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Standard Cyclopedia of Horticulture

Light. The various manifestations of energy which we term heat, light, electricity; gravitation, and the like, play an important ro1e on living matter, and none is more important than light. Photosynthesis or carbon-assimilation, a characteristic plant function, constitutes one of the fundamental processes in nature; indeed this process, which in remote ages may have been developed secondarily as a protoplasmic function, is as wonderfu as life itself, and a thorough comprehension of photosynthesis would appear to be as difficult as that of life. However, no form of energy affects plant configuration more than light, and in the growing of crops, whether out-of-doors or in the greenhouse, the factor, light, must always be given consideration.

Light is regarded as a form of radiant energy and is composed of various wave-lengths of exceedingly small size. Those which are especially active in photosynthesis or carbon-assimilation in our common green plants are in the red half of the spectrum, while the blue-green pigment organisms (marine alga;, and the like) utilize the more highly refrangible rays of the spectrum. Briefly, photosynthesis is the building up of carbohydrates from carbon dioxide and water through the chemical action of light on the chlorophyll, and since plants obtain most of their energy by means of this process, it is not difficult to realize its fundamental significance and importance. The ultimate product of photosynthesis is starch, but the complex chemical changes taking place in chlorophyll grains through the action of light and the building of starch from carbon dioxide and water is not definitely known, and there are doubtless many intermediate steps.

In any examination of flora is found much variation and adaptability to conditions, the result of innumerable generations existing under varied conditions. The light conditions in one section differ greatly from those in another part of the earth, but it is known from actual observation that some plants require much more light than others for their normal development. Even in the same location, such as the tropics, remarkable differences may be found in the light requirements. Many tropical plants require little light, and even when grown in northern latitudes in conservatories they require shading throughout the year. Palms, geraniums and other plants will develop normally under far less light than such plants as the cucumber or lettuce, but the latter grow much more rapidly and may perhaps for this reason alone show the ill effects of poor light regardless of their photosynthetic requirements. Within certain limits, however, it can be stated that photosynthesis or carbon-assimilation is proportionate to light intensity, and furthermore, that growth and development are correlated with photosynthesis.

The relation between photosynthesis and light intensity may be shown by using strong contrasting photographic negatives on leaves exposed to sunlight. By specially treating the exposed leaves, a positive can be produced which will show that little starch was formed under the thicker portions of the negative, and more under the thinner portions; in other words, the formation of starch under such conditions would be proportional to the amount of light received by the chlorophyll grains through the negative.

The general effect of fight on growth is to retard it, though Blaauw, McDougal and Vogt have been able to discover a stimulation of growth under certain conditions, while, on the other hand, lack of light or darkness accelerates growth. The yellow rays of the spectrum are more active in inhibiting growth than the violet rays, the latter having a similar effect on plants to that of lack of light. Plants grow the most in the night, the growth curve gradually rising during the night and falling in the daytime. While lack of light stimulates growth, plants grown entirely in the dark or under poor light conditions are abnormal. Etiolated plants, or those grown in the dark, are devoid of chlorophyll, possess thin stems, elongated internodes and very poorly developed leaves The mechanical or supportive tissue is little developed, and such plants possess small power of resistance. Light, on the other hand, develops mechanical tissue and induces firmness of texture. Moreover, without light there may be no increase in the weight of dry matter, hence seedlings grown in the dark may increase in size, but lose weight from respiration or loss of carbon dioxide.

Light is a factor in the dwarfing of alpine and arctic plants and in the development of hairs on some algae as a protection against too intense illumination. Some plants grown in a weak light fail to produce flowers (Insects attracted by the warmth of the sun are more likely to visit flowers growing in sunshine than in the deep shade.) Many fungi (Pilobolus) do not produce fruiting bodies (sporangia) until they reach the light. Wiesner, who has made a thorough study of the light requirements of plants, has classified some of our common species as follows:

(a) Light-requiring: Alfalfa, red clover, wild carrot and so on.

(b) Light-loving: Dandelion, plantain, lychnis and so on.

(c) Indifferent: Blueberry, poet's narcissus, the common brake, and the like.

(d) Light-shunning: Forget-me-not, violet, anemone and the like.

(e) Light-fearing: Wild strawberry, water violet, and the like.

