1922] GERICKE—ROOT SYSTEMS 209 
of approximately equal value, having increased approximately five 
times in weight during the six weeks’ period of growth. This test 
shows that these three solutions, markedly different in composition, 
must be considered as of equal value as media for the growth of 
wheat seedlings five weeks old having large root systems, when 
placed in the solutions and grown for six weeks. These same solu- 
tions, however, must also be considered markedly different physio- 
logically, when the test plants are wheat seedlings 6-8 cm. high 
with small roots 8-10 cm. long. 
All the cultures with large roots may be considered as having 
been injured by the treatment of the first five weeks’ growth in 
tap water, because they fell short in attaining the maximum 
growth rate obtained by the cultures with small roots (class B) in 
solution no. 1. Whether any treatment that can induce large root 
growth of a wheat plant, either at the expense of top growth or not, 
can subsequently be made to operate as a means to secure a greater 
growth rate for the plant as a whole, because of a greater root sur- 
face exposed for absorption, needs further investigation. It is 
probable that in the present experiment the treatment to obtain - 
large roots was too severe, and that exposure for a less time to the con- 
ditions by which large roots were obtained would have given a larger 
measure of growth when set into these solutions than was obtained. 
The extent of the root system appears as an important factor 
that affects the magnitude of growth obtainable from a given nutrient 
solution. It is conceivable, therefore, that extent of the root systems 
of plants plays an important réle when plants are grown in the field. 
That some plants are more resistant than others to certain untoward 
conditions, such as excessive amounts of salts in alkali regions, may 
not be due to any peculiar genetic factor of the plant, but simply be 
the response from differences in root development occasioned by 
certain conditions in the environment in the field. The common 
Observance in the field of a greater tolerance for salts of older 
plants than young ones apparently can well be accounted for in 
their root systems. This, however, does not mean that differences 
in extent of root systems any given kind of plant may have are due 
only to causes operative in the external environment. Differences 
in root systems may also be due to genetic factors. 
AGRICULTURAL EXPERIMENT STATION 
UNIVERSITY OF CALIFORNIA 
