The Clocks of Life

ANNUAL plants bloom and set their seed before frost or else their kind perish. Fur-bearing animals start the long process of change to winter pelage when the season is hottest. Birds prepare for molting and slowly accumulate fat for migration. The young are mostly born in season favorable to survival. Plants and animals survive by adaptation to seasonal change, but since they cannot instantaneously reproduce or become dormant, they must foresee the favorable or adverse period.

What is sensed by the plants and creatures in their surroundings, and what are the biological mechanisms that enable them to respond? Can we understand why chrysanthemums bloom in the autumn? Why does wheat mature in summer? And if plants and animals generally respond to the season, what might be expected of man?

I first faced some of these questions at the end of the war in 1945, having come to their consideration both from intellectual curiosity and from a desire to achieve the practical goal of improving crop production. I really did not know at first how to satisfy either desire, but thought the aesthetic and practical objectives would be found by the same pathway, whatever it might be. My method was to search for certain controlling principles — but how? In natural science a history of observations, of classification, and of recording sometimes extends back over a century or more. Let us first turn to this record of knowledge.

The change of temperature with the season is so evident as to have diverted attention from other possible stimuli for adaptations. About forty years ago, however, when H. A. Allard and W. W. Garner, of the Department of Agriculture in Washington, were working to improve the tobacco plant, they found a plant in a Maryland tobacco field that failed to bloom with the others in the cooler days when harvest approached, but instead kept on growing to a giant size. The plant was transferred to a warm greenhouse to bloom, on the chance that it might be the forerunner of a new variety. Some seedlings from this plant were grown in a heated greenhouse during the winter. Instead of growing into giants, they bloomed when very small. Others, planted in the fields at the usual time, again grew abnormally large in the next summer.

Since the greenhouse temperature was the same as that of the field in summer, it was clear that the blooming time was not controlled by temperature. The two scientists came to the radical conclusion that it was variation in the length of the day that controlled the plant’s reproductive cycle. They tested their assumption by lighting some tobacco plants at the end of the day, while others were darkened before night. The plants on short days soon bloomed, while those on long days continued to grow without blooming. Turning to other plants, the experimenters found that soybeans, poinsettias, and chrysanthemums also bloomed when the days were short. Many common grains, summer flowers, and vegetables, on the other hand, bloomed only on long days, while the flowering of still others, such as the tomato, was unaffected by day length.

Success with plants encouraged Allard and Garner to speculate that the time of migration of birds might also be controlled by length of day, instead of resulting from instinct or volition as some scientists believed. Tests were first made in the mid-twenties by W. Rowan, a zoologist at the University of Alberta. Juncos and crows were held in lighted cages in the open during the depth of the Canadian winter, and lowwattage lights were burned five minutes longer each day to simulate the lengthening days of spring. The birds soon began to develop sexually, and when the crows that survived misadventure were released after banding, migration was northward — to the confusion of the North Woods trappers.

Birds build their nests and lay eggs in the springtime when the days are long. Some other vertebrates, however, develop sexually on short days, as was found with field mice and is evident in the warm-blooded sheep and the cold-blooded brook trout that breed in the autumn. Many invertebrates, also, reproduce and migrate locally under control of the length of the day. Aphids, red spiders, fruit moths, shrimp, and snails are very responsive. The apple aphid is an example. Though it is born in apple trees, it lives and feeds on dockweed during the long days and bears flightless living young, but as the days shorten and the time approaches when cold will kill the adult aphids, winged migrant forms are born to return to the apple host-plant and lay dormant eggs along the branches to wait for spring. This was discovered in 1924 by S. Markovitch, an entomologist at the University of Tennessee. It was the first evidence that animals as well as plants were affected by the changing length of the day during the procession of the seasons.

WHAT is there about the length of the day that affects both plants and animals? Rowan held that the light simply disturbed the juncos and crows. Garner and Allard always spoke of the length of the day, and others, too, by natural predilection thought of the daylight period as the controlling factor. But in a twenty-four-hour period, there is both a time of light and a time of darkness, a day and a night. If the natural day is short, the night is long. The blooming of the chrysanthemum might result from the long nights instead of the short days in the autumn.

Actually, when the day and night lengths are varied in other than twenty-four-hour cycles, it is the length of the night that is found to be critical. The chrysanthemum is in fact a longnight plant instead of a short-day one, and the ability of the long night to induce flowering can be destroyed by an interruption with light. Thus, chrysanthemums or plants requiring long nights to flower will quickly come into bloom when growing on eleven-hour days and thirteen-hour nights, as in the autumn. If the plants are lighted for thirty minutes each night with ordinary incandescent lights near the middle of the night, they fail to bloom. The flowering of several hundred acres of chrysanthemums in Florida and California in winter is now controlled by stringing electric lights on poles across the fields.

