316 EFFECT OF CHEMICAL AGENTS [Cn. XI 



outside of organisms, and carry a part of that peculiarity with 

 them into organisms. All of them are of physiological interest ; 

 but so far as we know chlorine and iodine are most important. 



Chlorine. - -This element is probably of constant occurrence 

 in organisms, and indeed in rather large quantities, as Table 

 XXVII, p. 297, shows. It is not strange that it should be so, 

 since this element is very widely distributed in all waters. 

 This very fact, however, renders it possible that chlorine is 

 merely an accidental constituent of organisms, being unessential 

 to growth, and precisely this conclusion has been maintained. 



Of green plants it was early asserted that growth occurs as 

 readily in solutions containing no chlorine as in ordinary 

 potable water, but of late years evidence opposed to this view 

 has been accumulating. Thus, while it appears that growth 

 may occur in the absence of chlorine, ASCHOFF ('90) and 

 others have found that growth is not so vigorous as in solutions 

 containing this element. It has been thought that chlorine 

 makes an advantageous combination with the potassium neces- 

 sary for the plant, but the true significance of the favorable 

 properties of chlorine remains undetermined. 



Turning to animals, we find that chlorine occurs in the milk 

 of mammals, and is therefore probably necessary to them. 

 According to HERBST ('97, p. 709) it is a necessary food for 

 young echinoids. Certainly in the form of sodium chloride 

 it is an essential food of the higher animals, and some of them, 

 especially the herbivora, require large quantities of it, as " salt- 

 licks" testify. The function of chlorine is not altogether 

 plain. It must be used in the production of the hydrochloric 

 acid of the digestive juices. In addition, sodium chloride is 

 found widely distributed in the tissues. It has often been 

 asserted that it goes through the tissues unchanged ; but 

 NENCKI and SCHOUMOW-SIMAXOWSKY ('94) believe that it is 

 probably disintegrated and the chlorine built up into organic 

 molecules. 



Bromine. - - The normal occurrence of traces of this element 

 has lately been demonstrated by HOTTER ('90) for a great 

 variety of plants. It occurs most abundantly in various 

 fruits, apple, pear, peach, and also in the leaves and twigs of 

 many plants, as well as in various berries. Its normal occur- 



