34 THE PHYSIOLOGY OF PLANTS 



green chlorophyll granules, and do not form new cells very 

 readily. In the case of decomposition of succulent portions 

 of plants, very often only those parts which are very rich in 

 nitrogen will be found to contain crystals of magnesium phos- 

 phate. Magnesium, just like calcium, enters the plant in the 

 form of sulphates, phosphates, and other salts, especially in 

 combination with chlorine. 



Iron is necessary in the building up of chlorophyll. As it 

 is the function of chlorophyll to form new plastic material 

 under the influence of the sunlight, it is natural that the 

 absence of iron, which is shown by the paleness of the 

 leaves, should cause a cessation of assimilation. But as 

 iron is very widely distributed in the soil, such paleness 

 {etiolation) is of much less frequent occurrence than might 

 be supposed. In diseases of all kinds yellow foliage may 

 make its appearance, and we must not be surprised if in 

 many cases the addition of soluble iron salts remains without 

 remedial effect. 



To the subsidiary substances we must also reckon chlorine, 

 which is not looked upon as a nutritive substance by many 

 physiologists ; still many observations tend to show that this 

 substance is essential to the healthy growth of the plant. 

 Water-cultures devoid of chlorine compounds do not thrive, 

 and in some cases direct indications of disease make their 

 appearance. From the present data, we can only assume that 

 the presence of chlorine retards the absorption and ultimate 

 deposition in the tissues of calcium, but that, on the other 

 hand, it accelerates the passage of phosphates, and thus 

 increases the supply of food material at the growing 

 points. 



If much phosphoric acid is used up, the nitrogen and 

 the soluble carbo-hydrates are more rapidly utilised by the 

 protoplasm for the formation of albuminous substances. 

 Hence we find that a supply of chlorine salts (calcium 

 potassium and magnesium chlorides) causes long and suc- 

 culent shoots to be produced, which, however, are poor in 

 starch. 



