7 
Lastly, we have plenty of evidence that in addition to 
their need for water the roots of plants, just like the living 
parts of all plants or animals require for their growth that 
life-giving constituent of the air, which we also breathe 
in, namely, oxygen. This need of the roots for air is one 
of the fundamental facts which governs not only the dis- 
tribution of plants in nature, but dominates our whole 
agricultural and horticultural practice. It is this need for 
air which causes the farmer to drain his fields and the 
gardener to grow his plants in porous and well-drained 
pots. If we turn a plant out of such a pot we shall see 
by the dense felting of the roots in close contact with the 
sides of the porous pot and among the crocks at the bottom 
of the pot how eager the roots are for air. If on the other 
hand we do not secure drainage or if we over-water 
pot-plants we soon find that they show signs of ill-health. 
As a matter of fact their roots are being suffocated and 
may die away unless we alter our treatment. Probably 
more plants are lost by over-watering than by insufficiency 
of water. Normally, therefore, though water is a prime 
necessity of plants its provision must not interfere with 
the respiratory process of plants, and we must always 
provide a porous soil for our crops, that is a soil with 
sufficient air. This is one of the reasons for the practice 
of hoeing. For apart from the clearing out of weeds, 
hoeing prevents the ground from becoming caked, a con- 
dition which would prevent the free access of air into the 
soil. At certain stages in their growth this need seems 
greater than at others. Speaking generally we may say 
the more actively growth is procceding, the greater the 
need for air. Germinating seeds for instance require a 
large amount of air, and when the formation of new roots 
is proceeding in the case of cuttings and layerings a porous 
soil is essential. When a potato starts its new growth and 
is rapidly developing its new shoots and roots we. find 
that the skin, a hard and impervious layer, becomes inter- 
rupted by numerous breathing pores which enable an active 
respiratory process to take place. These pores can easily 
be seen with the naked eye on the skin of a sprouting 
potato. They are equally clearly seen on the twigs of 
trees such as those of the horse-chestnut. On the leaves 
the pores through which the plant takes in various con- 
stituents from the air are not visible except with such 
magnification as the microscope affords. 
