176 BOTANICAL GAZETTE [MARCH 
which is detrimental to plant growth, but there have been various 
theories as to its nature. Some years ago Livincston (23) dis- 
covered that bog waters have an effect on the growth of algae 
which is quite comparable to the effect of various toxic agents. 
More recently DAcHNowsk1, following the lead suggested by 
ScCHREINER and REED, has been making a careful study of bog 
toxins (27, 28). On account of the poor drainage of bogs, there 
is no other habitat where root excretions would be more likely to 
remain. Year by year these excreta would accumulate, thus 
making the bog more and more unfitted for the development of 
ordinary hydrophytes; hence for a time the dominating bog plants 
would be those which would be able to withstand the acids and 
other deleterious excreta given off by the roots or produced sub- 
sequently by changes in the accumulating humus. However, 
when these bog xerophytes bring the humus level well above the 
water table, the deleterious plant products will be more and more 
oxidized, and ultimately there will be produced a soil of such char- 
acter that ordinary mesophytes may flourish in it. While there 
is much in this theory which still requires confirmation, it certainly 
accounts for most bog phenomena and is not controverted by any 
known facts. It is likely also that some of the accumulating soil 
compounds may be of importance in neutralizing deleterious 
inorganic or organic soil constituents. In any event, the study of 
soil toxins and of their varied relations to plants is one of the great 
fields of investigation for the future. 
d. Food.—Perhaps there are some who would have supposed 
that the chief significance of humus accumulation lies in the in- 
creased amount of plant food that thus is made available. Once 
it was supposed that the well known luxuriance of plants in humus 
is due to the large amount of plant food which it contains. Long 
ago this luxuriance was shown to be in the main due to other causes, 
but recent experiments have demonstrated that ordinary gree? 
plants are able to absorb certain foods (as glucose), and it may be 
that such plants actually utilize in this way some of the substances 
of the humus. It is likely that the increasing food supply in accumu 
lating humus is an important factor in the succession of the soil 
organisms, but as yet this subject has never been investigated. 
