44 THE PHYSIOLOGY OF PLANTS 



about 0.64 lbs. of ammonia per acre, gypsum will absorb only 

 0.23 lbs., carbonate of lime 2.62 lbs., pure quartz 0.93 lbs., but 

 iron oxide 10 lbs. If, however, pure quartz is mixed with a 

 brown humus compound {idmin), it will absorb, by the addi- 

 tion of only I per cent, of ulmin, 1 4 lbs. ; by the addition of 

 5 per cent, 92 lbs. of ammonia. This demonstrates the indirect 

 influence of humus compounds. 



Although in artificial cultures in distilled water or sterilised 

 sand, plants thrive exceedingly well when supplied with in- 

 organic food substances only, and a proof has therefore been 

 given that these plants can grow without any decomposing 

 organic substances, still the figures just quoted indicate that 

 the organic substances of humus are of considerable value in 

 the nutritive processes. In certain cases humus is absolutely 

 necessary. To the plants which live entirely on organic food 

 matter belong all the fungi, which take their nourishment 

 either from living organisms {parasites) or from decaying 

 organic matter {saprophytes). Besides these cryptogamic or 

 flowerless plants, many more highly organised plants afford 

 us examples of plants which must necessarily be supplied 

 with organic food matter. The mistletoe illustrates the para- 

 sitic habit on a living host plant, while decaying organic food 

 matter is essential for the nutrition of the humus-growing 

 plants which have either very little chlorophyll in their tissues 

 or are completely devoid of it. Examples of these are the 

 Yellow Bird's-nest {Monotropa Hypopitys), and the yellowish- 

 brown Orchids {Neottia nidus avis, EpipogiLiii aphyllum, and 

 Corallorhiza innata). In connection with the mistletoe we 

 must not omit to mention those green plants, rich in chloro- 

 phyll, which are able to lead an independent existence, growing 

 and flowering in the soil, but which can on occasion further 

 their nutrition by absorbing the sap of other plants. To 

 these belong the well-known inhabitants of moorland meadows, 

 such as the Eyebright {Euphrasia), the Yellow Eattle {Rhinan- 

 thus), and the Cow-wheat {Melampyrum). The roots of these 

 plants, if they come into contact with other roots, develop 

 small suckers, which abstract substances from the roots 

 of the host plant. The desire to supplement the normal 

 nutrition by a parasitic absorption of organic nitrogenous 



