Orchid Mycorrhiza. J. Ramsbottom. 57 
Weevers* working from a chemical point of view on the 
presence of ammonia and ammonium salts in plants established 
the fact that although ammonium salts were found in abundance 
in the tubercles of the Leguminosae they were in small quantity 
or absent in mycorrhizal plants. He holds therefore that if 
fungus-roots really assimilate nitrogen it must be brought about 
in a manner different from that in the Leguminosae. Weevers 
is rather of the opinion that mycotrophic plants are, with the 
help’ of their fungus partner, able to utilize fully the organic 
compounds of the soil. 
McDougall}, working with ectotrophic mycorrhizas of forest 
trees formed by the association of toadstools with the roots, 
considers that they are not in any sense symbiotic associations 
but must be considered as instances of parasitism by the fungi. 
Rexhausent studied ectotrophic mycorrhizas by the micro- 
chemical method. He considers that the fungus and the root 
together form an osmotic unit for the absorption of nutrient 
salts. These are probably made soluble for the root by the 
fungus. This gathering up of nutrient salts is first used by the 
fungus for its own benefit. The mycorrhiza is not a fixed sym- 
biotic condition, but is dependent upon the biological condition 
of the soil. Where the conditions are not suitable for the growth 
of the fungus it acts as a parasite on the root and may damage 
it severely, as it cannot be kept in check. Where the fungus is 
well nourished it can be easily withstood by the root. Thus in 
good soils the mycorrhiza gradually disappears or, at all events, 
the fungus part becomes less. 
It will be apparent from the above that many somewhat 
diverse theories have been put forward to account for the 
fungus-root association and many modifications have been pro- 
posed. No purpose would be served here by entering on a de- 
tailed criticism: the only general one we would suggest is that 
no benefit can result from pushing the old idea of mutual and 
equal advantages of the two components to its extreme. Re- 
ferring only to orchids it seems most reasonable to regard the 
condition as having arisen from parasitic attacks by the fungus. 
This seems beyond doubt in the exceptional case of Armillaria 
and Gastrodia. The ability of the fungus to transport nutrient 
solutions has been made use of by the flowering plant. As in the 
case of Leguminosae and their nodules the tables have been 
* T. Weevers, Das Vorkommen des Ammoniaks und der Ammonsalze in 
den Pflanzen. Recueil des Travaux botaniques Néerlandais, XIII, pp. 63-104 
(1916). 
+ W. B. McDougall, On the mycorrhizas of forest trees. American Journ. 
Bot. 1, pp. 51-74 (1914). 
+ L. Rexhausen, Uber die Bedeutung der ektotrophen Mykorrhiza fiir die 
hdheren Pflanzen. Beit. z. Biol. der Pflanzen, x1v, pp. 19-58 (1920). 
