82 
Skene. — The Acidity of Sphagnum and its 
chalk cultures. So that, though his acidity values are not characteristic 
for the species, this does not invalidate, but rather enhances the value 
of his conclusions as to the connexion between acidity and sensitiveness. 
The lack of accuracy of his method of determining acidity is, however, 
a serious objection. 
When we turn to the determination of the fatal dose of chalk, we find 
that it is a matter of great difficulty. This is at once evident on reading 
Paul’s description of the behaviour of almost any species in different con¬ 
centrations of chalk solution. The damage increases with concentration, 
but quite gradually ; this applies to the decrease in the amount of growth, 
the discoloration, and even to the inhibition of the geotropic reaction. The 
effect on growth is exemplified in Table VII, and the other symptoms may 
be well seen in such cultures of recurvum. Even in the highest concentra¬ 
tions the tips may remain fresh, green, and alive. Besides the species 
mentioned, papillosum, subsecundum , squarrosum, and cymbifolium were 
examined, but only in the case of rubellum could anything like a sharp 
limit be obtained. The conclusion is inevitable that the designation 
of any particular concentration as initiating fatal damage must be largely 
arbitrary. 
As already mentioned, Paul states his chalk as amount per ioo grm. 
dry weight of Sphagnum , instead of as concentration of the solution 
employed. To test the validity of the assumption that it is the actual 
amount supplied, and not the concentration of the solution employed, that 
is determinative, two sets of recurvum were grown, one in 2,000 c.c., the 
other in 200; the chalk present in each was the same—200 mg., so that 
only the concentration was different. The first set showed a growth of 
37 per cent., a strong geotropic curvature, and a fairly healthy colour ; the 
second did not grow at all, showed no curvature, and was quite dead. 
From this it follows that it is the high concentration that is effective. 
Paul shows that even in the case of rubellum the amount of chalk 
supplied must be sufficient to neutralize the acid compounds before 
damage sets in. Were such small quantities of solution employed that the 
amount of chalk therein was not sufficient to neutralize the Sphagnum 
employed, then amount would enter as a factor. When, as in my experi¬ 
ments, an excess is present even in the dilute solutions, then only concentration 
will come into play. Paul’s paper gives no clue as to the amount of solution 
he employed. 
That contortum and recurvum are sensitive to different degrees is clear, 
but in the Acidity Table no less than four species lie between these two; 
it will be understood that to demonstrate a difference in sensitiveness 
between neighbouring members of that series of six would be a matter of 
very considerable difficulty. 
We must conclude that while the correlation which Paul finds can be 
