Popular Science Monthly/Volume 45/May 1894/The Guests of the Mayflower
|THE GUESTS OF THE MAYFLOWER.|
By Prof. CLARENCE M. WEED.
NO native plant has so endeared itself to the New England heart as the mayflower. For two centuries it has been to old and young the sweetest of spring's harbingers as it pushed its dainty blossoms through the fallen leaves beside the lingering snow. It has charmed those fortunate ones who have wandered over the hills to find it, and has carried glad tidings to those compelled to stay at home. It has been constantly used to carry Cupid's message from youths to maidens—a custom which I like to fancy may have originated when, in the infancy of Plymouth, John Alden brought to Priscilla Mullens bunches of arbutus blossoms that spoke not only for themselves but also for the hand that plucked them.
But Epigæa is a plant of decided interest in itself apart from its associations. It was not originally designed as an emissary of
Fig. 1.—The Mayflower.
the goddess of love, and its beauty was primarily developed without reference to the æsthetic needs of the Pilgrims or their descendants. Long before the Mayflower reached Plymouth or Columbus landed at San Salvador—probably before the Indians arrived, and possibly before the glaciers came down from the north—the arbutus blossomed with each returning season and carried on the cycle of her existence as tranquilly as she does today. But her fragrance was by no means "wasted on the desert air," for she received then, as now, the tributes of a host of insect visitors that went about to do her unconscious bidding.
Although the trailing arbutus has been developing for so many centuries, it is still in a state of transition, and appears to be looking toward a goal which probably will not be fully reached for centuries to come. Every one with the least knowledge of the vegetable world knows that the great majority of flowering plants have the stamens and pistils in the same blossom, although Nature generally devises some method of preventing self-pollination. Many species, however, bear the pistillate blossoms on one plant or part of the plant, and the staminate blossoms on another plant or part of the plant, relying on insects or the wind to carry the pollen from the latter to the former. But occasionally there occurs a species whose flowers are neither wholly one nor the other, being in a transition stage between the two. In this category we find the mayflower.
The examination of the structure of a dozen bunches of arbutus blossoms reveals a great variation in the relative conditions and positions of the stamens and pistils. In some specimens the anthers are completely abortive; in others only partially so; and in others in good condition, well filled with pollen grains. Two types of stigmas are also present: in some specimens the stigmas as a whole are broad and more or less flattened—spread out, so to speak—projecting at right angles to the style with the upper surface moist and glutinous; in others the stigmas are crowded into less space and project very little horizontally; they are drier and less glutinous, and evidently in a partially abortive condition. The perfect stigmas are usually associated with abortive anthers, and vice versa, so that many of the plants are already diœcious.
If the flowers are examined with reference to the length of the styles and filaments of the pistils and stamens, great variations will also be found. In some the stigmas are perfect and reach the mouth of the corolla; no anthers, and only rudiments of filaments are present. The variations I found on Blueberry Hill at Hanover, New Hampshire, may be epitomized as follows:
1. Stigmas perfect, reaching the mouth of the corolla; no anthers, and only rudiments of filaments present (Fig. 2, a).
2. Stigmas perfect, reaching the mouth of the corolla; anthers present, but abortive, reaching two thirds the way to the mouth of the corolla (Fig. 2, b).
3. Stigmas perfect, reaching half way to the mouth of the corolla; anthers abortive or absent, not reaching the stigmas.
4. Stigmas imperfect, anthers perfect; both reaching the mouth of the corolla.
5. Stigmas imperfect, anthers perfect; both reaching two thirds of the way from the base to the mouth of the corolla.
6. Stigmas imperfect, reaching slightly beyond the mouth of the corolla; anthers perfect, reaching to the mouth (Fig. 2, c).
The relative proportions of the different forms seem to vary with the locality. The majority of specimens I have studied belonged either in the first or fourth category. The arbutus at Hanover is evidently tending strongly to a more perfect diœcism When it finishes its task of eliminating the filaments as it has the anthers of the stamens in many of the pistillate blossoms, and gets rid of the superfluous pistils of the staminate blossoms, it will accomplish its purposes of reproduction with less waste than at present.
A plant in the condition of the arbutus may be said to be in a certain sense at a "parting of the ways." To attain the end of cross-fertilization—the carrying of the pollen from the stamens of one plant to the pistils of another—two methods appear to be open to it. It may, and in the case of many of the Blueberry Hill specimens evidently has, become more perfectly diœcious by aborting the stamens on some plants and the pistils on others; or it might become dimorphous by developing perfect sexual organs
Fig. 2.—Variations of the Mayflower.
in each blossom and having them at different heights—that is, having the stamens in one plant reach the mouth of the corolla and the pistil reach only half way to the mouth, while in another having the pistil long and the stamens short. The tendency toward dimorphism or trimorphism is shown by the varying lengths of the styles and filaments.
The blossoms of the common asparagus of our gardens show by their structure that they are in a transition stage somewhat similar to that of the arbutus. The staminate blossoms have rudimentary pistils and the pistillate blossoms rudimentary stamens, and sometimes a blossom is found which has both sets of organs in good condition—a reversion to an earlier condition of the plant.
