SCHREINER AND REED: EXCRETIONS BY ROOTS 281 
ment the effect of a good absorbing, agent was tried. Ferric 
hydrate was added to five pots of “exhausted” sand in which 
wheat was planted. This good absorbing agent renovated the 
sand to such an extent that the growth of wheat in it was only 6 
per cent. less than that in the fresh sand. It would be obviously 
incorrect to assume that the decreased growth of wheat in the 
sand was due to the depletion of nutrients. Neither can one 
maintain that the beneficial action of ferric hydrate consisted in 
supplying nutrients. The only explanation is that the ferric 
hydrate had a strong absorbent action upon some substance, or 
substances, which were toxic to the second growth of wheat and 
Which had resulted from the growth of the first crop. 
Facts like those cited above give indications that the toxic 
condition of unproductive and “exhausted” soils may be caused 
by some substances excreted from the roots of plants. These 
indications are further supported by the results of all experiments 
upon the nature of the toxic substances. Up to the present time 
they all indicate that the toxic substances are organic, and not inor- 
ganic, bodies. It has been shown, for example, that the extract 
from a poor soil may often be benefited by incineration or mere 
charring of the organic matter present in the residue obtained by 
‘vaporating the soil extract, and redissolving it in pure water. 
hen plants are grown in such solutions they show marked im- 
Provement over those grown in the original extract. In an experi- 
ment described in Bulletin 28 of the Bureau of Soils (p. 29 ff.), 24 
Wheat plants grown in such a solution showed an increase of 25 
Per cent. in growth over a control in the original extract. Here 
there has apparently been a destruction or an alteration of the 
Substances which were previously detrimental to the growth of 
plants, 
Turning to the literature, it will be seen that there are numerous 
instances in which deleterious excretions have been demonstrated 
M the lower plants, but in the higher plants there appear to be 
No definite proofs that such excretions are produced. 
Among the earlier botanists there prevailed an idea that roots 
“xcreted waste matters. Such ideas were a logical outgrowth of 
the efforts of their time to correlate the structures and functions 
of plants with those of animals. Brugmans (’89) alleged that he 
