Lastly, we have plenty of evidence thai 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 proceeding, 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. 



