THE LIGHT FACTOR IN PHOTOSYNTHESIS 1037 



loci to the concliisioii that these effects were the result of the previous 

 light treatment of the experimental plants. Plants cultivated in fairly 

 strong light showed an induction period when placed in strong light; 

 plants cultivated in weak light showed the decrease in photosynthesis 

 when experimented on in strong light. The shape of the curve depended 

 on the relative light intensities used in the cultivation and in the sub- 

 sequent experiments. These relations are schematically represented in 

 Fig. 8, prepared by Harder. Also Harder's results show that "shade" 

 and "sun-plants" have real significance, that one form of plant can be 

 transformed into the other, and that the change from weak-light plants 

 to strong-light plants is faster than the reverse change. 



Sudden changes in the intensity of illumination cause immediate 

 changes in the rate of photosynthesis. Thus Li (63) has shown that a 

 decrease in light intensity results in an immediate drop in rate which then 

 again rapidly rises. Similarly an increase in light intensity results in an 

 immediate acceleration followed by a rapid decrease in rate. Changes 

 from one spectral region to another of equal available energy produce no 

 appreciable variation in photosynthetic rate. Kostytschew (56) and his 

 associates have reported that under "natural conditions" the course of 

 photosynthesis during the day is extremely variable. In some plants 

 the rate increases to midmorning, decreases to midday when oftentimes 

 carbon dioxide is evolved, then increases to midafternoon and falls to 

 night. 



Compensation Pomts. — The compensation point is the light intensity 

 at which carbon dioxide is neither absorbed nor evolved. In general, 

 this decreases with decreasing temperature (Miiller, 81), but is very 

 different in different species, depending in a large measure upon the rate 

 of respiration of the plant. Ehrke (20) has shown that different species 

 of algae kept under similar conditions possess compensation points at 16° 

 which vary from 457 meter-candles for Enteromorpha compressa to 270.3 

 for Delesseria sanguinea. The highest compensation point yet reported 

 is 4200 Lux for the lichen Peltigera canina (816). Gregory and Richards 

 (35) found that when the respiration of barley plants, deficient in phos- 

 phate nutrition, is plotted against the compensation point a straight 

 line is obtained. 



The intensity of the light used to adapt the plant is a very important 

 factor in determining the compensation point of the individual. Harder 

 (39) showed that when one sample of Fontinalis sp. is separated into two 

 portions and one is kept in the sun and the other in the shade, the shade 

 plant had a compensation point of 37 meter-candles, and the sun plant 

 170. 



The irregularities in the rate of photosynthesis under natural condi- 

 tions which have been reported by Kostytschew (56) and the remarkable 

 fluctuations in rates of carbon dioxide evolution observed by Jaccard (45) 