This same phenomenon has been unwittingly observed by many people who have wondered why their Christmas poinsettias fail to blossom in the living room. The answer is the fact that our homes are artificially lighted in the evening, so that the plant does not receive the long, uninterrupted hours of darkness it needs for flowering.

How can plants unerringly measure the length of the night to foresee a change of season? The question was a tantalizing one to me and to Harry Borthwick, who had taken up the study of flowering in succession to Allard and Garner. Borthwick is a botanist and I am a student of the physical sciences at Beltsville. We pooled our intellectual resources some twelve years ago in an effort to find an answer.

Our approach was to go on to the next step and try to find out how light acts on plants. This might seem strange if the dark period or night is being measured, but darkness has only the property of duration, and the duration is ended by light. We turned our attention to how different wave lengths — or colors — of light varied in effect. A suitable prism spectrograph was built, giving a spectrum about four inches high and five feet long, from violet at one end, through indigo, blue, green, yellow, orange, and red, into the regions beyond visible light in the near infrared. The light source was a twelve-kilowatt carbon arc of the type used in motion picture projection, donated by Warner Brothers Theatres.

When artificial light was used to interrupt long nights for cockleburs, which are long-night plants like chrysanthemums, the red section of the spectrum most effectively prevented flowering. Next, plants of the opposite kind, like barley and henbane, which grow without blooming on long nights, were given long-night interruptions, using light from various sections of the spectrum. Red light again was most effective in producing a response: this time it caused flowering. In fact, each section of the spectrum had the same relative ability to change the flowering habits of the two types of plants, even though the changes were of opposite kinds. The conclusion was inescapable that both types perceive light and “measure time” in the same way.

In the next stage the most pertinent experiments developed in an unexpected direction. They were largely the result of the interest of two students of seeds, Eben and Vivian Toole, whose laboratory was down the hallway from the spectrograph. The starting point was the fact that many seeds germinate only if they receive some light after being moistened. It is for this reason that packets of flower seeds often bear the caution to sprinkle the seeds on the surface of the soil and press lightly for germination.

Of greater significance is the converse fact that seeds of this type, once buried without having been exposed to light, will lie ungerminated until they are exposed. This is why farmers plowing find new weeds coming up in place of the ones destroyed. During the bombing of London in 1941 the hairy epilobium, or fireweed, and other unexpected plants came up on sites that had been covered since the great fire of London in 1666. More authentic and controlled experiments have shown that seeds can be held in soil for more than eighty years and still germinate when exposed to light.

If seeds respond to a light stimulus for germination, what color of light causes this response? Lettuce seeds were placed across the fight spectrum after being moistened and held in darkness. They were exposed for only a few minutes and then returned to darkness to develop for several days before being examined for germination. Those in the red part of the spectrum were most responsive, and the relative action of the several colors in germination was exactly the same as in flowering control. One paused to think, for no obvious biological similitude exists between flowering and seed germination. Then, too, the flowering response is a periodic one resulting from the succession of days and nights, while the seeds require only a single brief exposure to light.

A most interesting phenomenon occurred among the seeds exposed to infrared light. When the lettuce seeds were placed across the spectrum, others were held in darkness as controls, and about 20 per cent of these germinated. In other words, one seed out of five normally germinated without any light. As we have seen, among those seeds exposed, the ones in the red part of the spectrum were helped in germinating. It was apparent, however, that those at the infrared end of the spectrum, just at the limit of visible light, actually germinated less than those that received no light at all. It seemed that this infrared radiation must act in just the opposite way to the red part of the spectrum.

To explore this finding, lettuce seeds accordingly were first exposed to a little red light to promote germination and immediately afterward to infrared radiation. The suppression of the first response was dramatic. In fact, if lettuce seeds are repeatedly exposed to red and infrared radiation in succession, they germinate if the last exposure is to red, and remain dormant if it is infrared. In nature, this type of reversal is not evident, since the plant receives infrared only as a part of daylight, when red light is also present and is dominant.

IF GERMINATION control was reversible to light of different parts of the spectrum, and if the response that produces germination was the same as that in flowering control, was not flowering control also reversible? The possibility had never been suspected. The prejudice had been to regard darkness as nothing. When, however, the cocklebur plants which had been stopped from flowering by exposure to red light were exposed to infrared, they were induced to flower. The response could be repeatedly reversed in the same way as for seed germination. The reaction to light that controls such processes as flowering in plants is, then, reversible.

