Chap, ix Nutrition of Heterotrophic Plants 439 



in 1896 and 1899, and by Rosenberg in the latter 

 year. 



A certain reserve was maintained by the earlier writers 

 as to the advantages accruing to the plants, but this was 

 dispelled by the investigations of Francis Darwin in 1878 

 and of Biisgen in 1883. Both writers proved the absorbed 

 products to be nutritive by a detailed comparison of the 

 development of two sets of cultivated specimens, one of 

 which was fed with insects, chiefly aphides, while the other 

 received no other nutriment than was supplied by the 

 air and soil. Similar, though less complete, researches on 

 this point were carried out by Von Raumer in 1878. 



The acid of the digestive secretion was said by Rees and 

 Will, in 1875, to be formic acid. These authors found 

 that the digestive constituent of the secretion can be 

 extracted from the leaves by glycerin, but that it is not 

 active until it is acidified. 



Drosophyllum, another member of the Droseraceae, was 

 examined by C. Darwin and found to agree in all essential 

 respects with Drosera. It was again the subject of a care- 

 ful research by De Wevre in 1895. 



Another allied plant to which considerable attention was 

 directed is Dionaea muscipula, the Venus' fly trap. Its 

 habit of catching flies was known to Ellis in 1768 and cer- 

 tain of its peculiarities were mentioned by several writers 

 prior to 1800, while Curtis in 1834 published an account 

 of its behaviour which was very complete for the time at 

 which he wrote. During our period its peculiarities were 

 first examined by Nitschke in i860. Mrs. Treat's researches 

 in 1871-3 were also concerned with Dionaea, but the most 

 accurate information we possess was afforded by the investi- 

 gations of Charles Darwin, published in 1875. Mention 

 should be made also of the work of Canby in 186S, in which 

 we have an account of the process of absorption by the leaf 

 following the actual digestion. 



