190 PART III. PHYSIOLOGY. [ 45 



however, being strictly parasitic upon them since they do not 

 destroy or injure them. This association of two distinct plants is 

 termed symbiosis. The best instance of it is afforded by the 

 Lichens, where a Fungus and an Alga are associated symbiotically. 



It is remarkable that certain plants which possess chlorophyll are never- 

 theless parasitic in habit; for instance, Viscum (the Mistletoe) which is 

 parasitic on various trees, Rhinanthus (the Eattle) and other Scrophu- 

 lariacete. also Thesium (Bastard Toad-flax,) which are attached to the roots 

 of other plants by their haustoria. The nutritive processes of these green 

 parasites are not yet fully understood, but it seems probable that they 

 absorb from their hosts the substances which they should normally obtain 

 from the soil, though in a somewhat modified form. 



The great majority of the saprophytes are Fungi, such as the various 

 Agarics which grow in the soil of woods (humus) which is formed by de- 

 cayed leaves and is rich in organic matter ; the Moulds and Yeasts which 

 grow in saccharine juices, or fruits, etc. ; and Saprolegnia which attacks 

 the corpses of animals. Some of these Fungi, notably the Yeasts and the 

 various kinds of Bacteria (Schizomycetes), are peculiar in that they not 

 only decompose the amount of organic substance which they require for 

 their nutrition, but they give rise to widespread decompositions which are 

 known as fermentation and putrefaction. Amongst the higher plants there 

 are many saprophytes which grow in soils rich in humus : they may 

 be almost destitute of chlorophyll (e.g. Monotropa ; Neottia and some 

 other Orchids) : or they may possess it in considerable quantity 

 (e.g. some Orchids ; Pyrola ; Ericaceae), in which case they are probably 

 only partially saprophytic ; plants of this kind grow mostly in the leaf- 

 soil of forests, or in peat on moors. 



Hydrogen. The hydrogen of the plant is mainly absorbed in the 

 form of water (H.,0), but it may also be absorbed in combination 

 with nitrogen as ammonia-compounds (NH 3 ), and also in combin- 

 ation with carbon when complex carbon-compounds are absorbed by 

 the plant. 



Oxygen is absorbed in combination with carbon, as C0 2 , and 

 with hydrogen, as H.,0, and in many of the inorganic salts of the 

 food, such as sulphates, phosphates and nitrates, as well as in more 

 complex carbon-compounds. Oxygen is also absorbed uncombined, 

 in connexion with the catabolic processes, in respiration. 



Nitrogen, which is an essential constituent of proteid substances, 

 is only exceptionally assimilated in the free form ; although it is 

 present in large quantities in the atmosphere, most plants perish 

 if the soil in which they grow contains no compounds of nitrogen. 

 Nitrates and compounds of ammonia are widely distributed, and it 

 is in this form that nitrogen is mainly taken up by plants ; it seems 



