The Bees and their relatives are particularly welcome to the flowers because they do the work of fertilization so well. Wingless insects are undesirable because they offer little guarantee that they will successfully carry pollen to some other flower of the same species. Even if it is not brushed off in the course of their laborious travels, they are not at all particular what kind of flowers they visit and so offer small hope of carrying pollen to its correct destination. Flying insects of the Bee family seem to have the work of cross-fertilization directly assigned to them. On each of their separate, pollen-gathering journeys, they are partial to one particular kind of flower. As they flit from blossom to blossom of the same species, going in and out of flower and flower, rubbing against a group of stamens here and brushing against a pistil there, they fertilize plant after plant in grateful acknowledgment of the store of sweets they are collecting.
Many and ingenious are the methods which flowers adopt to make sure that only invited and useful guests come to their nectar-feasts. The very Ants which guard the lower portions of a plant so well, might become mere greedy plunderers, if allowed to crawl within the flowers. It is not often that they do. Sometimes, the stalks and even the petals of flowers like the Rock-Lichens and the Butter-Wort are coated with some plant chemical exceedingly disagreeable for an insect to crawl over. Various alkaloids, resins and oils in the cell juices also make the flower and its leaves obnoxious to grazing animals. Many plants, like the Mullein and Stinging-Nettle, use bristles and prickles to repel Slugs and Caterpillars.
A common protective device is for a flower to place its nectar at the bottom of a long, narrow tube only accessible to a flying insect having a proboscis. In the Antirrhinum the entrance to the flower is closed to small crawlers by a very heavy corolla. Bees, because of their size and strength, can force their way through. It is said that as soon as the stigma of this flower has been fertilized, the corolla relaxes and Ants and their kind are free to enter and partake of such dainties as are left.
Nettles, Passion-flowers, and Lilies frequently line their interiors with stiff, in-pointing hairs which oppose a most effective palisade against anything that crawls, whereas a flyer provided with a proboscis can stand on the edge and, inserting his straw, drink up the best soda water in plantdom. This existence of proboscides in insects which help to cross-fertilize flowers is the very finest example we have of true mutualism. Here is a case where members of two supposedly different worlds of life have developed highly specialized organs in order that they might help each other.
It is said that Charles Darwin, after noting the extraordinary length of the spur of the Orchid Angraecum Sesquipedale of Madagascar predicted that some day there would be found in that country a moth with a proboscis ten to eleven inches long. Not many years after, Dr. Fritz Müller verified the sagacity of the famous scientist by finding an insect exactly answering this description.
The Birth-Wort (Aristolochia Clematitis) takes no chances with its insect visitors. In entering it, a Bee brushes easily by the down-pointing hairs only to find that, when he attempts to go out again, the bristles present stiff, unyielding obstacles against his egress. In his excitement at this discovery, he buzzes around quite angrily and, without noticing it, thoroughly showers the stigma with pollen and incidentally covers his own body with a good supply to be carried on to the next stop. When this process is quite complete, the flower graciously relents, relaxes its hairs and allows the exasperated insect to escape.
The Pedicularis family uses similiar coercive methods, and by sharp teeth, forces insect-visitors to take a course through the flowers which brings them in contact with both stamens and pistils.
The purple Loosestrife, pretty dweller by banks and meadows, sets a rich table and so always has plenty of insect visitors. It produces six different kinds of yellow and green pollen, and is therefore sure to suit every taste. Incidentally it has two different sets of stamens and stigmas of three different lengths.
Night-blooming flowers only entertain after the sun goes down. All day long they look withered and dead, but with the coming of the stars, they open up to show conspicuous white or light-tinted interiors. A flower like the Silene also exhales a rich, sensuous odor, which, with its light colour, serves to attract such insects as are abroad at night.
Sycamore and Lime trees have humble allies in the tiny mites which live in the retreats built of hairs to be found at the places where the veins of the leaves fork. During the day they hide away from sight, but at night they come out and scour the leaves clean of noxious bacteria and fungus spores.