SYMBIOSIS 269 



vincingly traces out the whole process. The normal plant 

 obtains its food from inorganic material. But when opportunity- 

 offers it easily lapses into a condition in which it takes the 

 material for metabolism ready made from the decay of others 

 and becomes saprophytic. Ward shows that it is only a step to 

 the attack on the living, and for the saprophyte to become a 

 parasite, and he further shows that it can be readily educated to 

 be so. He does not hesitate to suggest that the function of 

 conidia in the complicated cycle of fungal reproduction is to 

 form the cellulose-dissolving ferment. But now and again the 

 host does not succumb to its invader. A truce is sometimes called 

 in the struggle, and host and parasite are content to live together 

 in a mutually advantageous symbiosis or commensalism. 



Three years earlier, in 1887, Ward's attention had been 

 drawn by a happy accident to the physiological aspect of symbi- 

 osis, and it never ceased to occupy his mind. It was well known 

 that ginger-beer was made in villages in stone bottles. The 

 fermentation was effected by the so-called " ginger-beer plant " 

 which was passed on from family to family, but nothing was 

 known as to how or where it originated. It seemed to have 

 some analogy with the Kephir of the Caucasus. A specimen 

 was sent to me from the Eastern Counties, and it stood for 

 some time in the sun in my study. I noticed the vigorous 

 growth accompanied by a copious evolution of gas. Ward 

 coming to see me one day, I handed it over to him as a problem 

 worth his attention. At the same time Prof Bayley Balfour 

 had examined it and concluded that it was a mixture of a yeast 

 and a bacterium. Its study involved Ward in a very laborious 

 research which occupied him for some years, and of which the 

 results were published in \hQ Phil. Trans, in 1892. It proved to 

 be a mixture of very various organisms, every one of which 

 Ward exhaustively studied. This required not less than 2000 

 separate cultures. The essential components proved to be, as 

 Balfour had suggested, a yeast derived from the sugar and a 

 bacterium from the ginger. Both were anaerobic; the yeast 

 fermented cane-sugar with the copious production of carbon 

 dioxide but little alcohol; the bacterium also produced carbon 

 dioxide, even in a vacuum tube. 



