ABSORPTION OF FOOD, AND EXCRETION. 497 



like wood-charcoal, absorb gases and moisture from the atmosphere, the 

 expression of the fact in the text seems natural and warranted. This 

 subject suggests a beautiful series of experiments for the purpose of 

 determining the facility possessed by the root-sheaths of absorbing gases 

 and vapour from the atmosphere, and introducing the same to the 

 roots. 



Some observations of the earlier experimentalists, and which are per- 

 fectly correct, but which were employed for the purpose of forming 

 theoretical views, founded upon the false analogy between animals and 

 plants, and have led to the complete doctrine of the excrements of plants, 

 demand the broadest treatment in the history of our science. They have, 

 however, been recently misused, even by Liebig. The historically 

 important points in this subject are as follows : Duhamel* was the 

 first to observe that the earth adhered to the spongioles of plants. 

 Brugmansf described a brown substance in the water in which roots 

 were growing. Brugmans and CoulonJ drew the inference from this, 

 and the well-known fact, that certain plants, as Oats, Cnicus arvensis, 

 Polygonun Fagopyrum, Spergula arvensis, &c., will not grow near each 

 other, that all plants give out from their roots an excretion, which is 

 injurious to certain other plants. This theory was variously opposed 

 and supported, without any new facts being supplied, when DeCandolle 

 suggested to Macaire Prinsep the performance of a new series of 

 experiments to test its value. But these experiments were performed so 

 regardless of securing the essential fact of such a theory, a sound vege- 

 tation, that they are entirely worthless. When plants are removed from 

 their natural soil, as in the experiments of M. Prinsep, the roots become 

 injured, and thus the water in which they are placed penetrates the 

 tissues, and necessarily a part of the juices of the sap must flow into the 

 water ; and when M. Prinsep adds, that no adulteration of the water 

 takes places if a branch cut off a plant be placed in water, this is so 

 evidently an error that we lose all confidence in his experiments. The 

 worthlessness of the experiment of Prinsep has been pointed out by Meyen 

 (Physiologic, vol. ii. p. 528.), Treviranus (Physiologic, vol. ii. p. 117.), 

 and Hugo Mohl. || On the other hand, the experiments of UngerJ. and 

 Welser^l", which were performed with all proper care and accuracy, gave 

 a perfectly negative result ; so that there can be no doubt that an excre- 

 tion from the root, such as that believed in by DeCandolle, Prinsep, and 

 Liebig, has no existence at all. 



It appears almost a necessary conclusion, that if we regard endosmose 

 as the cause of absorption, that excretion from the spongioles, although 

 only to a small amount, must still take place. Such excretion would 

 consist of imperfectly assimilated matters, and some salts, as the special 

 assimilated matters are so bound to the cell that an excretion of them 

 appears almost impossible, as they are never found on the outside of the 

 spongioles, which yet perform especially the function of absorption. 

 But perhaps the assumption of such an exosmose is unnecessary, for 



* Natural History of Trees, vol. i. p. 107. 



f Dissertatip de Lolio ejusdemque varia specie L. B. 1 785 



\ Dissertatio de mutata humorum indole, &c., p. 77. 



Memoires de la Societe de Geneve, vol. v. p. 287. 



|| Dr. J. Liebig's Yerhaltniss zur Pflanzenphysiologie. 



j. Ueber den Einfluss des Bodens, p. 147. 



1 Untersuchungen iiber die Wurzelausscheidung. Tubingen, 1838. 



K K 



