HAVE you noticed the marvelous sense of timing of living things? Each year plants germinate, grow and flower according to schedule. Not all are on the same schedule​—some flower in the spring, others in the summer and still others in autumn and early winter. But each species knows the right time to carry out its various activities.

The Wondrous Timing Device

It is believed that the main timing device of living things is light. It used to be commonly thought that the changing temperatures of the season triggered the various responses in plants and animals. But temperature is variable; it is inconsistent from year to year. Light, on the other hand, is reliable. On any given day of the year the length of daylight will be the same. It never varies. Thus a living organism is provided with exact information on the advance of the seasons.

This is not to say that temperature or other factors may not also affect the seasonal rhythm of plants and animals. They apparently do. But the main timing device seems to be the length of daylight. That the activities of living things are scheduled by means of this wondrous clock is a relatively recent discovery.

An Important Investigation

In 1920 investigators were studying a certain variety of tobacco called Maryland Mammoth. They were trying to determine why it was late in flowering when grown near Washington, D.C. Although the plant was ready to flower for days, something prevented it from doing so until too late in the season for its seeds to mature.

Many experiments were conducted, but they failed to reveal the reason for the delayed flowering. Finally plants kept in a greenhouse were artificially given a shortened daily exposure of light. This did it! The plants bloomed earlier than those that were grown outside. This gave the clue as to why the Maryland Mammoth will not bloom until late in the season near Washington, D.C. It is because not until late summer has the daylight decreased to the proper length for this plant to flower!

Does light similarly affect the functions of other plants? Further research by these investigators showed that it does. It was discovered that plants can be divided into three groups, depending on their reaction to length of daylight.

First, there is the group that includes plants, such as tomatoes and cucumbers, that are not choosy as to the length of day. A second group are called “short-day” plants. These will not flower until the daily dose of light is below a certain number of hours. The third group are called “long-day” plants. These flower when daylight extends beyond a certain number of hours.

Consequence of Findings

These investigations answered many questions. They explain why plants of a given species can be planted at different times of the year, and yet all flower at the same time. And they reveal why certain plants bloom in particular districts, but will not flower at all in others.

Agriculturists now routinely determine the light requirements of plants. Some of them have very specific length-of-daylight needs. For example, various varieties of onions and soybeans do best only when grown within a belt of latitude of 150 miles. If they are grown either north or south of this region they may fail as a crop.

The daylight needs of plants can result in disappointment to flower lovers. While on a trip a person may obtain a colorful plant for his garden, but back home it may not bloom. Why? The daylight where he lives may not be of suitable length for the plant to flower.

For example, there is the rock-garden plant Sedum telephium, which grows in southern Vermont. But it needs a daily dose of sixteen hours or more of light in order to bloom. It receives this in Vermont. However, if one took it very far south it would fail to bloom because of insufficient daylight.

On the other hand, a person in northern Maine may be grateful that there is little or no ragweed there. Ragweed will not flower until daylight decreases to fourteen and a half hours. This does not occur in northern Maine until after August 1, so it does not allow enough time for seeds to mature before cold weather comes.

How Plants Detect Light

Learning these facts about the responses of plants to length of daylight made something else apparent. Plants must have something within them that detects the change in length of daylight and that causes them to respond accordingly. Just recently this substance, called “phytochrome,” has been isolated.

Phytochrome is a bluish, light-sensitive pigment that absorbs red light. It has been shown that many plants, when exposed to the red wavelength of light, mature more rapidly. Somehow the light acts on the phytochrome to regulate a plant’s growth changes, from seed stage to maturity. But it is not understood just how this is accomplished.

Manipulating the Light

Many gardeners now use to good advantage this knowledge about the responses of plants to light. By adjusting the length of exposure to light they can make a plant bloom when they want it to. Thus in winter they enjoy flowers that normally grow only in summer, and those that normally bloom in autumn they may have in other seasons.

A chrysanthemum, for instance, is normally an autumn-flowering plant. But it can be made to bloom in summer. Just cover it with a cardboard carton in the late afternoons, and remove the carton in the morning. The extended period of darkness will cause the chrysanthemums to react as if it were autumn, and they will bloom with the summer flowers.

On the other hand, a person may want to enjoy in winter flowers that normally bloom only in summer. By giving them daily doses of artificial light after the day has ended, these plants can be made to react as if the long summer days had arrived. Thus they will bloom during the short days of winter.