280 Hooker.—Hydrotropism in Roots of Lupimts albus . 
divided longitudinally down the middle, the two halves become concave on 
the inside and bend away from one another, when a drop of water is placed 
on them. This experiment shows that a region of negative tension is 
present in the centre of the growing root, and that it is surrounded by 
a zone of positive tension. In other words, the young root is in a condition 
of unstable equilibrium. When the positive tension is reduced on one side 
by evaporation, the root bends. 
It has been shown that if roots reacted solely to mechanical forces 
resulting from a moisture difference, they would bend negatively. That 
they bent positively .can only be explained by assuming the presence 
of a vital factor, which must be powerful enough to overcome the mechanical 
factor. The moisture difference produces a difference in the osmotic pres¬ 
sure of the cells on the opposite sides of the root. It is natural to conclude 
that this difference may cause the bending. If an increased osmotic 
pressure acts as a stimulus to growth, the explanation of hydrotropism 
is simple. Moreover, it is known that the growth of certain sea Algae 
is inhibited by transferring them from salt to fresh water. 1 Here the 
decrease in osmotic pressure acts as a stimulus and retards or stops growth. 
The data in Table II would indicate that this is not the case in roots, but 
as the roots in these experiments had no means of obtaining water, which is 
essential for growth, no conclusions may be drawn from them in regard to 
this question. 
In the following experiments roots were exposed to air of various 
degrees of dryness, in order to produce a stimulus by increasing the osmotic 
pressure through evaporation. The roots were then placed in moist 
sawdust, and the amount of growth compared with controls. The amount 
of relative moisture in the air was determined by using sulphuric acid 
solutions as in Table II. 
Experiment I. Forty-seven roots were exposed for four hours in 
an atmosphere of 85 per cent, relative moisture. After five hours in moist 
sawdust, the amount of growth was measured and found to average 3 mm. 
Forty-eight controls were placed for four hours in a saturated atmosphere. 
The average growth for five hours in moist sawdust was 6*5 mm. 
Experiment II . Ninety-six roots were exposed for 4J hours to 90 per 
cent, relative moisture. They averaged for four hours’ growth in sawdust 
3*6 mm. Eighty roots, after being the same length of time in a saturated 
atmosphere, averaged 3-7 mm. for four hours’ growth in moist sawdust. 
Eighty-five roots that had been germinated in the sawdust were measured 
and replaced. They averaged 3-3 mm. for four hours’ growth. 
Experiment III. Forty-eight roots were placed for one hour in 
a chamber having a relative moisture of 90 per cent. Their average growth 
for the following five hours measured 5-3 mm. Forty-eight controls exposed 
1 Jost’s Pflanzen-Physiologie, p. 348. Dritte Auflage. Jena, 1914. 
