92 PLANT PHYSIOLOGY 



true that all these benefits do accrue to the alga though 

 this is very far from being demonstrated but even if this be 

 true, are these benefits needed, are they not superfluous? 

 Man's association with his domesticated animals is bene- 

 ficial to them, but are these animals really any better off 

 than by themselves in nature? Even if man's association 

 with them is beneficial for the time, in the end it is fatal or, 

 at least, onerous. The absolute dependence of the fungus 

 component of the lichen upon some green plant for food, 

 and the damage and death to the alga wrought by the 

 fungus, furnish the strongest evidence that the association 

 is not equally beneficial to the two members, mutually bene- 

 ficial though it is sometimes claimed to be. 



In these cases, we have examples of the stages through 

 which parasitism has advanced first, green plants, incom- 

 pletely and only periodically parasitic; second, plants nor- 

 mally not green, permanently parasitic, and completely 

 parasitic when the host is suitable; third, plants never 

 green, permanently and completely parasitic. 



The bacteria living in the bodies of animals, either inter- 

 cellularly or intracellularly, absorbing already elaborated 

 foods, utilizing these by processes fundamentally like those 

 already discussed, present no new physiological principles, 

 and hence those who would know more of this, as of the 

 other special groups of organisms which we have just been 

 discussing, must turn to the treatises devoted to them. 



OTHER ELEMENTS ESSENTIAL TO PLANTS 



The physiological chemistry of the other elements con- 

 cerned in the nutrition of plants is still so vague that little 

 can be said about them till further investigations give 

 definite facts to deal with. These elements are obtained in 

 analyses of plants in the form of incombustible compounds 

 forming the ash resulting from combustion. For this reason 

 they are collectively termed ash-constituents, to distinguish 

 them from hydrogen, oxygen, carbon, and nitrogen, which 

 go off in drying and burning. Besides the salts containing 

 the elements absolutely essential to the normal nutrition of 

 the plant, others are invariably present in the ash because 



