434 



THE FOOD OF PLANTS 



SECTION 75. The Non-Essential Ash Constituents. 



Just as plants may absorb an excess of their essential constituents, so 

 also may non-essential mineral substances be absorbed in greater or less 

 amount. Thus, probably, no plant grown wild is without silicon and 

 sodium, and these elements may sometimes form the greater portion of its 

 ash. Many plants also accumulate perceptible amounts of aluminium, 

 manganese, and zinc, and although marine plants contain relatively but 

 little iodine or bromine (Sect. 22), still it requires a comparatively intense 

 absorptive power to obtain this amount from the minute traces of iodides 

 and bromides present in sea-water. Such accumulation is an example of 

 selective absorption, and is due to the fact that the substance absorbed is 

 converted into an insoluble form or into a non-diosmosing compound. 

 Whenever a diosmosing substance undergoes modification of this character, 

 it must necessarily accumulate in the plant whether it is useful or not, and 

 in this manner useless substances may be absorbed, which under normal 

 circumstances the plant never encounters, such as methyl-blue and other 

 aniline dyes. Similarly a plant may accumulate large quantities of poisonous 

 bodies, if they are presented in such dilute form that an injurious concen- 

 tration is never reached during the endosmosis through the plasma. 

 The poisonous metallic salts are retained by humus with considerable 

 tenacity and presented to the plant in very dilute form (Sect. 28). Zinc 

 salts may, for example, be accumulated in large amount, although even 

 a very dilute solution is extremely poisonous. 



Selective power is always a physiological problem, for it is determined 

 and regulated by the present or previous vital activity. Many other elements 

 may accumulate to a marked extent when presented to the plant, though, 

 owing to their rarity, they are usually absent from the ash, as for example is 

 the case with rubidium and beryllium. The fact that a substance is passively 

 secreted in this manner does not necessarily indicate that it takes part in 

 metabolism, for accumulation occurs whenever an insoluble compound is 

 formed by combination with any metabolic product, while the removal of 

 such solvents as carbon dioxide, &c. may cause a precipitation of the 

 penetrating substance, as silica or chalk for example. Nevertheless non- 

 essential elements frequently become involved in metabolism, and are 

 utilized to a certain extent, as is shown by their partial substitution for 

 essential elements and by other facts (Sects. 73 and 74). Thus the non- 

 essential elements, such as manganese, cobalt, or zinc, may in certain cases 

 favour growth. It is, however, still doubtful whether chlorine is necessary 

 or advantageous always, or only in special cases. Even sodium may 

 be of use to the plant when the supply of potassium is reduced to the 

 lowest possible minimum, and it has yet to be explained why a lower 



