756 BIOLOGICAL EFFECTS OF RADIATION 



there is, during the first few minutes, a pronounced increase in growth 

 rate followed by a decrease below the initial rate. Continuous light 

 causes a depression in growth rate which later on tends to return to 

 normal. It shortens the period of elongation and causes a reduction in 

 total length. Exposure to darkness after continuous illumination causes 

 at first an increase in growth, followed by a return to the initial rate. 

 Brief exposures to light induce a wave-formed response in the growth 

 curve, the period of which depends upon the amount of light to which the 

 plant is exposed. 



In low intensities of normal daily duration, plants develop chlorophyll 

 and the leaves unfold, but otherwise they present the appearance of 

 etiolated plants, viz., long internodes, weak, succulent stems, vegetative 

 growth only, poor root development, and thin leaves with loosely organ- 

 ized, thin-walled cells. As the intensity is increased, vegetative growth 

 increases to a maximum at about 25 to 50 per cent of normal summer 

 sunlight of temperate regions. The plants attain maximum height and 

 leaf area. The leaves are still comparatively thin and flowering is 

 considerably reduced. Such plants contain a higher percentage of 

 nitrogenous compounds, particularly in the soluble forms, than plants 

 developed under full sunlight. A further increase in light intensity causes 

 an increase in dry matter but a decrease in height and leaf area. Maxi- 

 mum fruitfulness and maximum mechanical and water-condvicting tissue 

 usually occur at about the same light intensity as maximum dry weight. 

 Still further increases upset the water economy and often the iron nutri- 

 tion of the plant and result in local injuries to leaves or even in some cases 

 death through excessive heating. 



Transpiration, the osmotic concentration of plant sap, and alkalinity 

 of sap increase directly with light intensity. The ash content per plant — 

 but not always per unit dry weight — increases with increasing light 

 intensity, as does also the ability of the plant to use strong ions. 



Light favors the hardening of plants against cold, provided it is 

 accompanied by proper temperature conditions, viz., warm days and cold 

 nights. Light also favors the development of drought resistance in 

 plants. Both of these effects depend to a considerable extent upon the 

 accumulation of a carbohydrate reserve. 



In natural habitats many factors aside from light affect the rate of 

 photosynthesis and the growth of plants. Frequently plants, especially 

 in desert regions, may perform their entire daily photosynthesis in a few 

 morning hours. Therefore it is difficult to study the minimum light 

 requirements of plants in natural habitats. Furthermore, it must be 

 borne in mind that a plant, to maintain itself successfully in a given 

 habitat, must have sufficient light not only to enable it to exist, but 

 sufficient for active growth and for the accumulation of a food reserve 

 for withstanding unfavorable seasons and for reproduction. 



