266 Hooker.—Hydrotropism in Roots of Lupimis alhns. 
fibres acted in exactly the reverse manner. Thus roots bent down and 
stems up. 
Half a century later, Bonnet ( 1 , p. 272) recorded an instance of 
hydrotropism when he admired the movement of roots which follow the 
undulations of a wet sponge. 
Duhamel ( 9 , vol. i, p. 86 ; vol. ii, pp. 137-45) was led to believe 
-that large bodies of water might influence the direction assumed by the 
roots of near-by trees. To decide this, he made experiments. He placed 
an acorn upside down between two sponges, and again in pipes filled with 
earth and laid at various angles. As the radicle grew down and the 
plumule up in every case, Duhamel concluded that moisture had no effect 
on the directions assumed by the root and shoot. 
Just a century after Dodart, Erasmus Darwin ( 5 , p. 144) wrote: ‘The 
plumula is stimulated by air into action and elongates itself where it is most 
excited, and the radicle is stimulated by moisture and elongates where it is 
most excited.’ 
The following year, Lefebure (21, p. 50) demonstrated nicely the 
existence of hydrotropism, although he did not realize it. A moist sponge 
was placed over some seeds in a nutshell, which was then inverted. The 
roots grew down and soon reached the air, whereupon they turned back 
into the sponge. 
Knight ( 20 ), in an article read before the Royal Society ten years 
later, gave the first complete proof of hydrotropism in roots. Vicia Faba 
seeds were half embedded in the mould of inverted flower-pots. The 
radicles of the germinating seedlings had earth above and air below them. 
When the mould was kept moderately moist, the radicles extended hori¬ 
zontally along the under surface of the mould and sent side rootlets up into 
it. When the earth was supersaturated with moisture, the roots grew 
straight down into the air beneath. Knight held to the physical explanation 
of Dodart and E. Darwin, and emphatically denied the existence of 
anything resembling sensation or intellect in plants. 
In endeavouring to test E. Darwin’s statement, Keith ( 19 ) substantially 
repeated Duhamel’s experiment. He used a kidney bean and a grain of 
wheat planted upside down in a glass tube. Although the radicles assumed 
a horizontal position after descending perpendicularly into the open air 
below the mould in the tube, Keith passed over this fact, because germina¬ 
tion was then past. He merely wished to show that the primary cause of 
the descent of the radicle at germination was not sensitiveness to moisture. 
In 1824 Dutrochet ( 10 , pp. 59-60) made several experiments, including 
a repetition of Keith’s, with Phaseolus multiflorus. He distinguished care¬ 
fully between mass and moisture. In one experiment he fastened beans to 
the roof of an excavation where the earth above them was several metres 
thick. As the roots bent down, he concluded that mass of itself had no 
