PHOTOSYNTHESIS 91 



Disappearance of Starch from Green Tissues in Darkness. This may be 

 demonstrated either by Sachs' method, described in Darwin and Acton, 32, 

 or by the reverse of the preceding experiment, viz., taking a suitable plant 

 from the bright light about ten o'clock of a bright day (by which time it 

 usually contains an abundance of starch), placing it in a dark warm (25°- 

 30 ) place, and taking discs from the leaves at regular intervals, these 

 being marked and later tested for starch. This Translocation of starch 

 will be considered later under that subject. 



In connection with the foregoing experiments the student 

 has been brought into touch with questions concerning the opti- 

 mum temperature and light for promoting the process under 

 study. Our knowledge of this matter is now in a transitional 

 state, and the subject is set forth with great clearness, under 

 the name Assimilation, by Blackman in the Annals of Botany, 

 19, 1905, 281. This paper the student should here consult, 

 noting particularly his temperature-curve and its meaning. 



Practical Optima of Temperature and Light in Photosynthesis. 

 As Blackman shows, in the paper just cited, the rate of photosynthesis prob- 

 ably rises with increase of temperature to near the death point, the optimum 

 and maximum thus lying near together. Since, however, at temperatures 

 above 25°-3o° the rate is not maintained when the temperature is held con- 

 stant, but falls off (and the faster, the higher the temperature), there is no 

 experimental advantage in working with temperatures above about 25 , 

 the more especially as respiration increases so rapidly with higher tempera- 

 tures as soon to quite equal and then overbalance photosynthesis. Further, 

 if one is working with starch formation in the leaf, it is in any case necessary 

 to keep it below 25°, since above that temperature, with most leaves at least, 

 the translocation is so fast that no starch appears in the leaf even though 

 an abundance of the photosynthate is being formed. All things considered, 

 for experimental purposes the optimum may best be taken as about 22 , 

 certainly not above 25°. 



As to the optimum of light for photosynthesis, Blackman has shown that 

 this depends upon the percentage of carbon dioxide available to the leaf. 

 Where, however, the percentage is as low as it is in the atmosphere, the leaf 

 cannot use, especially in summer, all the light of direct sunlight. A strong 

 diffused light, therefore, not only contains all the light the leaf can use, but 

 it is better for the leaf in other ways, particularly if the tissues are enclosed. 



The foregoing observations upon the appearance and dis- 

 appearance of starch must suggest to the thoughtful student the 

 query, whether this substance represents simply a transforma- 

 tion of some leaf material already present, or a new substance 



