FUNGI AND MODERN AFFAIRS — RAMSBOTTOM 319 



trate for only a certain distance and are then held in check. An un- 

 infected tuber sends out a dropper and the process is repeated each 

 year until a tuber is formed which is too small to grow. An infected 

 tuber, however, sends out a dropper which produces an inflorescence 

 the following year. It is obvious that the orchid obtains nutriment 

 from the rhizomorphs, which presumably act as conducting strands. 

 It may be that in some such reversal of parasitism we have a clue to 

 the origin of the more typical mycorrhiza or fungus-root. Orchids 

 are the classical example of obligate symbiosis, the seeds not normally 

 germinating unless infected by the fungus present in certain cells of 

 the roots. In modern methods of commercial orchid growing, either 

 the seeds are infected artificially with the appropriate fungus, or its 

 action is replaced by sowing sterilized seeds on a medium containing 

 sugar. 



Forest trees have a layer of fungal hyphce surrounding many of 

 the absorbing rootlets, the fungi concerned being mainly of the toad- 

 stool type. Many perennial plants also have mycorrhizas, but it is 

 not yet certain which fungi are concerned: the usual appearance is 

 suggestive of Phycomycetes. Similar associations with fungi occur 

 throughout the plant kingdom. 



The association of fungus and alga has resulted in the large homo- 

 geneous class Lichens. 



Many insects have internal yeasts: indeed, these are supposed to 

 have played a definite part in the evolution of some insect groups. A 

 more obvious harnessing is that of the leaf-cutting ants of South 

 America which, as first described by Bates and amply confirmed since, 

 cultivate fungi of the toadstool type in their fungus gardens. An 

 association in which the fungus seems to be less in subjection is that of 

 bacteria and yeast which are active in various fermented drinks — 

 Mexican tibi, koumiss, kephir, leban, tea cider, ginger-beer plant — 

 some fermenting sugary liquids, others milk. 



In addition to these combined masses, which are usually in the form 

 of grains, man in his early history noted that fruit juices or other 

 sugary fluids underwent a change if left for some time: honeycomb 

 washings became mead, grape juice became wine. These much-appre- 

 ciated changes, the work of yeasts, were not to be left to blind chance, 

 and in the course of centuries the conditions controlling the changes 

 and finally the reason for them became known. Nowadays distillers, 

 brewers, many wine growers and cider manufacturers no longer rely 

 on some general supply or on casual wild yeasts, but maintain pure 

 cultures of special strains of the appropriate species which have proved 

 to give the best results in the conditions of production. Here we have 

 the breaking down of organic matter by fungi to give a desired result. 

 It must be stressed, however, that a fungus acts only in a certain way 

 in definite circumstances. 



