94 PLANT PHYSIOLOGY 



Still other bacteria inhabiting sulphur springs or places 

 where sewage is abundant obtain the necessary energy 

 by oxidizing US to SO2, sulphur frequently being stored 

 up as a reserve food supply. It is held by some investi- 

 gators that other bacteria obtain their energy by oxi- 

 dizing certain iron compounds, others by oxidizing 

 methane and still others hydrogen. 



142. In the foregoing processes of photosj^nthesis 

 and respiration (including fermentation) many other 

 substances are produced besides those mentioned. Some 

 of these are perhaps nothing more than waste products, 

 or at least by-products, but others are reserve food of 

 various kinds. Still others perhaps serve for special 

 functions such as protection of plants from attacks of 

 insects, covering of wounds, etc. Among the substances 

 thus produced and whose functions are not certainly 

 known, are the alkaloids of which a great many have been 

 studied, e.g. caffein, nicotine, etc. Besides these may be 

 mentioned resins, rubber, gutta-percha, glucosides, etc. 

 Many of these are of great use to man. Many are very 

 poisonous. The organic acids mostly stand in another 

 category. They are either directly reserve stuffs, re- 

 placing carbohydrates, or are stages in the respiration 

 of carbohydrates, or in many cases are the substances 

 which produce the requisite osmotic pressure within the 

 cell. The commonest organic acids are the following: 

 maUc, (C4H6O5) found in the apple and many other 

 fruits as well as in the leaves of many succulent plants, 

 citric (CeHsO?) in the fruits of lemon, orange, etc., 

 tartaric (C4H6O6) in fruit of grapes, oxalic (C2H2O4) 

 in the leaves of many plants, e.g. Oxalis, Rumex, etc., 

 and tannic acid (C14H10O9) and its derivatives which ap- 

 pear to play a very important but little understood part 

 in the energy relations of the plant. ]\Iany of these 



