MANUFACTURE AND UTILIZATION OF CARBOHYDRATES 165 



same degree. Hence, it appears that mature leaves contain an excess 

 of chlorophyll and some other factor limits the rate of assimilation. 

 In autumn the chlorophyll content decreases but as chlorophyll is not 

 usually the limiting factor, assimilation does not decrease in proportion 

 at first. If leaves remain green, they maintain their assimilatory power 

 until they fall — a matter of no small importance in food storage. 



Variation with Light Supply. — The development of chlorophyll in most 

 plants depends on the action of light in which the red rays seem the most effect- 

 ive.", 189 In all probabiUty precursors of chlorophyll are present and exposure 

 to light effects certain chemical reactions necessary for the complete development 

 of the pigment. Light is not always essential to chlorophyll development, how- 

 ever, for conifer seeds germinate and become green even in the dark. 



Light not only aids in the development of chlorophjdl but at higher intensities 

 brings about its destruction probably through oxidation. The decomposition of 

 chlorophyll occurs outside the plant as well as within its tissues. This can be 

 demonstrated by exposing a test tube containing a solution of chlorophyll to 

 the light and comparing it with another kept in darkness. Red and yellow 

 light are most effective in destroying chlorophyll. Consequently it is found 

 that at low light intensities plants grown in yellow light contain the most chloro- 

 phyll but that at higher intensities plants grown in blue hght contain the most, 

 owing to the destructive effect of the yellow light at these higher intensities.^"^ 

 Similar effects from red light have been observed. !*■* The double effect of Hght 

 in stimulating the development of chlorophyll and in bringing about its destruc- 

 tion, leads to noticeable differences in the chlorophyll content of plants growing 

 in different latitudes and altitudes. It has been found that the minimum amount 

 of chlorophyll necessary for growth is approximately the same at all latitudes 

 but that the maximum amount increases toward the equator. ^^"^ Hence, a 

 plant may have twice as much chlorophyll in the tropics as at 60° north latitude. 

 For a given species, however, the amount is less at both extremes of its range 

 than at the center and it has been suggested that this may be due to greater 

 oxidizing action at the limit on the equator side where the light would be more 

 intense. The significance of these discoveries lies in the relation of carbohydrate 

 accumulation to fruitfulness. Plants growing at high altitudes contain less 

 chlorophyll than those growing in the lowlands. ^^ In Alpine plants carbon assimi- 

 lation requires greater light intensities but lower temperatures. 



Temperature. — At low and medium temperatures in the absence of 

 other limiting factors, the rate of assimilation is a coefficient of the tem- 

 perature. Assimilation has been detected at — 6°C. and from this point 

 to 25°C. the rule stated above has been found to hold with the plants 

 investigated. Above 25°C., the rate of assimilation does not remain 

 constant at any given temperature. The higher the temperature, the 

 more rapidly it decreases; at any given temperature the initial decrease 

 is greatest. This "time factor" that enters at higher temperatures 

 probably is indicative of the interference of another factor, namely, 

 enzymes. 



