on the Growth of Barley and Wheat in Water Cultures . 83 
Second Series. 
Seeds sown, March 27. 
Plants put in solutions, April 5. 
Plants harvested, May 24 (Plate II, Figs. 1, 2, 3), 
Solutions changed 4 frequently at intervals of four days. 
Solutions changed 4 once ’, on April 30. 
The course of events was the same as in the first series, though growth 
was more rapid owing to the more favourable season for growth, and 
differences due to the varying concentrations were more marked than in the 
earlier experiment (Curve 2). The thick rootlets in 4 frequently changed ’ 
plants were less strongly developed. It seems probable that the frequent 
renewal of the nutrient salts caused the plants to put out the abnormal 
rootlets for some unexplained reason, particularly at the time of year when 
growth was fairly slow. When growth was more rapid, the root development 
remained more normal in type, though thick rootlets did appear to some 
extent. Later on in the year they 
were only produced by plants in r*~s 
high concentrations, the others 
bearing quite normal fibrous roots. 
It may very tentatively be sug- 
gested that the thickened rootlets 
provide a means of protection at 
certain periods of growth against 
the constant change of balance due 
to the frequent renewal of the food 
solution. As they are so badly 
provided with laterals, it may be 
that they are able to prevent the 
ingress of too great and sudden an 
influx of food material at the time 
of the change of solution, so that 
they act as a kind of control. 
When growth is more rapid, the 
plant can deal with extra food Curve 2. Mean dry weights of ten barley-plants 
... . . growing in nutrient solutions of different concen- 
more readily, SO that the con- tvations. Dotted lines show limits of probable error. 
trolling function is of less impor- (April 5-May 24.) 
tance, and the thick roots are cor- 
respondingly less developed. This idea is also borne out by the fact that 
in the first series the N-plants produced very thick rootlets in quantity, 
rendering the root thick and much less fibrous than usual, thus indicating 
possibly that at that time of year the plant was never able fully to cope with 
such a constant renewal of food solution of high concentration, owing to the 
relative slowness of growth which entailed the utilization of a lesser quantity 
of plant food (see Table I). 
