plants having parents belonging to different varieties or species. ”

“ Bees, wasps, flies butterflies, and beetles could not be kept away from an entertainer so generous; for while the nectar in the deep, tubular brown florets may be drained only by long, slender tongues, pollen is accessible to all. Any one who has had a jar of these yellow daisies standing on a polished table indoors, and tried to keep its surface free from a ring of golden dust around the flowers, knows how abundant their pollen is. The black-eyed Susan, like the English sparrow, has come to stay--let farmers and law-makers do what they will. ”

“ Even now poplars, alders, birches, and oaks rely chiefly upon the wind to carry their pollen. These plants were amongst the first of our modern flora to appear upon the earth. Some of them possess very neat contrivances suited to the wind. The catkins of the alder, for example, hang downwards, so that each little male flower is protected from rain by a little scale or bract above it. The pollen is very light, dusty, or powdery, so as to fly a long distance. The Scotch fir (Pinus sylvestris) has male flowers in little cones. ”

“ On other trees the catkins are greenish, and they stay on after reaching full size. They are the fertile flowers, which develop into seed pods. Pollen brought to them by the wind or by visiting insects in search of nectar, insures the setting of seed in these flowers. Though the gayer flowers fall, they are quite as necessary to the making of seeds as the fertile ones. In all the willows and poplars, it requires two trees, bearing the two kind of flowers, to make the seed. And the wind and nectar-seeking insects are necessary as pollen-carriers. ”

“ Butterflies are by far the most abundant visitors; honey-bees also abound, bumblebees, carpenter and mining bees, wasps, a horde of flies, and some destructive beetles; but the short tongues can reach little nectar. Why do the pistils of the florets protrude so far? Even before each minute bud opened, all its pollen had been shed on the tip of the style, to be in a position to be removed by the first visitor alighting on the ball of bloom. After the removal of the pollen from the still immature stigma, it becomes sticky, to receive the importation from other blossoms. ”

“ For, of course, flowers do secure a mate, and they are aided in this enterprise by the most formidable array of helpers, one might almost call them conspirators. The chief of these are insects, thousands of different kinds of which are constant flower visitors. Some of the smaller birds, and even snails, also help flowers to meet their mates. The wind, too, bears pollen through the air to some expectant bride-to-be. And, finally, in the water, by a series of acts the like of which no one could improve for cunning, the cross-fertilization of certain aquatic plants is consummated. ”

“ Each pollinium consists of two leaves of pollen united for about half their length in the middle with elastic threads. As the pollinia are attached parallel to the disk, they stick parallel on the bee's tongue, yet she may fold up her proboscis under her head, if she choose, without inconvenience from the pollen masses, or without danger of loosening them. Now, having finished sucking the newly-opened flowers at the top of the spike, away she flies to an older flower at the bottom of another one. ”

“ The plant has, nevertheless, not yet developed submersed pollination, the pollen-grains rising to the surface, where they are caught by the stigmas of floating female flowers. It follows that the individuals rooted in the deeper water, though growing vigorously, do not mature seed, for the production of which the species has to rely on plants which, at least at low water, are rooted sufficiently near the surface to allow the flowers to rise above it. ”

“ The observer will also notice that a bee makes the rounds from flower to flower, taking all of one kind and passing by other species. While this habit undoubtedly avoids some complications, even should he mix his drinks and visit in succession flowers of widely different species, confusion would not be apt to result, for the stigma of one species is usually not responsive to pollen brought from blossoms of another family. ”

“ The wild red or yellow plum forms dense thickets in moist woods and along river banks from New York to Texas and Colorado. Its leafless, gnarled, and thorny twigs are covered in spring with dense clusters of white bloom, honey-sweet in fragrance, a carnival of pleasure and profit to bees and other insects. In hot weather this nectar often ferments and sours before the blossoms fall. The abundant dry pollen is scattered by the wind. The plum crop depends more upon wind than upon insects, for the pollination period is very brief. ”

“ Pollination is accomplished by means of the wind. At the period of pollination the leaves are always so widely separated from one another, that the ovules can catch the pollen-grains carried to them by the wind; this is often effected by the mucilaginous drops which appear at the micropyle, and by the evaporation of which the pollen-grains are brought in contact with the nucellus. ”

“ But the meadow-rue, which produces a super-abundance of very light, dry pollen, easily blown by the wind, is often fertilized through that agent also, just as grasses, plantains, sedges, birches, oaks, pines, and all cone-bearing trees are. As might be expected, a plant which has not yet ascended the evolutionary scale high enough to economize its pollen by making insects carry it invariably overtops surrounding vegetation to take advantage of every breeze that blows. ”

“ Any pollen they may bring in on their bodies now rubs off on the sticky stigma lobes, already matured at the bottom of a newly opened flower, in which they buzz, crawl, slide, and slip, seeking an avenue of escape. None presents itself: they are imprisoned. ”

“ In species where the pollen is wind-borne as in most of our trees—firs, oaks, beech, ash, elm, etc., and many herbaceous plants, the flowers are as a rule small and inconspicuous, greenish, and without either scent or honey. Moreover, they generally flower early, so that the pollen may not be intercepted by the leaves, but may have a better chance of reaching another flower. And they produce an immense quantity of pollen, as otherwise there would be little chance that any would reach the female flower. Every one must have noticed the clouds of pollen produced by [Pg 123] the Scotch Fir. ”