As all output of energy depends upon intake of oxygen, there must be some supply of oxygen available for theand a discharge of carbon dioxide from it. This latter gas, dissolving in soil-water, forms carbonic acid and, like the acid from the -hairs, helps to bring into solution some of the mineral substances in the soil.
Unless they are liberally supplied with oxygen,cannot carry on their very active work. If they are unfavourably placed in the soil with regard to this gas, they die. Farmers, therefore, drain and plough land, so that air may be drawn into the soil for the use of roots. Gardeners dig with the same object.
Pyrogallic acid has the power of absorbing oxygen with great rapidity and is, therefore, a ready means of absorbing oxygen from a limited supply of air.
If a bean, whose root has grown about 2 inches, is suspended from the cork of a gas-jar that contains a solution of pyrogallic acid, its development is arrested. Probably it will not die for some little time, because plants have a faculty, denied to animals, of making use of the oxygen of their own tissues for a limited period. This is known as intramolecular respiration and accounts for the lingering life of the suspended seedling. Even so, the seedling lives only for a 1,p1 week or two at the longest. Its death is largely due to the fact that its root is deprived of oxygen and therefore cannot breathe. During this period a similar seedling, suspended over water, has grown freely ; the shoot has emerged and is growing upwards, while the root has elongated and given rise to lateral branches.
The imperative need of oxygen is seen in the roots of “ pot-bound “ plants. The main root grows downwards until it is hindered by meeting the base of the flower-pot. After this it, like the outwardly extending branch roots, grows round and round between the soil and the pot, which is porous. Through the pores a small amount of air enters and the roots are getting into as close proximity as possible with this incoming air. When a plant is in this condition its roots should be gently separated before it is transferred to a larger pot. It is well to let the plant stand for a little time in water before putting it into the new pot. Then, after the addition of the necessary extra soil, it should be well watered, so that in the damp atmosphere root-hairs may develop in the shortest possible time.