98 PLANT PHYSIOLOGY 



as sulphurous oxide and dioxide, gases poured forth in con- 

 siderable quantities from chimneys in which inferior sorts of 

 coal are burned.* A few species of bacteria use as food- 

 material, as well as their source of energy by respiration 

 (see page 20), the sulphuretted hydrogen and metallic 

 sulphur occurring in considerable quantities in mineral 

 springs and set free in decompositions taking place in the 

 ooze under bodies of water. 



Sulphur occurs in plants in the elementary condition only 

 in well-fed sulphur bacteria. In these, as in all other plants, 

 it occurs also as a constituent of protoplasm. A few plants, 

 notably the Crucifersa, contain sulphur in mustard oil, and 

 it is a constituent of the garlic oil in the various species of 

 AUmm. 



Nothing whatever is known of the stages through which 

 the amides are elaborated into protoplasmic matters by the 

 addition of sulphur and phosphorus. 



Potassium, although not a constituent element of proto- 

 plasmic substances, is an indispensable food-constituent. 

 The compounds of sodium, frequently much more abundant, 

 fail to serve as complete substitutes, although in the pres- 

 ence of an abundance of sodium salts plants demand less 

 potassium than otherwise. In the total dry weight of plants 

 potassium occurs in much larger amount than sulphur and 

 phosphorus — t 



Potassium salts are most abundant in young and growing 

 parts, least abundant in those which have ceased to grow 

 or to be otherwise active, and from which the potassium 



* The striking absence from certain cities in which one might otherwise 

 expect to find them, of lichens and other plants especially sensitive to 

 gaseous poisons, may be attributed to the poor coal. 



f Frank. Lehrbuch, I., p. 589. 



