268 Hooker.—Hydrotropism in Roots of Lnpinus albus. 
of the sieve-bottom nearest it. Sachs mentions several less satisfactory 
substitutes for the sieve, such as peat bricks, plaster of Paris plates, sponges, 
or bags of moist earth. His attempt to explain hydrotropism as caused by 
thermotropism failed (see 16 , pp. 135-6), so he was forced to conclude 
that the bending was produced directly by the moisture difference. 
Duchatre’s and Ciesielski’s results he considered to be produced by altera¬ 
tions in the turgidity of the roots. Shoots were found to be hydrotropically 
insensitive, for they grew straight out of the sawdust in the tilted sieves. 
Van Tieghem ( 33 ) in 1876 observed the formation of arcades by the 
stolons which give rise to sporangiophores in Afrsidia, and attributed their 
bending to 4 somatropism ’ or the attraction of mass. He found stolons of 
Circinella , Mortierella , Mucor , Pilobolus y and Phycornyces all positively 
somatropic. Three years later Sachs ( 28 , p. 224) showed Van Tieghem’s 
somatropism to be untenable. He suggested that the vegetative hyphae of 
Phycomyces and Mucor were positively, and their sporangiophores negatively, 
hydrotropic. 
Charles Darwin ( 4 , pp. 180-6) held that the hydrotropic sensitivity 
of radicles resided in their tips. He used Sachs’ method for obtaining 
the bending with seedlings of Phaseohis, Vicia , Avena , and Triticum . To 
check the bending, from one to two millimetres of the root-tip were coated 
with a mixture of olive oil and lamp-black. Of fifty-nine roots so treated, 
twenty-nine remained straight. One millimetre of the tip was cauterized 
with silver nitrate in other experiments. Of twenty cauterized roots, eleven 
remained straight after twenty-four hours. Darwin wrote that ‘ a greater 
number of experiments than those which were actually tried would have 
been necessary had it not been clearly established that the tip of the radicle 
is the part which is sensitive to various other irritants ’. 
Wiesner ( 35 , pp. 130-4) objected to Darwin’s method, on the ground 
that the roots were in an abnormal condition. His own experiments with 
decapitated roots led him to the conclusion that hydrotropic sensitivity was 
not confined to the root-tip. The term ‘ hydrotropism ’ appears here for the 
first time in the literature. 
Detlefsen (6, pp. 646-7) also criticized Darwin’s method and results. 
An experiment with six decapitated pea seedlings, four of which reacted, 
convinced him that the entire growing region of the root perceived the 
stimulus. 
Mer ( 22 ) offered a ‘ more natural ’ explanation for hydrotropism. After 
detailed observations on germinating lentil seedlings, he decided that roots 
were pulled to one side by the attachment of root-hairs, and that they 
possessed no special hydrotropic faculty. 
The same year Wortmann ( 36 , pp. 368-74) showed that Phycomyces 
nitens sporangiophores were negatively hydrotropic. A sporangiophore 
projecting through a hole in a glass plate was found to bend away from 
