Hooker.—Hydrotropism in Roots of Lupinus albus. 269 
a wet piece of cardboard, but to grow straight by a dry piece. The 
mycelium was not found to be hydrotropic. 
In 1883 Molisch (24) published a comprehensive work on hydrotropism. 
He constructed the funnel apparatus for obtaining hydrotropic bendings. 
The projecting root-tips of seedlings placed on the top of the solid plaster 
of Paris funnel bend first down and then inward to the moist side of the 
funnel. Molisch demonstrated that hydrotropic bending resulted from 
unequal growth of the opposite sides of the root. Roots of Zea and Pisum 
seedlings were marked with ink every millimetre, and the region of hydro¬ 
tropic bending was thereby found to be the growing region. Reactions 
were inhibited below the minimum temperature for growth. Hydro- 
tropically bent roots were plasmolysed, and remained bent, showing that 
the reaction was not a phenomenon of turgidity. He observed that 
disturbances of the turgidity caused by a psychrometrical difference 
frequently bent the root away from the source of moisture. Molisch 
agreed with Darwin that the root-tip alone received the hydrotropic 
stimulus, and gave positive proof that 1 *5 mm. of the root-tip was sensitive. 
The part of the root above the tip was wrapped in wet tissue-paper, and 
the wrapping was shoved down as the root grew, so that more than 1-5 mm. 
was never exposed to the moisture difference. Molisch found side rootlets 
especially sensitive to hydrotropism. The rhizoids of Marchantia poly - 
morpha , Lunularia, and Fegatella were found to be positively hydrotropic; 
the sporangiophores of Mitcor and Coprinus and the hypocotyl of Linum 
usitatissimum were shown to be negatively hydrotropic. 
Elfving (12) in 1890 observed that sporangiophores of Phycornyces nitens 
bent towards pieces of iron and various other substances, and described the 
process as physiological action at a distance. Two years later Errera (14) 
pointed out that all of these substances were hygroscopic, so that the real 
cause of the bending was negative hydrotropism. He found that roots 
bent away from iron even in a saturated atmosphere, ‘ which shows that 
hydrotropism is not due, as generally admitted, to differences in the 
hygrometric state of the air. Hydrotropism itself is the bending of a plant 
towards a point, not where it will find a minimum or maximum of moisture, 
but where it will, within certain limits, transpire most or least.’ Elfving (13) 
replied in 1893 that potash, although hygroscopic, did not cause the 
sporangiophores to bend, and that various other inactive substances were 
rendered active by exposure to sunlight or to heat. Errera (15) wrote in 
1905 that although sure of the exactness of his former experiments, 
Elfving’s more recent experiments should be repeated in the light of our 
recent knowledge concerning the radiation of metals. 
In 1894 Miyoshi (23) demonstrated that pollen tubes of Epilobium 
angustifolium , Oenothera biennis , Oe. fructicosa , Digitalis grandiflora , and 
D. purpurea were positively hydrotropic. 
