226 



PHYSIOLOGY OF GROWTH AND CONFIGURATION 



It is self-evident that such methods of observation can give but inaccurate 

 and merely approximate results." Plant growth is not proportional to tem- 

 perature. On a certain day, for example, a temperature of 35° may occur, while 

 the best temperature for the growth of the plant in question may be 25". The 

 additional 10° may not only be useless in promoting growth but it may even be 

 injurious to the plant. Because a plant has developed under conditions giving 

 a certain sum of daily temperatures, it is not safe to conclude that the same 

 plant might not have developed equally well under conditions giving a smaller 

 temperature summation. The birch grows near Kiev at a higher temperature 

 than it experiences in the neighborhood of Petrograd. The following table, by 

 which the course of development of the vegetation at Brussels and at Petrograd 

 are compared, substantiates this conclusion. Six groups of plants are considered, 

 the first group consisting of the earliest-flowering plants (Anemone, Corylus) 

 and the other groups being composed of progressively later-flowering forms. 

 The temperature measurements were begun in Brussels on Jan. 16, and in 

 Petrograd on Apr. 8. The date of flowering for Brussels is given for each group 

 of plants and also the number of days between this date and the corresponding 

 date for Petrograd. The temperature summations, above o°C., are also given 

 for each group at the two stations, up to the time of flowering in each case. 



These observations show that the plants at Petrograd came to flowering with 

 a smaller temperature summation than did those at Brussels. It is also note- 

 worthy that the date of flowering at the northern station is very markedly later 

 than that at the southern only in case of the early-flowering forms, and that the 

 difference between the two dates decreases as the date for Brussels becomes 

 " On the general problem of integrating temperature values to obtain a measure of the 

 effectiveness of temperature conditions for plant growth and development, see: Livingston, B. 

 E., Physiological temperature indices for the study of plant growth in relation to climatic con- 

 ditions. Physiol, res. i : 399-420. 1916. Other references are there given. Also see: 

 McLean, F. T., A preliminary study of climatic conditions in Maryland, as related to plant 

 growth. Ibid. 2 : i2g~2o8. 1917. It must be remembered that many environmental con- 

 ditions besides temperature are influential in determining plant behavior, and that these also 

 vary from day to day and from place to place. Blackman's discussion of limiting conditions 

 for plant processes has a bearing on this general problem. See: Blackman, 1905, 1908. (See 

 note If, p. 35.) — Ed. 



