ROOT GROWTH AND TEMPERATURE 331 



''grand period," and that this period may have been passed 

 during the experiment. However, it is impossible to differen- 

 tiate between the two, and other factors, peculiar to the method 

 used, may have been in operation as well. 



VARIATION IN THE RATE OF GROWTH 



Three types of growth variation in the roots of the mesquite 

 were observed. Of these two are associated with every root 

 whose growth was studied, and one type appears to be depend- 

 ent on the length of the root. The first two types may be called 

 the major and minor fluctuations in rate. They are of few or 

 many hours duration, and do not determine the distinguishing 

 character of the growth curve, which is determined by the third 

 type of variation. This can be characterized briefly. Generally 

 speaking roots whose initial length at the commencement of 

 observations are 50 mm., or less, in length, show a gradual accel- 

 eration in the rate of growth for several hours, after which the 

 rate decreases. In roots which are 50 mm. in length, or more, 

 on the other hand, such acceleration appears not to be present, 

 but on the other hand, the rate either remains fairly constant, 

 or it decreases gradually. It should be noted that the length 

 50 mm. is only approximate and probably varies considerably. 

 Another characteristic of the growth curve of the young mes- 

 quite roots, as contrasted wdth that of the older roots, is the 

 greater amplitude of the major and minor growth variations 

 as above described. 



So far as the variation in rate of root growth in mesquite for 

 the temperatures used is concerned, this can be summarized in 

 a moment. In table 7 the average hourly maximum and mini- 

 mum rate for the same individual is given. In all cases the 

 maximum rate is the highest observed in the experiment, but, 

 per contra, the minimum may, or may not be the least ob- 

 served. The figures give, however, the actual range in the 

 variation rate for the individual and at the temperature given. 



Although the leading conclusions as above given do not 

 harmonize clearly with the results of recent workers in this 

 field, whose results also are not in close accord, they, however, 



