94 PLANT PHYSIOLOGY 
Still other bacteria inhabiting sulphur springs or places 
where sewage is abundant obtain the necessary energy 
by oxidizing H.S to SOs, 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 photosynthesis 
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, ete. 
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: 
malic, (C4H.O;) found in the apple and many other 
fruits as well as in the leaves of many succulent plants, 
citric (CsH;07) in the fruits of lemon, orange, etc., 
tartaric (CsH»O¢) in fruit of grapes, oxalic (C2H20u.) 
in the leaves of many plants, e.g. Oxalis, Rumex, etc., 
and tannic acid (Cy4H00¢) and its derivatives which ap- 
pear to play a very important but little understood part 
in the energy relations of the plant. Many of these 
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