288 Brenchley . — On the Action of Certain Compounds of 
tend to be very slightly below that of the control, though with 1/20,000,000 
zinc sulphate the plants are rather above the normal in weight. No 
evidence of stimulation of peas was obtained with any strength of zinc 
sulphate down to a limit of 1/200,000,000. 
Arsenic. 
Arsenic is one of the most interesting of the inorganic plant poisons. 
Inasmuch as very small quantities of arsenic are used in medical practice 
as stimulants, one is naturally led to wonder whether the action on plant 
protoplasm is parallel to that on animal protoplasm. Much work has 
been done with arsenic, but naturally enough the toxic action has claimed 
most attention, as it is by far the more obvious. Certain discrepancies 
occur in the results obtained, but further investigations make it probable 
that these are due to the fact that the common compounds of arsenic are 
derived from two distinct acids, arsenious (H 2 As 2 0 4 ) and arsenic (H 2 As 2 0 6 ), 
which differ in their action upon plant protoplasm. These two acids, with 
their compounds sodium arsenite and sodium arsenate, have been studied, 
chiefly in their action on peas and barley. 
Arsenious Acid. 
Some of the earlier experiments at Rothamsted were made with 
arsenious and arsenic acid instead of with their compounds. 1 Arsenious 
anhydride is not very soluble in water, so considerable care and much time 
were needed in making up the stock solutions of the poison. 
Barley. Barley is very susceptible to the action of arsenious acid, 
1/50,000 As 2 0 3 killing the plants without allowing any preliminary growth. 
Little growth occurs in any concentration down to 1/100,000 or 1/250,000 
As 2 0 3 . Below 1/100,000 As 2 0 3 ‘Pedigree Plumage’ barley (grown in June 
and July, 1910) improved considerably in growth, while with ‘ English 
Archer ’ (grown from March to May, 19 11) the increase in dry weight did 
not set in until concentrations below 1/250,000 As 2 0 3 were reached. The 
toxic action rapidly increases with the concentration, but there is evidence 
to show that a slight depression is still caused by as little as 1/10,000,000 
As 2 0 3 , though the growth of the plants with this strength approaches the 
normal very closely (Curve 6). According to the dry weights no stimula- 
tion is induced even with 1/50,000,000 As 2 O a , but if stimulation is judged 
by the appearance of the green plants, then concentrations of arsenious acid 
from 1/1,000,000 to 1/5,000,000 cause improvement in growth. The curious 
point is that certain of the plants with poison appear to be much stronger 
than the controls, whereas the dry weights of the same plants fall considerably 
below those of the controls. 
1 In actual practice one always deals with arsenious and arsenic anhydride (As 2 O s , As 2 0 6 ) 
which on solution in water give the corresponding acids. 
