1922] ROBBINS—ROOT TIPS 73 
increases in length for one of these roots, no. 32, for the five periods 
were as follows: 11.2, 6.5, 4.9, 4.5, and 1.3 cm., and the secondary 
root production was 58, 12, 3, 4, and o. For the other, no. 39, 
the increases in length were 13.0, 11.5, 8.3, 6.9, and 5.4 cm., and the 
secondary root production was 82, 51, 25,16,and 17. These figures 
show a continuous decrease in length. They also emphasize the 
fact, which has been observed continuously in this work, that the 
root tips act consistently as individuals period after period. A root 
tip which develops well in the first and second periods usually con- 
tinues to show a superior growth in the later periods. 
TABLE V 
APPROXIMATE ACTUAL MATERIAL, ASH, OR ORGANIC MATERIAL 
N AUTOLIZED YEAST EXTRACT ey Cpe URE 
SOLUTIONS IN EXPERIMENTS 12 AN 
Concentrations of | Total dry 
autolized yeast matter added 
in culture solutions! Total dry Ash ( ) Organic rs 
— as _|matter (ppm)| ‘*S” ‘PP™m (ppm) ts rl 
bott rot ‘dese ESE Sori (gm.) 
TO. a4 0.5 3.0 0.00016 
40... 13.5 2.0 LIcS ; 5 
Boe fe) 29.6 4.0 23.0 0.0013 
200. . 67.0 10.5 56.5 0.0032 
400. . 134.0 Bi 113.0 0.0067 
800. . 268.0 42.0 226.0 0.0134 
The difference in the effect of the various concentrations of yeast 
was not great, and appeared most sharply in the later periods. 
Thus, so far as length is concerned, the gain in 80 ppm of yeast was 
as great in the second and in the third period as in any of the other 
concentrations. The secondary root production and dry weight 
produced, however, were greater in the higher yeast concentrations 
even in the second period. It was not until the fifth period that 
200 ppm of yeast showed its inferiority to 400 ppm; 800 ppm of 
yeast apparently was somewhat too concentrated. The addition 
of 200 ppm of a colloidal material like gelatine to the culture solu- 
tion did not favorably affect the development of the roots. It 
should be noted that root hairs were found on the root tips in the 
gelatine solution at the end of the third period. Excised roots which 
did not have their root tips severed and transferred at intervals 