With this background, the flowering plants and the germinating seed were studied further to find which of the two pigment forms — the one responding to the red part of the spectrum or the other that responds to infrared — triggers biological activity. The answer found from studies of lettuce seed germination was that biological activity is connected with the infrared form. The seed lying dormant in the ground for a hundred years is dormant because shortly after burial the pigment permitting growth changed from the form that can produce biological action to the more stable but ineffective red-absorbing form. Only when the seed is exposed to red light can the pigment change back to the other, biologically active form.

Plants use the reversible reaction to light for purposes other than anticipating the change of season. If grass is covered with a board the leaves turn yellow and the stems lengthen greatly in the course of several days. The stems of seedlings from deeply planted corn or wheat lengthen in this way until the surface is reached, where normal growth starts. When seedlings are grown in darkness and then exposed across the spectrum, lengthening is suppressed by the various colors in the same relative way as is flowering. Again, the initial controls must be the same.

Another example is shown by trees. Trees grow rapidly during the early summer, but when the nights lengthen in July and August, the buds become dormant in preparation for the winter.

When the nights are even longer, a layer of cells grows across the petiole of the leaf and cuts it off the tree. This is autumn. This whole process is controlled in part by the reversible light reaction, and in part by the fall in temperature.

WHAT about the animal’s sense of time and season? Consider the weasel, the snowshoe rabbit, and the ptarmigan. Each has a protective white coat in the winter which we might reasonably think is induced by the cold that brings the snow. When the skin is examined in midsummer, however, the hair or feather follicles are found to be already losing the dark dye of the summer coat, and fat is accumulating even though the days are still warm. By early August the change to the winter coat is well under way and the summer coat starts to drop. The effect of alternating periods of light and darkness on such changes is shown by experiment. When the snowshoe rabbits are blindfolded for part of the day, simulating the change of season, the shift to the white winter fur is hastened. The ptarmigan, lighted in the winter, changes back to the speckled feathers of summer and lays her eggs in the snow. The winter coat of the weasel in the north is the white of the ermine. Further to the south, where little snow falls and the nights arc shorter than in the north, the guard hairs of the fur change but the color remains brown. The weasel in a sense both anticipates the winter and measures the latitude, with its implications about snowfall, by the length of the night.

Sheep sent out from England to settlers in Australia and New Zealand were confused by crossing the equator to the south temperate zone with its reversed seasons, and bred again six months after breeding in England. They were then on schedule with the seasons and resumed yearly lambing.

Sheep, with a five months period of gestation, are long-night animals for coming into heat. As a result, they conceive in autumn and bear their young in spring, which gives the lambs time to develop before the next winter. Horses of the Scottish Isles, on the other hand, are short-night animals, breeding in the spring. Since their gestation period is eleven months, their foals are born the following spring.

Borthwick and I, knowing the superficial similarity of response of plants and animals, thought that they might be alike in their way of measuring time. We hesitated, though, to try sheep or birds in the spectrum experiments because of the difficulties of handling them. We turned instead to snails, which arc seasonally responsive in egg-laying and easy to deal with. Charles Jenner and Oscar Paris, zoologists of the University of North Carolina, who were studying egg-laying of snails induced by short nights, arranged to supply a number for testing in the spectrum. Only a few snails survived, and these responded poorly without any evident similarity to plants. For the time being we face a blank wall in attempting to correlate the seasonal responses of plants and animals.

We know that seasonal changes of animals are chiefly associated with reproduction, or change of form as in insects. In vertebrates, these changes are controlled by the hormonal secretions of the front lobe of the pituitary body and by the action on the pituitary of hormones from the reproductive system. The part of the central nervous system that is called the hypothalamus, closely associated with the pituitary both by tissue connection and through blood vessels, is also part of the controlling system. In insects, a group of large cells in the brain secrete hormones that act on a gland around the thorax to release hormones that control change of form.

All hormonal controls are complex. They slowly go through a rhythmic cycle similar to sleeping and waking. The rhythms are usually repeated about every twenty-four hours and can be shifted by changing the timing of day and night, but the shift often requires a number of days for its accomplishment. This very resistance to change indicates the importance of the rhythm for warmblooded animals, while the eventual response shows how the environment is ultimately controlling. If the day and night lengths match the hormonal rhythm, the animal responds in one way, but if the light cycle and the hormonal cycle conflict, a modification of either the response or the rhythm must occur.

The sensing of light in the environment is in the region of the eye, as was shown by blindfolding the rabbit, but whether vision is involved, or the eye is merely a window to the optic nerve and the hypothalamus as part of the central nervous system, is still very uncertain. The most revealing experiments were done on ducks that developed sexually in response to light after their eyes were removed. The best evidence is that light acts as a stimulus to the hypothalamus.