Wiesner found that the beech, for example, reaches its normal development in one-tenth part of the light intensity required by the larch and other sun-loving plants.

Light requirements are affected by a decrease in temperature: e.g., the maple in Norway requires ten times as much light as in Vienna for its normal development, and in general one may say, the farther north or the higher above sea-level a certain plant grows the greater becomes its light requirement.

The size of leaves is very much influenced by light. Too intense light as well as etiolation tends to reduce the size of the leaves, which reach their maximum in a medium light intensity. This is shown in the growth of such crops as cucumbers under glass during the winter.


Phototropism or heliotropism is the term applied to the response of plants to a light stimulation from one side which causes movements toward or away from source of light Phototropic movements are of much biological significance to the organism. Most aerial parts of plants (stems and branches) are positively phototropic, i.e., they bend toward the source of light, while roots are usually negatively phototropic, i.e., they bend away from light. For a clear understanding of the processes of plant response to light stimulation from one side, it is best to consider the reaction of a simple orthotropic plant such as the cotyledon of an etiolated oat seedling. This plant, which is remarkably sensitive, has been studied extensively by Darwin, Rothert, Fitting, Blaauw and others. It has been found in this plant that the apex of the cotyledon is the most sensitive to light stimulus, thus demonstrating, as in other forms of response, the localization of the perceptive or sensitive zone.

The bending toward the source of light as the result of the light stimulus begins at the apex, and it proceeds toward the base until the cotyledon coincides with the incident ray of light. The reaction time or latent period following stimulation is one-half to one hour in most cases, and the rate of transmission of the stimulus is from 0.7 centimeters to 1 centimeter an hour. The minimum time of exposure necessary to induce a reaction to a light stimulus is called the "presentation time," and this is dependent on the amount of light applied, which is a product of the light intensity, the exposure and distance from the plant. For example, an exposure to a light of high intensity (26,520 meter candle power) for one-thousandth of a second produced the same reaction as an exposure of forty-three hours to a light of very low intensity (0,00017 meter candle power).

An organ may be positively phototropic at one period of its development, and negatively phototropic at another, and it has been found that an oat seedling will respond positively and negatively or remain indifferent, depending upon the amount of light used. Many complicated forms of phototropism are observed in the movements of leaves and flowers (begonia and sunflower) which adjust themselves to certain angles as regards the source of light.

Phototaxix and pholotonastie.

The reacting or orientating response of motile (bacteria, swarm spores, and the like) or free-moving organs (chloroplasts) to a unilateral light stimulation is called phototaxis. This is a common mode of light reaction in plants. Plant movements which are not dependent upon the direction of light stimulus but are due to changes in the intensity are called photonastic movements. Under this category may be placed the movements of stomata which open under illumination and close in the darkness, also certain movements of etiolated seedlings when subjected to light.

The pathological effects of light.

The pathological effects of light on plants are a much more important factor than is generally realized. Many pathological conditions of plants are brought about by lack of light, and in some cases excess of light may produce injurious effects. This is shown in the case of sun-scald, which occurs on various trees. Some plants are so perfectly adapted to forest conditions that they cannot endure direct sunlight without injury. When they are exposed to direct light such as is caused by forest thinning, they are likely to sun-scald badly, but more often difficulties arise from lack of light. Tissue developed under poor light conditions is more likely to be affected with winter-killing due to non-ripening of the wood, and burning from fumigation with gases is induced by lack of light in poorly lighted greenhouses. Lack of light during the dark months often develops inferior tissue, which, when exposed to the more intense light of spring becomes susceptible to wilting. The exclusion of light from part of the plants resulting from crowding, often gives rise to various stem-rots such as are characteristic of parsley, water-cress, lettuce, and the like, and there are a large number of leaf-blights and spots such as occur on cultivated plants which are induced by insufficient light. Lack of light induces the formation of various mildews on plants and is conducive to damping-off in many cases. It often causes disease of plants growing in dry soil as a result of excessive transpiration; in short, every greenhouse grower must regulate the growth of his crops according to light conditions in order to eliminate the possibility of disease.

Light as a factor in greenhouse construction and management.

