388 BOTANICAL GAZETTE [MAY 
tip in the third period also would indicate that the stoppage of 
growth is not due to the toxicity of the solution. The dextrose 
may penetrate too slowly to furnish sufficient carbohydrate to the 
root cells for continued growth. This would not appear to be a 
feasible explanation, however, because the early growth of the root 
tip is rapid, and the decrease in growth appears progressively 
greater. The early growth must occur at the expense of glucose 
which penetrates the root cells, because the root tips furnished with 
no glucose in the nutrient solution grew very little. It would seem 
that if the rate of penetration of the glucose determines the stoppage 
of growth one would have to assume a continuously increasing 
difficulty of penetration, terminating in entire impermeability of 
the root cells to glucose. The limited oxygen supply in the solution 
cultures may account for the growth stoppage. Here again, 
however, no better supply of oxygen is supplied in the first period 
than in the later periods, and yet the growth in the first period is 
the most rapid. If the limited oxygen supply is the factor which 
eventually causes the stoppage of growth, it must be a cumulative 
effect, due either to the development of deleterious materials or 
failure to synthesize some necessary material. It should also be 
noted that, although the aeration of water cultures of entire corn 
plants favors root and top development (ANDREWS and BEALS 1), 
the roots of an entire plant in unaerated water cultures do not show 
the stoppage of growth evident with the excised roots in solution 
cultures. It would seem reasonable to assume, therefore, as a 
working hypothesis from the experiments described, that oxygen, 
the mineral salts of Pfeffer’s solution, glucose, and water are insuf- 
ficient for the continued growth of excised corn roots. 
Summary 
1. A simple method of growing the isolated meristematic tissue 
of higher plants, excised root tips and stem tips, under sterile 
conditions is described. 
2. The excised root tips of peas, corn, and cotton make consider- 
able growth in the dark in solution cultures containing mineral 
salts and glucose or levulose. 
