68 BOTANICAL GAZETTE [SEPTEMBER 
roots in any series always develop abnormally. The root tip be- 
comes clear and glassy, sinks to the bottom of the solution, and 
almost ceases growth. When this occurs in the early periods of an 
experiment the secondary roots become unusually long and well 
developed, frequently attaining a length three or four times that of 
the secondary roots of an excised root whose tip is developing nor- 
mally. The appearance of a root whose primary tip became abnor- 
mal and whose secondary roots show marked development is shown 
in fig. 5. A normal root is shown in fig. 6. The latter figure also 
illustrates the fact noted before that the secondary roots develop 
from the convex side of the main root. 
Effect of different concentrations of autolized yeast 
While autolized yeast and peptone exert a favorable influence 
on the continued growth of the excised root tips in solution cultures 
containing glucose and the mineral salts of Pfeffer’s solution, never- 
theless the increase in length and the production of secondary roots 
and dry matter continuously decrease, until eventually growth 
stops. If we are to determine the complete nutrient requirements 
of a root by the method used, it will be necessary to find a set of 
conditions in which the excised root tips will grow continuously and 
produce approximately the same increase in length, number of 
secondary roots, and dry matter period after period as the root tips 
are cut off and transferred. 
It was hoped that increasing the concentration of the favorable 
factor already found would accomplish this. Experiments were 
therefore carried out in which concentrations of autolized yeast vary- 
ing from 10 ppm to 800 ppm were used. The results of these experi- 
_ ments indicated that while 400 ppm were somewhat more favorable 
than any of the other concentrations used, there was no marked dif- 
ference in the effects of concentrations as low as 10 ppm and as high 
as 800 ppm so far as the growth in the early periods was concerned. 
In the later periods, however, the higher concentrations of yeast 
were more beneficial. Two experiments were performed dealing 
with this phase. The first extended from February 5 to April 16, 
the second from April 3 to May 17, 1921. 
