126 
IOWA ACADEMY OF SCIENCE 
had been kept free from burs for two years, was in oats the first year and a 
thin stand of clover the second year, was 45 per cent better for clover growing. 
The soil from plot 208, given clean culture, corn the first year and oats the 
second, was 22 per cent better. The soil from plot 308, given clean culture and 
in corn both years, was 18 per cent better. 
The burs in one soil produced a gain of 64 per cent. In this case the stand 
was light and as the gain was calculated per plant, it gave the burs an advan- 
tage. In the other soil the gain was 33 per cent. This shows that the burs 
did nourish the clover, but not to the full extent of the plant food found in 
them. They had practically twice the plant food that the manure had, but the 
gain was much smaller than that produced by the manure, which was 105 per 
cent. Either the plant food in the burs was not available, or the burs con- 
tained a poison which counteracted much of the plant food action of the 
material. 
This pot method is somewhat crude and results obtained from it are not 
entirely satisfactory. The trouble is partly that results are based on weights 
of clover several months old. The burs are decidedly toxic to clover at germ- 
inating time, but after the plant is once started, the bur has a much smaller 
effect on it. Field observations established the fact that clover and cockleburs 
may grow side by side, if not in harmony, at least with tolerance-. 
The effect of the burs on the germinating clover is shown by the following 
notes: On October 25th it was noted in the series just described that the clover 
was coming very good and fairly even in all pots except in 236 and 237, which 
had none sprouted, and in 242 and 243 which had six and five respectively. 
These pots were the ones treated with cockleburs. Pots 236 and 237 were 
replanted with ten seeds each. This was six days after the clover was first 
planted. Five days later, pots 242 and 243 had several plants each which had 
come through the soil but the cotyledons ware burnt off. Three days later all 
these pots treated with burs had additional seeds planted in them to supply 
the lack of stand. 
In spite of this extra seeding it is seen by the table that only one of the 
four pots had a full stand. Just about one-fourth of the seeds planted in the 
pots treated with .5 per cent of burs succeeded in producing clover plants. 
In the other pots, from 90 per cent to 95 per cent of the seeds produced 
vigorous plants. The burs undoubtedly hindered the germination of the 
clover by poisoning it. 
The nature of the poison in the burs was not ascertained. They were 
tested for alkaloids but none were found. 
Work in physiological chemistry during the last two decades has been char- 
acterized, by a thorough and systematic study of solutions from both the theore- 
tical and experimental points, of view. The discoveries made in this work 
have been a great help in giving an insight into some of the problems of plant 
physiology. A vdde field of research along physiological lines opened up by 
applying to the field of biology the dissociation theory that has proved so 
fertile in chemistry and physics. A desire for a deeper and clearer insight 
into the subject of the nutrition of plants has lead many botanical investigators 
to study the poisonous or stimulative qualities of a large number of com- 
pounds. The literature on this subject is very large. These men have devel- 
oped a method of research, which, although the matter of detail varies con- 
siderably, in the main features may be briefly described as follows: Seeds of 
