The Supposed Reversal of Geotropic Response 287 
The plants continued from one to four days in carbon dioxide 
atmospheres. It frequently happened in the concentrations of 12 to 
20 per cent, that the shoots grew long and then, after having made 
an upward bend, swung laterally by their weight, twisting the axis 
and so bringing the shoot into a pendent position. 
Series IV. With ammonia vapour. In the attempt to surround the 
roots with an alkaline atmosphere, ammonia vapour was used—the 
same means as employed by Small, but by a different method. To 
start with, one-twelfth normal ammonium hydrate was prepared. 
From this solution were made 0-5, 0*25, 0-12, 0*06 and 0-03 per cent, 
of the twelfth normal solution. The seedlings used were those of 
Zea mais and Vicia fab a. Sixty of Zea and 52 of Vida were success¬ 
fully carried through the tests, after favourable strengths of vapour 
were determined. The seedling roots were 1 to 3 cm. long when the 
seedlings were fastened to wooden bars and placed in desiccators 
made into damp chambers. The tubulature of the desiccator was in 
the top and was closed with a rubber stopper through which passed 
a glass tube for the supply of ammonia. The glass tube was, of 
course, kept closed except when introducing the ammonia. Red 
litmus paper was suspended in each damp chamber as an indicator. 
When the seedlings were first suspended in the damp chambers 
5 c.cm. of the ammonia solution was inserted, and the same amount 
was introduced every five or six hours thereafter to the end of the 
experiment. 
In the vapour from the 0-5 per cent, ammonia, the roots did not 
grow but soon became brown and died. In vapour from 0-25 per cent, 
the roots grew but a few millimetres, some remaining straight, and 
some becoming twisted and distorted, bending upward, downward, 
and in all directions. After 24 hours in this strength of ammonia, 
the roots were brown and dead. In the chambers with 0-12, 0-06 
and 0-03 per cent., the growth of roots was good, and all roots showed 
positively geotropic curves. The majority of seedlings were kept in 
vapour from 0-12 per cent, ammonia, as this was the greatest strength 
in which a fair amount of growth could be made. 
It would seem that the conclusion to be drawn from the examina¬ 
tion of the work of Small and his associates, and from the results of 
similar experiments recorded in the foregoing pages, is that there is as 
yet no evidence for the reversal of geotropic response by such means 
as those used by those investigators. The conclusions reached by 
these authors seem to the writer to have been due to misinterpreta¬ 
tions of distortion figures and to the behaviour of sagging shoots, 
too old and too weak to hold themselves erect. 
