Hooker.—Hydrotropism in Roots of Lupinus albus . 279 
towards the source of moisture, and in the other away from the side 
wounded. Molisch ( 24 ) considered that the roots bent away from the dry¬ 
ness, rather than towards the moisture, and so came to the conclusion that 
hydrotropism was merely a special kind of traumatropism. That this 
conception is fallacious is evident by careful examination of the data 
given. Hydrotropic reactions are obtained best between 90 and 100 
per cent, of relative moisture ; they cease slightly above 80 per cent. ; 
while roots become noticeably injured from dryness below 80 per cent. If 
roots are injured, they bend away from the source of moisture, because the 
turgidity of the exposed side is lost. It is therefore not possible to main¬ 
tain that hydrotropism is due to reaction from injury. Molisch based his 
idea on experiments made in an atmosphere of 72 per cent, relative moisture. 
Without doubt many of the roots sustained injuries. 
To gain a correct conception of the nature of hydrotropic reaction 
in roots, it will be necessary to analyse the factors involved. First will be 
considered the mechanical effects resulting from exposing a root to a 
moisture difference in the air. As the root contains more water than the 
surrounding atmosphere, evaporation will result. This loss of water will 
produce increased osmotic pressure and decreased turgidity of the root-cells. 
The length of the cell decreases with the lessening of the turgidity, just as 
it does before plasmolysis. Since evaporation is greater from one side than 
from the other, the drier side will shorten more, and the root will bend 
away from the source of moisture. This reaction was obtained experi¬ 
mentally by pressing seedlings against the vertical surface of an agar block. 
The air contained 50 per cent, relative moisture. In fifteen minutes 
11 out of 20 root-tips had bent away from the block, and in thirty minutes 
more all but two had reacted. The controls placed in a saturated atmo¬ 
sphere remained straight. The negative bending described in Experi¬ 
ment IV was of this nature. 
But it is still a question if this mechanical tendency to bend negatively 
is present under those circumstances which produce positive hydrotropic 
reaction. In order to eliminate the vital factor, which will be discussed 
presently, roots were anaesthetized by exposing them twenty minutes 
over 3 per cent, ether water. The roots were then exposed to a hydro¬ 
tropic stimulus before a moist filter-paper. Of the 25 roots exposed, 
12 bent away from the paper within an hour, and 10 more within four 
hours. Sixteen out of 20 controls reacted hydrotropically. The exposure 
to ether water rendered the roots hydrotropically insensitive, without 
stopping their growth. This experiment shows that even those minimal 
differences of moisture content which induce hydrotropic bending produce 
a tendency for the root to bend negatively. This is readily explained 
by considering the mechanical tensions normally present behind the root- 
tips. If a thin radial section of a root, taken 5 to 8 mm. from the tip, is 
