136 LECTURE VIII. 



therefore, an important part in the formation of the green 

 colouring-matter, chlorophyll, but, as will be subsequently 

 shewn, it does not enter into its chemical composition. It 

 doubtless affects in some way the processes in the cell which 

 lead to the formation of the chlorophyll and also of the 

 chlorophyll-corpuscles, for Arthur Gris observed that no 

 differentiation of corpuscles had taken place in the cells of 

 chlorotic leaves. 



We have now concluded the consideration of those con- 

 stituents of the ash of plants which have been found to be 

 essential to their nutrition : we will now consider some of 

 those which, though universally present, have been found to 

 be unessential. 



Sodium. This element, one of the most widely distributed 

 is, as might be expected, never absent from the ash of plants, 

 and in some cases, especially in maritime plants, it is present 

 in considerable quantity. It was thought that possibly it 

 might serve as a substitute for potassium in the nutrition of 

 the plant, but this has not been found to be the case (see 

 Fig. 21, 3). A glance at the table given above (p. 129) will 

 suffice to shew how small a quantity of this element is 

 absorbed, and from this an inference may be drawn as to its 

 uselessness. 



Chlorine. Chlorine also is a very constant constituent of 

 the ash of plants, but it does not appear to be essential to 

 their nutrition. It has been indeed observed that plants of 

 Buckwheat, Barley, and Oats, did not flourish when grown in 

 solutions containing no chlorides, and, as in these plants the 

 chlorophyll-corpuscles became overfilled with starch-grains, it 

 was thought that chlorine was of importance in connexion 

 with the translocation of carbohydrates (Nobbe, Leydhecker, 

 Beyer). On the other hand it has been observed that Maize- 

 plants will grow well in solutions containing no chlorine, and 

 further that the accumulation of starch in the chlorophyll- 

 corpuscles may be induced by various abnormal external con- 

 ditions (Knop and Dworzak). It seems probable, therefore, 

 that chlorine has no direct influence upon the metabolism of 

 plants, but only an indirect one, the chloride being the com- 



