Oct. 1, 1921 Effect of Soil Temperature on Development of Nodules 25 



Now it would be expected, and it is clearly true, that plants grown in 

 soils held at such widely different temperatures would show in the given 

 time considerable difference in size and degree of maturity. The first 

 question which will be asked regarding this difference in nodule develop- 

 ment will be whether it does not correspond more or less approximately 

 with corresponding differences in root or shoot development. Does it 

 show a trend distinctly different from that of other portions of the 

 plant ? 



When the figures given in this table are presented in graphs, the 

 differences in trend become obvious. A comparison of the weights of 

 the nodules with those of the roots (fig. i, 2) will show that in both 



/s 



/s 



2/ 24 27 30 



33 



36 <^ 



Fig. 2. — Comparison of dry weights of tops, roots, and nodules of soybeans given in Table III, grown 



during April and May. 



series the maximum development of nodules occurs at 24° C, with 

 very slight development at the extremes, 15° and 36°. Root develop- 

 ment, on the other hand, rises much more rapidly at the lower temper- 

 atures and is maintained at the the higher temperatures, reaching a 

 maximum in the second series at a point 9° higher than that of the 

 nodules. Root development is far more uniform at all temperatures 

 than is nodule development. 



A comparison of nodule development with shoot development (fig.i, 2) 

 shows that the effect of temperature upon the development of the two 

 structures is quite different. As with root development, shoot develop- 

 ment is relatively more vigorous at 15° and 18° C. than is nodule develop- 

 ment, which increases greatly at 21° and reaches a maximum at 24°. 

 "When at 27° the v/eight of nodules is beginning to diminish, that of 

 shoots maintains its level or increases. Through the higher temperatures 

 weight of nodules falls off rapidly, while that of shoots remains at the 

 high level. 



