1922] ROBBINS—ROOT TIPS AND STEM TIPS 387 
The maximum gain in the first period was that of a root tip 
originally 4.3 cm., which increased to 18.0 cm. and produced 
ninety-seven secondary roots. The maximum gain in the second 
period was that of a root tip 2.7 cm. long originally, which increased 
to 7.4 cm. and produced twenty-one secondary roots. In the third 
period only three out of thirteen roots showed any increase in length, 
the maximum being 0.9 cm. This experiment has been repeated 
many times, and many root tips of corn have been carried through 
the three periods of culture given. Five varieties of corn have 
been used: a dent variety from Alabama; Longfellow flint from 
New York (kindly furnished by Dr. J. K. Witson); Boone County 
White and Reed’s Yellow dent from Missouri; and Funk’s Yellow 
dent from Illinois. The stoppage of growth in the third period in 
Pfeffer’s solution plus 2 per cent glucose has occurred in every case. 
No single root tip thus far has made more than a slight amount 
of growth in the dark in the third period. 
Not only does a diminution of growth rate, and a reduction in 
the production of secondary roots ending in a cessation of growth 
in the third period take place in the course of these transfers, but 
the diameter of the root tip continually decreases, until in the 
third period its diameter is one-fourth or less that of the original 
root tip. When stoppage of growth takes place, however, the root 
tips may apparently be normal in macroscopic and microscopic 
appearance, showing vascular bundles and root hair development. 
The failure of an excised root to continue growth when repeated 
transfers of the root tip are made, at once suggests that the seedling 
root contains some material derived from the seed other 
glucose, the mineral salts of Pfeffer’s solution, water, and free 
oxygen which are necessary for continued growth and which the 
root cannot synthesize in the dark in solution cultures from the 
material supplied. Such material or materials would be fraction- 
ated by the continued transfers of the root tips. 
Other explanations may be suggested. The stoppage of growth 
may be due to an unbalanced condition of the nutrient solution 
in which the roots are grown. The root tips, however, at the 
time of growth stoppage may show no macroscopic evidence of 
injury, and the fact that root hairs may be present on the root 
