Pollen of different plants, when examined under the microscope, reveals wonderful facts about the reciprocal relations which exist between plants and insects. Wind-fertilized plants are nearly always without any special beauty of form, colour or scent, while plants which are fertilized by insects are most always conspicuous, brightly coloured and highly scented. In the same way, pollen of the Hazel, Birch, and Balsam Poplar, which is carried by the wind, is small, light, practically spherical and devoid of protuberances. Pollen of the Primrose, Cowslip and Polyanthus, often carried by insects, is deeply furrowed, covered with spines and knobs, strung together by sticky threads and, in other ways, provided with apparatus which enables it to adhere to any object which it touches.
The pollen of the Hollyhock and the Dandelion consists of large, beautiful, spherical grains covered with spikes. The Rhododendrons, Azalias, and Fuchsias produce great masses of grains bound together by viscid threads. Many of these bits of life-principle are geometric masterpieces. A pollen grain of the Cobaea Scandens is one of the most fascinating objects of the microscopic world. It is perfectly spherical and cut into small hexagonal facets like the eyes of a fly. Grains of pollen of all kinds vary between one two-thousandth and one two-hundredth of an inch in diameter.
Alliances between plants and birds are more important than we imagine. The tropical Humming-birds and the eastern Sun-birds are in habits exactly like the pollen-carrying insects. To watch one of these brilliantly coloured creatures hovering over a flower or flying directly into a blossom after nectar, is to almost always mistake it for a Butterfly.
Many birds are invaluable allies of the plant world. They devour thousands of leaf-eating insects per day and so keep down the army of enemies which would otherwise destroy whole forests. Birds like the Woodpeckers rid tree bark of wood-boring crawlers.
In the human world every partner does not always live up to his agreements. And there are evidences that both plants and their allies sometimes engage in questionable practices, bordering on deception and chicanery.
The insects are often enough the offenders, and their crime is most frequently one of robbery. If they can get the sweets they are after without carrying out their share of the bargain, they will do so. Bumble Bees have been observed to cut through the flower-walls of a Nasturtium and so extract its nectar without coming near the pollen-producing stamens. Sweet Peas frequently ignore the insects and fertilize themselves. The Hawkweed (Hieracium) has so little faith in insect allies that it produces seeds parthenogenetically, that is, without the union of sex elements.
Alliances which start out advantageously for both parties sometimes degenerate into mere sinecures for one or the other. The naturalists Ihering, Ule and Fiebrig, working in South America, a few years ago concluded that the association of the plant Cecropia and the Aztecan Ants, long regarded as a classic example of mutualism, is by far of greater benefit to the Ants. The openings which the Ants make into the hollow interiors of this plant also allow the entrance of certain destructive insects, and the Ants themselves attract Woodpeckers which damage the plants. It is also alleged that these same Ants, and the ones which inhabit the Humboldtia Laurifolia, are often so busy feasting on nectar that they do not stop to repel invasions of foliage-destroying insects.
While man is the greatest enemy of the plant world, he is also at times its greatest friend. When it is to his advantage or when he is prompted by a sincere love of Nature, he becomes a strong and helpful ally. He aids his fellow creatures of the vegetable world when they are sick or injured and, by improving their environment and protecting them from attack and danger, enables them to develop to best advantage. A wizard like Luther Burbank helps them in their efforts at race improvement and development.
In Egypt and Arabia, man has acted as carrier of pollen for centuries, and has thus insured an abundant Date crop. The same thing is often done in other parts of the world with Apples, Pistachios, Melons, Cucumbers and other plants having unisexual flowers.