III. TOXIC EXUDATES AND SOIL TOXINS. 



Early views. The early observations upon root excretions were 

 made at a time when physiological knowledge was still of the vaguest, 

 and are merely of historical interest. The first mention of root 

 excretion seems to have been that of Hales (1727), who assumed that 

 albumen as well as carbon dioxid was secreted by roots. Duhamel 

 (1755) noted that the earth about the roots of old elm trees was 

 darker and more greasy than usual, and concluded that this was the 

 result of root secretion. Brugmans (1786) thought to see small drops 

 exuded from the ends of roots of Viola arvensis grown in sand, and 

 assumed that this peculiar sap was injurious to neighboring plants. 

 He ascribed to Lolium temulentum the power of corroding the roots 

 of nearby plants, and decided that it was a specific excretion that 

 made Cirsium arvense so harmful to oats, Euphorbia peplus and 

 Scabiosa arvensis to flax, Erigeron acris to wheat, Spergula arvensis 

 to buckwheat, and Inula helenium to carrots. 



Senebier (1791) and Cotta (1806) supposed that roots excreted a 

 substance which often accumulated to the point of bringing about the 

 decomposition of bulbs in the soil, and Simon (cf . linger 1836) thought 

 that the roots of hyacinth served only for purposes of excretion. 

 Plenck (1795) believed that plants excreted refuse more or less after 

 the manner of animals, as shown by the drops exuded at night through 

 the "openings" of the roots. He regarded this "excrement" as partly 

 useful, partly injurious to the plant itself, as well as to its neighbors. 



Sprengel (1812) regarded the moisture about the roots of grasses as 

 an excretion, and thought that it aided in increasing the fertility of 

 dune-sand, a view supported by the observations of E. Meyer (1830). 

 John (1819) found that the malic acid of hyacinth bulbs was ex- 

 creted into a solution, where it converted sodium carbonate into 

 sodium malate. 



DeCandolle (1832 : 248) regarded the excretions of roots as of 

 great importance in their economy, and believed that cockscomb and 

 other weeds injured adjoining plants in consequence of harmful root 

 secretions. He followed Humboldt and Plenck in assuming that 

 such secretions were the basis of the supposed attraction and repul- 

 sion of plants as expressed in plant communities. He also ascribed 

 the benefits of crop rotation to root excretions, on the assumption 

 that the excretions of one crop would be harmful to the same crop, 

 but harmless or even beneficial to a different one. He supported 

 his conclusions by the results of Macaire (1832), who thought to 

 demonstrate that the roots of grains, grasses, and other plants 

 excreted gummy substances, calcium carbonate, etc. These excre- 

 tions were assumed to free the plant from substances that could not 



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