1922] ROBBINS—ROOT TIPS 61 
Pfeffer’s solution, oxygen, and water are insufficient for continued 
root growth, the natural place to look for the materials lacking is 
the grain or the plant. Extracts of the young embryos including 
both roots and tops, and of young seedlings a week or ten days old 
did not benefit the excised root tips. In fact, these extracts used 
in the proportion of the extract of one embryo or seedling to one 
root tip showed a slight injurious effect upon the growth of the root. 
Fic. 1.—Corn root tips originally about 2 cm. long: ( 1) excised root tip in Pfeffer’s 
solution plus 1 per cent agar; (2) excised root tip in Pfeffer’s solution plus 1 per cent 
agar and 2 per cent glucose; (3) root tip of root attached to grain in Pfeffer’s solution 
plus 2 per cent agar. 
Neither did creatinine, gylcocoll, or asparagin used at the concen- 
tration of 100 ppm, 79 ppm, and 50 ppm respectively produce any 
better growth. The first two substances were slightly injurious, 
the last one no better than the check. Two experiments in which 
the Ca(NO,), and KNO, of Pfeffer’s solution were replaced by 
CaCl, and KCl, and in which the roots were carried through three 
periods of culture, showed that the lack of nitrate somewhat 
decreased the total amount of growth in length instead of increasing 
it, as generally occurs in water cultures lacking nitrogen when the 
