298 Brenchley . — On the Action of Certain Compounds of 
(1) Variation at different periods of the same year. 
(2) Variation in different years. 
As a general rule plants are able to withstand a higher concentration of 
poison in the summer months than in the early spring and late autumn. 
It is quite conceivable that the limit of toxicity is lower at the latter times 
than in the late spring and summer. As the concentration of the poison 
approaches the indifferent point, the balance between the toxic action of 
the external agent and the normal functioning of the plant itself becomes 
more finely adjusted. If the plant is growing vigorously, as it is in 
summer — the optimum growing period — then its vitality is sufficient to 
enable it to successfully withstand rather more of the poison than it can 
endure in the early spring and autumn, when growth is more sluggish and 
life activity is at a lower ebb. 
As a similar variation is observed between plants grown at the same 
period in different years, it may be deduced that physiological factors, such 
as temperature and light, play their part in influencing the degree of toxicity 
of a given poison. 
Table showing the Highest Indifferent Strengths 
of Poison in Different Series of Plants. 
Zinc Sulphate. 
Boric Acid. 
Barley. 
Peas. 
Barley. 
Peas. 
1910 
March-May 
1/20,000,000 
t) 
May- June 
June-Aug. 
1/10,000,000 
1/5,000,000 
1 /5o,ooo 
>> 
Aug.-Oct. 
1/ 1 00,000 
1911 
March- April 
1 / 5,000,000 
» 
March-May 
1/25,000,000 
May-June 
1/250,000 
1/25,000 
1912 
Sept.-Dee. 
1/20,000,000 
1/100,000 
1913 
March-June 
1/100,000 
5} 
5’ 
April-May 
April-June 
1 /5, 000, 000 
1/10,000,000 
Arsenious Acid. 
Arsenic 
Acid. 
I9IO 
June-July 
1/10,000,000 
1/10,000,000 
Still toxic 
Still toxic 
I9II 
>> 
Feb.- April 
March-May 
1/50,000,000 
1 /5Qo,ooo 
Still toxic 
Sodium A r senile. 
Sodium Arsenate. 
1911 
March-May 
1/50,000,000 
Still toxic 
>> 
May-July 
June-Aug. 
1/50,000,000 
Still toxic 
** 
Aug.-Sept. 
1/2,500,000 
1/25,000,000 
x 9 l 3 
Jan. -March 
1/100,000,000 
Still toxic 
>5 
1 / 1, 000, 000 
Feb. -April 
1/2 50,000,000 
Still toxic 