Perhaps the parts of the timepiece are clearer to us than is the running of the whole, but this partial understanding is all that we have at the present time. Egg-laying of the chicken illustrates the total operation. My information comes from our colleague, Richard Flaps, who has been studying egg release for many years.

The commonplace but startling fact is that chickens lay eggs during the light period of the day, usually between 8 A.M. and 4 P.M. Eggs are laid one a day in sequences of two or more consecutive days. Typically, the first egg of a sequence is laid early in the day, the eggs that follow on successive days somewhat later, until on the day that an egg is laid in the late afternoon the sequence is complete and no egg at all is laid on the following day. On the second day following, a new sequence is initiated. The four-to-eighthour period in which eggs are laid can be shifted to any time of the twenty-four-hour day by shifting the time of the light and dark periods, though the response is not immediate.

The timing is determined by a complicated sequence of events in the hen. Egg-laying is stimulated by the onset of darkness about thirty-six hours before the first egg appears. The absence of light apparently causes the hypothalamus to act on the front lobe of the pituitary so that this gland some eight hours later releases an ovulationinducing hormone, which causes the expulsion of a yolk from the follicle of the hen’s ovary. This yolk takes about twenty-four hours to develop into an egg ready to be laid.

While the egg is developing, the timing mechanism is being reset. This is done by hormones from the ovary acting on the hypothalamus and the pituitary. It takes a little more than twentyfour hours to reset the release of the ovulationinducing hormone. As a consequence, the relaying of each following egg occurs a little later in the day. As the action of the ovary on the pituitary gets more and more out of phase with the rhythm of the hypothalamus, finally an egg is laid so far out of phase with the cyclic sensitivity of the hypothalamus that the whole process stops for a day, and there is no egg. This timing scheme is the seasonal controlling system for birds and other warm-blooded vertebrates.

Careful breeding of poultry, incidentally, has made it possible to bring the action of the ovary on the pituitary so nearly in phase with the rhythm of the hypothalamus that certain hens will lay eggs for as many as a hundred successive days without a break. In contrast, an ordinary hen will take a day off after every two or three eggs.

AND so we come to man. He is only mildly seasonal, although poets sing of the springtime and interesting stories have been told about the Eskimos. I know of only one certain seasonal response, the cyclic variation in the ratio of male to female births. The average percentage of male births in the United States is 51.36. The monthly variation is from 51.20 to 51.50, which is a tremendous amount for the 61 million births in the thirtyyear period studied. The maximum is in June, with a subsidiary maximum in January and a minimum in February.

If a response to light or some other feature of the environment is to be seasonal, it must first result in a daily cyclical change of the animal. If the daily change is either very weak or very strong, the seasonal change might be obscure or absent. Man is diurnal in this sense rather than seasonal. This is most evident by his sleeping and waking, but it is also apparent in other cyclic changes, particularly those in response to stress. One of these is the level in the blood stream of the compounds related to cortisone that arise from the interaction of hormones from the pituitary and the cortex of the adrenal gland. The temperature of the body also has a slight daily variation.

Perhaps of greater significance is the daily variation in the frequency of cell division in the epithelial and muscular tissues. The numbers of dividing cells in the skin and in muscle are greatest in the middle of the night, and few can be found in the afternoon. The hypothalamus is rhythmically changing and shifts with the day and night, as is seen when one travels quickly by air to the Orient, but the seasonal control is absent.

Many experiments are in progress on plants and animals, either to explore more deeply into causes for seasonal change, or to make any of the myriad possible applications to growing of things. I am trying to isolate the pigment in plants that absorbs light in the reversible reaction controlling flowering and seed germination, and to find the next reaction that it controls. Harry Borthwick is trying to understand more fully the responses of a large number of varieties of barley, wheat, and chrysanthemum to the length of the night and to light. Others, in Australia, are finding how cattle shed their coats to adapt to warm seasons. In India rice is under observation, in Japan germination of forest tree seeds and the life cycle of silkworms, in England the dormancy of tree buds, the reproduction of sheep, and the change in form of red spider mites, in Hawaii the prevention of flowering of sugar cane, in Russia the growth of cotton. France, Portugal, Belgium, Holland, Germany, Italy, and Rumania — each has some contributors to a fuller understanding of these plant and animal responses.

The day, the night, the seasons, higher animals, insects, man, and plants are bound together in a most intricate way, but through this intricacy runs the simple thread that we have followed. Remember this when you next buy an unseasonal bouquet, or when you swat the bothersome mosquito, or watch the southward flight of geese, or carve the Thanksgiving turkey, or relish the spring lamb.