Oct. 1, 1921 Effect of Soil Temperature on Development of Nodules 2 1 



ment and size. In fact, in some instances, volume of nodular tissue 

 seemed in inverse ratio to number. With most legumes it is not easy to 

 get an accurate dry-weight determination of small nodules, because these 

 occur as swellings so closely attached to the root that it is hardly feasible 

 to separate them from the true root tissue. The one species tried which 

 gave least trouble from this source by reason of the distinct separation 

 of its nodules from the root, even at early stages of development, was 

 the soybean. For this reason, it alone was used in the final series recorded 

 here. 



Of course it was soon realized during the progress of the work that vol- 

 ume of production of nodules was only an easily observed index, significant 

 chiefly in so far as it revealed important eff"ects of temperature upon the 

 physiological processes which are dependent upon the nodular structures. 

 It would be of greater interest, for example, to measure the amount of 

 nitrogen fixed in these nodules produced at different temperatures and 

 that portion which becomes available to the plant for use in its vital pro- 

 cesses. The demonstration of an important limiting effect of temperature 

 upon nitrogen fixation would be of no inconsiderable importance. Such 

 an effect would probably be indicated by large differences in size of 

 nodules, though it might occur without the appearance of such difference. 

 In any case it seems easily possible to determine approximately the effi- 

 ciency of nodules in the fixation of atmospheric nitrogen by growing 

 parallel series of plants, inoculated and uninoculated, in the same kind 

 of soil and at the same soil temperatures. If the amount of nitrogen in 

 the inoculated plants (aside from that foimd in the nodules on those 

 plants) is greater than in the uninoculated, the gain must be credited to 

 the efficiency of the nodules. The gains thus found should be an accurate 

 measure of the effect of soil temperature upon the fixation of available 

 nitrogen in the legume used in the experiment, and a comparison of this 

 gain with the weights of the nodules found on the inoculated plants 

 should give an approximate idea of the relation existing between effici- 

 ency of fixation of available nitrogen and volume of nodules. In the 

 last series recorded an attempt was made to carry out this experiment 

 with the soybean plant. Unfortunately some of the uninoculated plants 

 in the series became inoculated during the experiment and developed a 

 few nodules, thus making it necessary to discard the data so far as these 

 controls are concerned. Thus an exact determination of the extent of 

 the effect of soil temperature upon the fixation of nitrogen in the nodules 

 of legumes remains to be made. For the present we can only ascertain 

 the dry weights of the nodules themselves as they are found at the end 

 of a period of time and determine the amount of nitrogen found within 

 them. 



