270 Hooker.—Hydrotropism in Roots of Lupinus albus. 
The same year Rothert ( 26 ) published a critical study of the literature 
on the function of the root-tip. He pointed out that Darwin’s theory was 
as yet unproved. The converse of Molisch’s experiment would determine 
the point, however. This had been done in Pfefifer’s laboratory, but as no 
details were given definite conclusions could not be drawn in view of numerous 
possible sources of error. 
In 1901 Steyer ( 31 ) thoroughly investigated the hydrotropism of 
Phycornyces nitens. He found that the sporangiophores were both 
negatively and positively hydrotropic according to the percentage of 
relative moisture. Steyer was unable to find any foundation for Elfving’s 
c physiological action at a distance ’. Older sporangiophores were found to 
be more sensitive than younger ones, and exposure to light lessened their 
sensitivity. 
Voechting ( 34 , pp. 98-102) in 1902 experimented with potato sprouts 
and decided that they were hydrotropic. The following year Singer ( 29 ) 
showed that Voechting’s results were probably due to impure laboratory 
air, and that potato sprouts were not hydrotropic. 
Sperlich ( 30 ) in 1908 observed that the stolon of Nephrolepis was 
positively hydrotropic, thus enabling it to reach a moist substratum in 
spite of the absence of geotropism. 
The last contribution to the literature on hydrotropism was made by 
Jost and Stoppel ( 18 , p. 210) in 1912, and dealt with the limitation of 
hydrotropic sensitivity to the root-tip. Of eighteen decapitated Lupinus 
albus roots arranged a few millimetres from a wet filter-paper, thirteen 
bent to the paper. This indicates that although the strongest hydrotropic 
sensitivity resides in the tip, it is not confined to it. 
Up to the present time hydrotropism has been found in the sporangio¬ 
phores of various fungi, in the rhizoids of hepatics, in roots and pollen 
tubes, and in rare cases in the hypocotyl of the spermatophytes. 
Experimental Part. 
1. Method. The simplest contrivance for obtaining hydrotropic 
bending in roots was constructed in the following manner: A glass plate 
covered on both sides with wet filter-paper was inserted in a small rectangular 
glass jar parallel with the longer sides. Roots were fastened to pins which 
pierced strips of cork at regular intervals. Two strips were laid across the 
top of the jar on either side of the glass plate and parallel with it. The 
roots thus brought into the jar were exposed on one side to the moisture 
of the filter-paper, and their distance from the paper could readily be 
controlled. The bottom of the jar was covered with water to keep the 
filter-paper wet and to prevent the roots from drying out. The jar was 
left open that the air in it might not become saturated and so prevent 