In the growing of plants under glass, which constitutes a large and constantly increasing industry, the problem of light is intimately associated with the location, construction of the greenhouses, management, and so on. Improvements in the line of greenhouse construction have been based very largely on the effects of light. The early houses in the United States were very crudely constructed, and in the modern, improved types of houses some crops are grown in one-half the time formerly employed, a fact due largely to improvement in methods of greenhouse construction. The old tvpe of houses were chiefly sash-houses cumbered with shadow-producing material and glazed with small glass of inferior quality and often dirty, and widely lapped. The houses were in some cases so poorly constructed that they excluded from 40 to 60 per cent or more of light-rays. The modern house need not exclude more than 12 to 20 per cent. Some of the more or less modern types of houses which have been built for a number of years exclude as much as 30 per cent of light. Poor light conditions alone greatly retard growth, not to mention the frequent losses in poorly constructed houses from pathogenic organisms which find most favorable conditions for their development.

The amount of light to be found in any particular location depends upon the latitude, but more particularly upon the meteorological conditions which may prevail, and the variation in this respect throughout the United States is quite notable. Numerous meteorological observatories, without actually measuring the light intensity or amount of light, have given data as to the number of hours of sunshine, which is valuable in comparing light conditions in various localities. These records have been gathered for a considerable period of time and reliable averages are at hand. The average total number of hours of sunshine during the year based upon data covering a long period of time is as follows: Hours.

Chicago, Illinois 2,617 Cleveland, Ohio 2,000 Milwaukee, Wisconsin 1,865 Boston, Massachusetts 2,493 Nashua, New Hampshire 1,948 Ithaca, New York 2,273 New York, New York 2,510 Philadelphia, Pennsylvania 2,575 Phoenix, Arizona 3,742 Modena, Utah 3,354 Los Angeles, California 3,219

This data shows great variation in the hours of sunshine which cannot be attributed to latitude alone. It should be pointed out, however, that elevation constitutes a very important factor as regards light intensity, the higher the elevation the better the light conditions, and even the light intensities at an elevation of 500 to 600 feet are better than those at the surrounding low country, at least during the early parts of the day, but this difference gradually decreases toward night. From the commercial florist's point of view, the critical months in the year are November, December, January and February, and even a few days of cloudy weather when the crop is maturing often make much difference in the financial returns. Much more variation in light intensity exists in the dark winter months than in other seasons of the year. This is true as regards the differences existing between morning and afternoon light, for during the darker months the light may average 30 per cent more intense during the morning than m the afternoon. The percentage of possible sunshine recorded during November, December, January and February averages 22 per cent for Cleveland, 44 per cent for Chicago, 54 per cent for New York, and 75 per cent for Los Angeles. From such variations in the amount of sunshine found in the various territories during the critical months, it is evident that there must be corresponding differences in the period of development of the crops, the growth of a crop being in general proportional to the amount of light it receives.

In the construction of greenhouses, therefore, it is important they should be designed to produce the maximum results during the critical light seasons.

From a study of the light conditions in greenhouses, it has been found that large glass is superior to small glass because of the smaller amount of light-obstructing framework, but with present methods of construction there is a limit to the size of glass that can be safely employed. Moreover, high-angled roofs are much superior to low roofs from the light point of view, but their practical utility is somewhat limited. Experiments with different types of glass have shown that there may exist 18 per cent difference in the light-transmitting properties of No. 1 and No. 2 quality glass, and third quality glass is 33 per cent less effective than No. 1 quality. A slight annual deterioration in greenhouse glass must be expected owing to the formation of a film of oil, but this can be obviated to some extent. The nature of the reflecting surface of the greenhouse, degree of lapping, and other factors influence light. In the modern large house more uniform light conditions are obtained than in the early, smaller houses.

As regards the direction of the greenhouse, for most purposes the east and west house is preferable for obtaining light, but some crops are able to thrive better, especially in the spring months, in a house running north and south. Morning light being superior to afternoon light, an east and west house should be tilted somewhat toward the northeast, thus exposing the plants more directly to the morning light and making it possible to syringe with less liability to fungous infection of the plants.

The above text is from the Standard Cyclopedia of Horticulture. It may be out of date, but still contains valuable and interesting information which can be incorporated into the remainder of the article. Click on "Collapse" in the header to hide this text.

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