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76 BOTANICAL GAZETTE [SEPTEMBER 
growth in 10 ppm of autolized yeast in the second period compares 
very favorably with that in 4o ppm or 80 ppm of yeast in the 
same experiment. In the third period the development in the 
solution containing 1o ppm of autolized yeast was better than in 
either 40 or 80 ppm of yeast. If, however, we compare it with the 
previous experiment in which a larger number of root tips were 
grown in 80 ppm yeast, we can see that the development was con- 
siderably less in to ppm than in 80 ppm of yeast. This substan- 
tiates the general expression of the effect of autolized yeast in these 
experiments, namely, that in the first period it has no beneficial 
effect, in the second period there is little difference between the 
effects of 10, 40, 80, 200, 400, and 800 ppm; but the later the period 
the more evident does the beneficial action of the higher concentra- 
tions appear. 
Discussion 
While excised corn root tips which are grown in the dark for 
about two weeks in Pfeffer’s solution plus 2 per cent glucose, and 
which have their root tips cut off and transferred to fresh solutions 
at intervals of two weeks, show a continued decrease in the rate of 
growth and stop growth in the third period, the addition of small 
amounts of peptone or autolized yeast permits them to grow for as 
long as six periods before growth ceases. 
Two possible explanations for the action of the peptone and 
autolized yeast suggest themselves: (1) the autolized yeast or the 
peptone supplies something which the root requires for its continued 
growth and which is not included in the salts of Pfeffer’s solution, 
water, glucose, and free oxygen; (2) the autolized yeast or peptone 
balances the solution, performing a function which could be accom- 
plished by a readjustment of the relative quantities of the salts of 
Pfeffer’s solution or their equivalents, of water, of glucose, and of 
free oxygen. A definite decision between these two possibilities 
cannot be made from the data at hand. The majority of the results 
reported in this paper, however, would seem to be explained best on 
the assumption that the mineral salts of Pfeffer’s solution, glucose, 
free oxygen, and water are insufficient for the continued growth of 
the root cells of corn, and that this deficiency can partially be sup- 
plied by autolized yeast or peptone. 
