THE NUTEITION OF THE BOOT 37 



Of the metals which are widely distributed in the vegetable 

 kingdom, we must mention first of all copper, which has been 

 found in the wood of various trees (oak, beech, lime, &c.), and 

 in some herbaceous plants. Not so widely distributed, but 

 occurring in higher percentages in the ash of plants, we find 

 zinc, which changes the entire appearance of some plants 

 (e.^., Forma calaminaria of Viola lutea), but only acts detri- 

 mentally when occurring in larger quantities. The leaves of 

 plants which suffer from zinc-poisoning are discoloured by 

 small rusty-looking spots, which after a while spread over the 

 entire surface of the leaf. This metal generally enters the 

 plant as a soluble sulphate. Lead is less harmful to plants, 

 but arsenic is poisonous in very small quantities, decreasing 

 very rapidly the absorptive power {osmotic activity) of the roots. 



From the above account, which is by no means exhaustive, 

 it will be seen what a number of elements can be present in 

 plants. Some of these substances are stored in large quantities 

 without injuring the plants in any way, while others prove 

 harmful when present in very much smaller quantities. Prom 

 this fact we may gather that roots are not able to choose their 

 nourishment ; if, therefore, poisonous substances reach the roots 

 in a soluble form and can penetrate through their cell-walls, 

 the plant will gradually be poisoned. But even if the sub- 

 stances are harmless in themselves, they will always have some 

 effect on the general economy of the plant; sometimes this 

 influence will become apparent externally. Thus we have 

 already seen that zinc plants possess a different appearance — 

 in some cases a differently coloured flower. The same may be 

 said of plants growing in salty regions (halophytes) ; they are, 

 it is true, able to flourish without absorbing any salt, but they 

 only develop their succulent character when they have absorbed 

 some sodium chloride. Similar experiments with regard, to 

 silica have been made on cereals, and they tend to prove an 

 indirect influence of this substance on the general growth. 

 The grains were better developed and the other food substances 

 were more completely used up. 



We must, therefore, not look upon the substances only 

 occasionally absorbed by the roots as entirely useless for the 

 general nutrition of our cultivated plants. Their influence, 