The partridge berry, a plant which has to contend with much the same external conditions as the arbutus, living in similar situations and remaining close to the ground, has adopted dimorphism as its method of securing cross-fertilization. The beautiful white blossoms of this species open early in summer. The stamens of some individuals are exserted, with the stigmas below the mouth of the corolla, while in others these conditions are reversed. Another common example of a low-growing plant with dimorphous sexual organs is that of the familiar bluets. The dainty blossoms of this species are small individually, but grow so abundantly on New England hillsides as often to color them like a light fall of snow. A sectional view of the two forms of flowers is shown in Fig. 3: a represents the long-styled form with the stamens in the lower portion of the corolla tube and the stigma exserted, while in b the stamens are near the mouth of the corolla and the stigma is below. These blossoms are mainly pollenized by small bees and butterflies, one of the commonest New England visitors being the meadow fritillary. When an insect
Fig. 3.—Long-styled and Short-styled Forms of Houstonia
sucks the nectar from the base of the corolla of the short-styled blossom (b), it will get at a certain place on its tongue some of the pollen from the anthers. If next it visits a long-styled blossom (a), it will be likely to brush some of this pollen on to the exserted stigma, while a point near the tip of the tongue will receive a fresh supply of pollen grains. If now it again visits a short-styled blossom, this last-received pollen will be at the right elevation to be deposited on the included stigma. Consequently, cross-fertilization will almost certainly occur.
In the case of the mayflower it is evident that the structural conditions described above will necessitate for the production of seed the transportation of the pollen from the staminate to the pistillate blossoms. The only agents to be called into play for this errand are the insects and the wind. The structure of the plant shows that under any ordinary conditions the wind would not serve the purpose, so that the insects only are left. It might at first seem that so early in the spring as the may flower appears there would be few insects abroad not enough to accomplish the desired results. But centuries of experience have taught the plant that the nectar hidden beneath her blushing petals will attract many visitors. On Blueberry Hill the most useful and abundant visitor is the beautiful orange-banded bumblebee. Dozens of the large females, which have wintered over in some sheltered nook, are usually present, busily gathering the nectar concealed in the bases of the corollas. Each bee stops but a few seconds at a flower, and visits on an average three or four bunches of blossoms a minute. After alighting either on a flower or the leaves, or the ground between, the bee crawls from blossom to blossom, poking its nose, so to speak, down under the leaves that none shall be missed, and often visiting a dozen heads before taking to wings again. When the wind blows hard a frequent occurrence on such hilltops Madame Bombus (these early spring forms are all females, the so-called queens) flies still more rarely, crawling long distances instead. The tongue of this bee is two fifths of an inch long, and its tip readily reaches the bottom of the corolla, being thrust quickly down between the hairs. There are generally several blossoms in a single head, and, as a rule, each is plundered before the visitor departs. I saw one bee visit six heads in ninety seconds, and another seven heads in the same length of time. On the supposition that there were five blossoms per head, the first bee was plundering twenty flowers a minute. Supposing that half of each hour was spent between the plants or going to the nest, the bee at this rate would visit six hundred blossoms an hour, or six thousand in a ten-hour day, if she should work so long. On the five acres of hilltop where my observations were carried on I judged that at least one hundred of these bees were at work each day. Supposing that they all worked at the above ratio, the mayflower would receive six hundred thousand daily visits. No doubt many of the blossoms are visited more than once each day, and the chances are certainly very good that each blossom will be visited at least once during the two weeks of its existence.
Although the orange-banded bumblebee is much the most abundant visitor, two other related species are often seen. The commoner of these is a large and handsome Bombus black, except for two broad yellow bands one on the thorax and the other on the abdomen. The other, which is seldom seen, is called by entomologists Bombus consimilis; the thorax and front half of the abdomen are yellow, with the hinder portion of the abdomen black.
By watching any one of these bees closely, one can see it stop every few minutes to brush the pollen grains off from its tongue and head, but no attempt appears to be made to collect the pollen in the beautifully developed pollen baskets on the legs (Fig. 5). These bees evidently visit the arbutus for the nectar it furnishes. Although the bumblebees are much the most numerous and important of the mayflower's invited guests, a few other insects are found among the visitors. A rather small, two-winged fly, with a hairy, yellow body and black-banded wings, often flashes, meteor-like, from blossom to blossom. This is the handsome bee fly of the genus Bombylius, one of the earliest spring insects. It has a very long tongue, which readily reaches the bottom of the mayflower corollas. I saw one of these flies stop twenty seconds at a single flower; it thrust its tongue down on one side of the pistil, then drew it out and pushed it down in another place, repeating the operation four times.
During the warmest portions of the brightest days the beautiful sesia moths appear. They are sometimes called hummingbird moths, because of their resemblance when flying to a humming bird, though the smaller of our species, the one I find visiting the arbutus, is more suggestive of a bumblebee. They have long tongues, curled up when not in use, through which they suck the nectar of flowers. Unlike most moths, which fly at dusk or after dark, the sesias are abroad in the bright sunlight.
Occasionally one of the early spring butterflies, especially the American tortoise-shell and the mourning cloak, may be seen hovering over the blossoms.
The insect visitors so far considered are all useful to the mayflower. They fly rapidly from head to head and plant to plant, carrying the pollen which sticks to them from the anthers of the staminate blossoms to the stigmas of the pistillate ones, thus causing the fertilization of the embryos and the development of seeds. But the surface of the rocky hillsides where Epigæa is most thoroughly at home swarms with ants of various species—wingless creatures that dearly love the nectar of flowers. These insects wander everywhere in search of food, and are often seen trying to get at the honey at the bottom of the mayflower corollas. Could they succeed, little would be left for other visitors, and consequently the ants would not only be of practically no value as pollen-carriers—for rarely would one chance to wander from a staminate to a pistillate blossom—but they would also prevent the visits of the useful bees and flies. The plant, however, has fenced out these and other similar unbidden guests by an elaborate chevaux-de-frise, composed of hairs projecting slightly upward from the inner surface of the corolla and the outer surface of the ovary and style. It is easy for a bee, moth, or fly to push its slender tongue down through these hairs to the base of the corolla, but an ant finds it very difficult to force its body down till its mouth is at the bottom.