BOT.-VOL. II.] PEIRCE- ROOT-TUBERCLES. S^I 



merismatic quality, and that they form the bowl -shaped 

 layer or layers of meristem which continue the growth of 

 the tubercle (see fig. 7)- The infection threads grow out 

 toward the tip of the tubercle (fig. lo), but the meristem 

 continues to form new cells between itself and the cells 

 containing bacteria and infection threads. By the layer or 

 layers of uninfected daughter-cells the meristem protects 

 its own cells from infection. Perhaps because they escape 

 infection, they retain their ability to divide. If they can be 

 prevented from forming a sufficient number of daughter- 

 cells to enable them to escape infection, what will be the 



result? 



To answer this, and some other questions, I tried the 

 following experiment. I imbedded young tubercles on 

 growing roots of potted plants of Bur Clover in plaster of 

 Paris, according to the method devised by Pfeffer (1892) 

 and used by his pupils (Newcombe, 1894, Richter, 1894, 

 etc.). The roots were disturbed as little as possible and 

 were put back carefully in the soil as soon as the plaster 

 had been applied to the tubercles. The plants were grow- 

 ing in coarse sandy gravel, so that it was not difficult or 

 dangerous to lift out one or more roots when the soil was 

 well loosened by being soaked with water. After the lapse 

 of twenty-nine days, I again took up the roots, cutting them 

 off for careful examination. Two tubercles had been firmly 

 held at the base by the plaster, but had broken the casts 

 sufficiently at the tips to grow fresh and pink out of their 

 investments. New tubercles seemed to have been formed 

 since the roots were operated upon. One tubercle, which 

 had stayed in its cast, was taken out and sectioned by hand. 

 It was evidently dying. The bacteria in it seemed active, 

 but were fewer and much smaller than in unconfined tuber- 

 cles. There was no meristem; the cells of the tubercle 

 were in their definitive condition; there were smaller quan- 

 tities of starch in this than in ordinary tubercles. What 

 was true of this tubercle was equally true of others which 

 had failed to break and grow out of their casts. 



Tune 18, 1Q02. 

 (2) 



