Apr., 1921] 
DUNN — HISTOLOGY OF GRAIN ROOTS 
211 
an explanation of the difference observed in winter, when the temperature 
was relatively low and when growth, respiration, etc., were not as rapid 
as in summer. It appears as if the higher temperatures of summer, with 
higher respiratory activity, etc., may have rendered the better aerated, 
solid-medium cultures more like those in liquid media. 
Several other experiments, not here described, seemed to support the 
idea that temperatures that gave slow growth in soil cultures retarded or 
prevented the development of the cortical openings, while temperatures 
that gave rapid growth resulted in numerous well developed openings. 
This suggestion seems to be along somewhat the same line as that expressed 
by Laetitia M. Snow^ regarding the formation of gas cavities in the stems 
of Scirpus. 
It is to be noted, however, that the development of cortical openings in 
corn roots frequently occurs after the root tissue has ceased to elongate. 
These openings may form, apparently, either in the elongating region or in 
the older part of the root where elongation has stopped. 
One other experiment is worthy of mention here. On June 12 two soil 
cultures of white dent corn were prepared, one being placed in the green- 
house (higher temperatures) and the other in the open (lower temperatures). 
At the end of 8 days the greenhouse culture had roots about 26 cm. long, 
with large cortical openings (as in fig. 3) throughout the older portion, to 
within about 5 cm. from the tip. The openings in the upper part of the 
root were larger than those nearer the tip. At the same time the out-door 
culture had roots only about 18 cm. long, with all the pronounced openings 
(as in fig. 3) confined to the 3 cm. just below the seed. In the remainder 
of the root occurred only very small openings, merely intercellular spaces 
such as those shown in figure 4. 
General Conclusion. It appears that the cortical openings considered 
in this note generally appear in solution cultures of both Marquis wheat 
and white dent corn, under greenhouse conditions of both winter and sum- 
mer. They were not observed in the sand or in the soil cultures of wheat 
in either winter or summer, but none of these cultures were continued for 
more than thirteen days. The time of appearance of the cortical openings 
in sand or soil cultures of the white dent corn seems to vary with the rate 
of growth of the roots. 
Washington, D. C. 
2 Snow, Laetitia M. Diaphragms of water plants. II. Effect of certain factors upon 
development of air chambers and diaphragms. Bot. Gaz. 69: 297-317. 1920. 
