278 PLANT BIOCHEMISTRY 



others grow noinially under conditions of high light intensity. Most 

 plants, on the other hand, show growth in complete darkness just 

 as long as they have a reserve food supply. Contrasted to green plants 

 growing in the presence of light, those grown in darkness have charac- 

 teristically small leaves which are white or yellow in color, and 

 poorly developed root systems. Such plants, said to be in a state of 

 etiolation, cease to grow as soon as the reserve food supply is exhausted. 

 In most cases, the absence of light increases and the presence of light 

 decreases the rate of stem elongation. Plants which have the benefit 

 of full light produce shorter, thicker stems than plants grown under 

 low-intensity light. Thus, plants that are crowded have a tendency 

 to be spindling and consequently more fragile than those which have 

 been given ample room for development. 



The effect of different wave lengths of light on the photosynthetic 

 process was discussed in Chapter 9. Some general conclusions can be 

 stated with regard to light quality and plant growth. Thus, light of 

 short wave length (blue region) retards growth in length, whereas 

 long wave lengths (red region) have a tendency to produce spindling 

 effects. 



Many plants react quite characteristically when exposed to varying 

 periods of light and dark. The response of plants to the relative 

 length of light and dark periods is called photoperiodism. Thus, it 

 has been found that certain plants do not flower until the photoperiod 

 reaches a certain critical value for an extended period of time. For 

 instance, spinach does not form flowers until the day length reaches 

 at least 13 hours for a period of not less than 2 weeks. Plants that 

 react in this way to long photoperiods are said to be long-day plants 

 and include henbane, spinach, dill, plantain, sugar beet, and winter 

 barley. 



Other plant species, such as Clirysanthetmin}, flower only when the 

 daily period of illumination is shorter than a certain critical value. 

 Such plants are called short-day plants. Thus, chrysanthemums remain 

 vegetative if the period of exposure to light exceeds 14 hours and 

 produce flowers only when the day length is less than this critical 

 \alue. Such plants as (hrysanthcmum, ragweed, poinsettia, and 

 cocklebur are all dependent on long periods of darkness. 



Finally, there are certain species of plants such as tomato, snap- 

 dragon, and buckwheat that bloom throughout the growing season. 

 Such plants are indifferent to the relative lengths of day and night. 



In addition to flowering, the photoperiod is known to affect many 

 other plant characteristics, among which are leaf shape, leaf thickness, 

 the bulbing of onions, and the tuberization of potatoes. 



