1922] ROBBINS—ROOT TIPS 67 
continued to grow. They also grew in the fourth period, and of 
seven root tips transferred from the solutions containing autolized 
yeast, one grew from an original length of 1.8 cm. to 2.6 cm. in the 
fifth period. 
It can be noted in table III and in fig. 4, where the average gains 
in length of the root tips in this experiment are represented graphi- 
cally, that there is a continued decrease in the amount of gain in 
length in each successive period in both the yeast and peptone 
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Fic. 4.—Growth in length of excised corn root tips in dark for five periods; solu- 
tions dot were modified Pfeffer’s solution plus 2 per cent glucose (check), same solution 
plus approximately 200 ppm or 400 ppm peptone (p 200, p 400) or 200 ppm of autolized 
yeast (Y 200); numbers within circles represent number of root tips still growing in 
each period in yeast extract. 
solution. The decrease which occurs in the first period is the most 
marked. The same holds true for the production of secondary roots 
and for the dry weight produced in each period. The decrease is 
most marked in the dry weight. The dry matter produced in the 
second period, even in the presence of yeast or peptone, is only ;'; 
or ;'y of that produced in the first period. A consideration of the 
data of both experiments shows that autolized yeast is superior in its 
effect to peptone. 
An observation made in this experiment as well as generally in 
others should be recorded. The root tips of some of the excised 
