16.6 Espino: Salt Requirements of Young Rice Plants 477 
the time is ripe for it, continuous renewal will surely be re- 
sorted to. With the present state of knowledge and technic 
it may be remarked that the solutions in culture bottles are 
probably always in a state of convection, so that the solution 
next to the roots is probably never very different from that 
at a greater distance. It may therefore be supposed that the 
culture solution alters as a whole and at a much lower rate 
than would be the case close to the roots if convection were 
prevented (as by the presence of sand, agar, etc.). Sufficiently 
rapid convection would amount to continuous renewal of the so- 
lution around any given root; and, if the volume of solution 
held in the culture bottle is great enough (or if the alteration 
in the solution, caused by the activity of the roots, is slow 
enough), then the rate of alteration becomes insignificant, so 
that the solution about the roots may be considered as being 
practically maintained constant as to its properties. It has 
frequently been mentioned in the literature that, if continuous 
and rather rapid renewal of culture solutions is not employed, 
the desideratum is more nearly approached with greater volume 
of the solution (size of culture jar) and greater frequency of 
renewal. In the present studies 250 cubic centimeters of so- 
lution were always employed for six plants, and solution renewal 
occurred at intervals of three or four days after the first five 
days, as has been said. The experimental conditions would 
of course have been more nearly ideal with a larger volume 
and with more frequent renewal. It may well be that the 
nature of the results obtained would have been insignificantly 
different had the conditions been more nearly ideal, but the re- 
sults presented below are to be regarded as applicable only 
with reference to the general method here employed. With more 
time and more work other and farther-reaching results might 
surely be obtained, along this line as well as along other lines. 
A chemical condition that is more or less definitely effective 
within the liquid media of solution cultures, but that is seldom 
or never adequately dealt with in this kind of work, is the 
oxygen supply of the root systems. It is clear that most roots 
require free oxygen — ^that they are aerobic — and there is 
reason to believe that rice belongs in this class. It may be 
conjectured, however, that the roots of young rice plants do 
“ Free, E. E., The effect of aeration on the growth of buckwheat in 
water-cultures, Johns Hopkins Univ. Cir. (March, 1917) 198 and 199. 
“ Nagai, I., Some studies on the germination of the seed of Oryza sativa, 
Journ. Coll. Agr. Imp. Univ., Tokyo 3 (1916) 109-158. 
