1913] RIGG—EFFECT OF BOG WATERS 325 
to low temperatures only does not seem to be settled. Evidently 
extremely low temperatures must be reckoned with as one of the 
factors that determine the characteristics of these plants in past 
ages, and the same is true of bog plants growing in the extreme 
north in post-glacial times. We certainly are not justified in 
concluding that bog conditions as they exist today in temperate 
regions are the cause of xerophily in bog plants. There does seem 
to be ground for the belief that certain plants having hairless roots 
and other xerophilous structures are able to live in bogs, while 
other plants that normally have root hairs and possess in general 
a mesophytic or tropophytic structure are kept out of the bogs by 
these toxins. 
It now seems well established that the inhibition from undrained 
bogs of plants other than xerophytes is not caused by acidity as such 
(H ions) (Livincston 7), nor by low osmotic pressure (LIVINGSTON 
6), and that it cannot be correlated with low temperatures or strong 
drying winds (DAcHNowSsKI 5), or directly with lack of aeration. 
Although the toxic effect of bog waters does disappear with con- 
tinued aeration (DACHNOWSKI 2), it seems evident that the presence 
of air destroys the toxic substances that are present in bog water, 
and that the mere absence of air from water does not render it 
toxic. The fact that DacuNnowskI (5) found that the toxic effect 
of bog water can be removed by filtering it through agricultural 
soils and that the toxic effect was then present in the soil used as 
a filter seems to settle the point. Whether the toxic effect of bog 
waters is due to one substance or to several we do not know. Nor 
do we know positively that it is always due to the same substance 
or mixture of substances. Undoubtedly the toxic substances are 
organic, and the problems of organic analysis involved are beyond 
us at present. 
What the source of the toxin (or toxins) is we do not know 
definitely. There seem to be at least three possible sources: 
(a) excretion products coming into the substratum from plants 
growing in the bog, (b) products resulting from decay in the absence 
of oxygen, (c) excretion products of bacteria. Since it is probable 
that many other fungi are associated with the bacteria in bogs, 
it seems scarcely possible to distinguish sharply between (a) and (c). 
