1856 



LIGHT 



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 regard- 

 less of their photosynthetic requirements. Within cer- 

 tain limits, however, it can be stated that photosynthe- 

 sis or carbon-assimilation is proportionate to light 



2145. Showing cross section of an east and west house, about 

 36 x 200 feet, showing method of growing cucumbers. The light 

 is much superior in a house of this sort. 



intensity, and furthermore, that growth and develop- 

 ment 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 light on growth is to retard it, 

 though Blaauw, McDougal and Vogt have been able 

 to discover a stimulation of growth under certain con- 

 ditions, while, on the other hand, lack of light or dark- 

 ness accelerates growth. The yellow rays of the spec- 

 trum 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 chlor- 

 ophyll, possess thin stems, elongated internodes and 

 very poorly developed leaves. The mechanical or sup- 

 portive 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 J 

 deep shade.) Many fungi (Pilobolus) do not produce / 

 fruiting bodies (sporangia) until they reach the light. L 

 Wiesner, who has made a thorough study of the light 

 requirements of plants, has classified some of our com- 

 mon 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 com- 

 mon 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 develop- 

 ment, 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. 



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 under- 

 standing 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 coty- 

 ledon 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 demon- 

 strating, 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 mini- 

 mum 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). 



2146. Showing cross section of a greenhouse used tor lettuce 

 and cucumbers. The slanting side toward the right (north) is 

 boarded and used for forcing rhubarb. The cucumber plants are 

 trained vertically. 



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 indif- 

 ferent, depending upon the amount of light used. Many 

 complicated forms of phototropism are observed in the 



