206 



are no longer able to obtain their necessary supplies of water 

 from the soil and are thus in danger of desiccation. At the 

 same time plants vary in this respect and there are many 

 (usually known as halophytes) which can thrive on saline 

 soil and are able to recover the necessary water from con- 

 centrated solutions of salts. There are also many substances 

 which have a more directly poisonous effect on plants and 

 such are as a rule the salts of the heavy metals, e.g. sulphate 

 of zinc and others. Free acids and alkalies are also very 

 poisonous even when dilute. Carbon dioxide is continually 

 excreted by the roots of plants and, in water-logged localities, 

 the accumulation of this gas may have a poisonous effect. 

 The escape of coal-gas from underground pipes, or chemical 

 solutions from factories, such as dye-works, may also kill and 

 injure plants. Again many essential mineral salts, and others 

 which, though not essential, are often found to be taken up 

 by plants, may have a poisonous effect if present in large 

 quantities. A trace of iron is thus essential for plants, but 

 a very dilute solution of an iron salt may be poisonous, the 

 quantity of the salt present being above the optimum. Cal- 

 cium appears to be injurious in some cases, while as regards 

 common salt it is not clear if its injurious effect is merely 

 due to its osmotic properties or to a more direct poisonous 

 action. In the majority of cases the exact way in which 

 the poison acts is not known, but sometimes at all events 

 the poisonous substance appears to enter into chemical 

 combination with some constituents of the protoplasm, much 

 as in the case of animals carbon monoxide combines with 

 haemoglobin. Humus by its absorbing action has a most 

 beneficial effect in protecting plants from poisons. Poisonous 

 metallic salts may thus be strongly held by the humus and only 

 allowed to pass into the soil in very dilute, harmless solutions. 

 The principal product which is excreted in large quantities 

 by the roots of higher plants is carbon dioxide, and this being 

 a gas rarely accumulates sufficiently to be injurious. 



Many plants are also able to accumulate considerable 

 quantities of poisonous substances without injury, by 

 depositing them in dead tissues, or in living ones in such a way 

 that they cannot penetrate the living protoplasm. 



185 ' Umess the root-hairs are 

 Water and able to obtain a sufficient supply of water and oxygen in the 

 Oxygen. soil they cannot perform their functions properly, and a good 

 soil, besides being sufficiently moist, must, therefore, be open and 

 well aerated. The amount of available water in a forest 



