202 SECTIONAL ADDRESSES. 
rhizome, to which the plant is periodically reduced, is only periodically 
attacked when fresh roots are formed. Bletilla, however, behaves in 
an exceptional manner. In other orchids (Ophrydee, Cattleyee, Cypri- 
pede@, &c.) the fungus is needed for germination, and the adult plant is 
fungus-free except when the orchid produces fresh roots. Therefore, 
in such cases symbiosis is intermittent. In higher orchids like the 
epiphytic Sarcanthinee the fungus is needed for germination, and, the 
roots being persistent, symbiosis is maintained continuously. Finally, 
in Neottia nidus-avis the symbiotic condition is maintained throughout 
the life-cycle of the orchid, the fungus being found in the roots, rhizome, 
and even in the flowers and seeds, and it is transmitted hereditarily. 
The activity or ‘virulence’ of Rhizoctonia, according to Bernard, 
diminishes when the fungus is kept apart from the orchid, being prac- 
tically lost after two or three years. An attenuated fungus regains its 
activity in a measure after a sojourn of some weeks in a young orchid 
plant; a full degree of activity under symbiotic conditions is, however, 
only regained slowly. 
The germination of orchids in the absence of fungi was successfully 
induced by Bernard through cultivating them in concentrated nutrient 
solutions of a kind that does not occur in nature; such solutions, more- 
over, except under carefully carried out experimental conditions, would 
be rapidly vitiated through serving as a medium for the multiplication 
of different micro-organisms. The effect of increasing the concentration 
of the solution, offered to plants reared without fungi, corresponds to 
that obtained by raising plants with fungi of increasing activity or 
‘virulence.’ It may be added here that when Rhizoctonia are cultivated 
on a medium containing saccharose and the substance of orchid tubers— 
namely, salop—they cause an increase in the molecular concentration of 
the medium. It is possible that the fungi, when associated with the 
orchids, bring about a similar increase in the molecular concentration 
of the sap of the invaded plant. 
The Origin of Tubers in Various Plants. 
The occurrence of endotrophic Mycorhiza in the roots of species of 
Solanum has been recorded by Janse (1897) for S. verbascifolium in 
Java, by Bernard (1909-11) for S. dulca-mara, by Mme. Bernard and 
Magrou (1911) for S. maglia collected in Chili, the last-named species 
having been regarded by Darwin as the wild type of S. tuberosum, our 
edible potato. 
Experimenting with the potato, Molliard (1907, 1920) found that 
tubers were not formed in aseptic cultures in a poor nutrient medium, 
and that raising the concentration of the sugar in the sap artificially (as 
with the radish) led to tuberisation; concentrating the culture-medium 
did not induce tubers. Magrou (1921) placed potato seeds in a poor 
soil and close to S. dulca-mara, which always contains fungi, and found 
that only when the fungus invaded the potato plant were tubers formed. 
Magrou also investigated tuberisation in Orobus tuberosus (Legu- 
minose) and in Mercurialis perennis (Euphorbiacee), and from his 
collective studies the following conclusions may he drawn :— 
