34 PLANT PHYSIOLOGY 



as a substitute for CO 2 (BOTTOMLEY, 1903), on the other hand (RICHARDS 

 and McDouGAL, 1904) this has been denied ; at the same time CO has been 

 stated to be a virulent poison. 



11. 4-5, for that, at the commencement . . . observable read that assimi- 

 lation may start in the absence of demonstrable traces of oxygen, 



1. 19, after inactive, read We speak of inactivity of the chloroplasts (WART, 

 1896 ; PANTANELLI, 1903) when C0 2 -decomposition is inhibited by any factors 

 which do not suspend other processes in the plant, e.g. respiration, and which 

 at the same time do not visibly injure the chloroplasts, so that when the in- 

 hibiting factors are removed, they are capable of again exhibiting their normal 

 functions. A whole series of substances act in this way, anaesthetics and 

 antipyretics, acids and alkalis. Carbon-dioxide itself in higher concentration 

 has the same effect, and an accumulation of carbohydrates formed in the 

 process of assimilation leads to temporary inactivity of the chloroplasts. 

 Hence, quite incidentally, such an inactivity of the chloroplasts may occur 

 in assimilation experiments, as, for example, when these are made on isolated 

 leaves, where translocation of the assimilata is impossible (compare SAPO- 

 SCHNIKOW [1895] and the startling statements made by BROWN and ESCOMBE 

 [1905, p. 50]). Finally, extremes of temperature and high insolation must be 

 alluded to, but of these we shall speak presently. That the substances men- 

 tioned above, when present in small amount, may also act as stimulants, is 

 not to be wondered at after what has been already said on the subject of 

 poisons. Such a stimulatory action, e.g. due to ether and chloroform (KEGEL, 

 1905), dilute acids (TREBOUX, 1903), has been already demonstrated, and no 

 doubt will be so for other substances also. 

 1. 20, for Heat read Temperature, 



1. 27 P. 125, 1. 8, for KREUSSLER (1890) . . . still more sudden.] read 

 MATTHAEI (1904). These were carried out with artificial light on isolated 

 leaves by means of the method adopted by KREUSSLER. The leaves are, 

 first of all, kept for i hours at the temperature to be investigated, and then 

 the amount of CO 2 -decomposition taking place per hour is next determined. 

 Fig. 25 a gives the results in graphic form. The temperatures are indicated 

 on the abscissa, while the ordtnates give the amount, in milligrams, of C0 2 

 decomposed per 50 sq. cm. of leaf surface. The continuous line gives the 

 results of the first hours of the experiment. It shows that the amount of 

 CO decomposed rises at first as the temperature rises and falls so rapidly 

 abo've 37-5 C., that at about 45 no assimilation at all takes place ; below 

 o C. feeble decomposition goes on, and may be observed even down to 5 C. 

 Many other functions in the plant depend in a similar manner on temperature, 

 and SACHS (1860), who was the first to draw attention to the point, described 

 the temperature at which the function in question began as the minimum, 

 that which gave the best results as the optimum, and that at which the func- 

 tion ceased as the maximum. The minimum, optimum, and maximum he 

 termed the cardinal points. Many investigations have been carried out since 

 his time on carbon assimilation (the only function we are at present concerned 

 with), especially by KREUSSLER (1890), with the view of determining the 

 position of these cardinal points. From MATTHAEI'S researches it would 

 appear that the cardinal points are by no means fixed. At all low tempera- 

 tures the values obtained for CO 2 -assimilation in successive hours on the whole 

 correspond ; the leaf, in other words, works with constant energy. Above 

 23-7 one notices, however, a steady fall in assimilation from hour to hour. 

 This is expressed by the dotted lines II, III, and IV in the figure, which repre- 

 sent the assimilation taking place i, 2, and 3 hours after the first determina- 

 tion. The result of this rapid fall at higher temperatures is that the optimum, 

 which lay at 37-5 in the first reading, and which would certainly have been 



