434 The Physiology of Plants BOOK in 



gradually shown to be capable of living and thriving in 

 the juices of the animal body. 



Observations like these needed explanation, which was 

 only gradually and slowly obtained. We may appreciate 

 the perplexity which was felt at the apparent impossibility 

 of reconciling them with the old hypothesis when we find 

 the most prominent of British botanists, in an address to 

 the British Association at Belfast in 1874, saying that 

 these plants ' seem to invert the order of Nature, and to 

 draw their nutriment in part at least from the animal 

 kingdom, which it is often held to be the function of the 

 vegetable kingdom to sustain '. l The trend of the thought 

 of the time was indicated in the words, ' though the pro- 

 cesses of plant nutrition are in general very different from 

 those of animal nutrition and involve very simple com- 

 pounds, yet the protoplasm of plants is not absolutely 

 prohibited from availing itself of food such as that by 

 which the protoplasm of animals is nourished.' 



As we have seen, further work showed that the old 

 hypothesis was erroneous, and that the processes of nutri- 

 tion, strictly so called, are alike in animal and plant. 

 Now that the true significance of the observations we have 

 referred to has been realized, they are seen to help materially 

 to the understanding of the actual nature of the nutritive 

 processes of the green plant, and to a discrimination 

 between the manufacture of food and the subsequent 

 processes of its assimilation. 



The various aspects of heterotrophic nutrition which 

 came prominently into notice during the years 1860-1900 

 were associated with symbiosis, metabiosis, saprophytism, 

 parasitism, and the capture and digestion of lowly forms 

 of animal life. 



The phenomena of symbiosis received a great deal of 

 attention. The part which it plays in connexion with the 

 1 Hooker, Brit. Ass. Reports, Belfast, 1874. 



