206 ENVIRONMENTAL CONDITIONS. 



other conditions do not always vary in the same way and they are never 

 constant, not even as related to criteria mthout the plant-body. Even 

 though they were constant for any plant, progressive alteration within 

 the organism would assuredly produce great variations in the relations 

 of the external world to internal conditions as criteria. It is therefore 

 quite hopeless to contemplate an accurate causal interpretation of 

 plant states merely on the basis of integrations of temperature effects. 

 Nevertheless, the same incubus of hopelessness overshadows similar 

 attempts along lines of approach based upon other external factors, 

 and since we are sure that no single criterion alone will lead very defi- 

 nitely toward the solution of our problem, progress may be sought only 

 by treating the different factors separately and studjdng the results, 

 after which artificial and natural combinations of factors may be 

 attacked. Furthermore, the study of the temperature conditions of 

 plant environments appears promising in this particular, namely, that 

 temperature not only directly influences plant activities, but also 

 influences all of the other effective environmental conditions to a 

 greater or less degree. 



There appear to be, in general, two possible criteria for comparing 

 the temperature intensities of several different localities. By one of 

 these the comparison is made of the extremes, merely, of the yearly 

 maximal and minimal temperatures for the several stations. Thus the 

 duration factor is completely left out of account. It is, however, 

 possible to consider and compare maxima and minima, not for the 

 entire year, but for shorter seasons, and these seasons may be of 

 different length at the different stations. The difference between the 

 maximum and minimum temperatures may then be obtained, giving 

 the range of normal temperature for each particular season and place 

 to be considered. The last-named function of the temperature condi- 

 tions does take some account of the duration factor (since the season is 

 described in each case), and may, in certain instances, have an impor- 

 tant relation to plant activities. By the second general criterion the 

 intensity conditions are summed or integrated, in some manner, 

 through a given length of time, and the duration factor is thus seen to 

 play a direct and important part in this procedure. 



Mean temperatures for long periods of time are not apt to be of value 

 in studies of the relations between plant activities and the environ- 

 ment; seasonal or yearly means, which comprise such an important 

 part of the usual meteorological reports, seem never to have given any 

 real promise in this direction. Such means do not take account of the 

 duration factor; the summed daily means are divided by the number 

 of days considered, so that either this number of days must be the 

 same for all localities compared, or else the numbers obtained can bear 

 no relation to the corresponding extent of plant growth and other 

 activities. Thus, a long growing-season with a given mean tempera- 



