THE NUTRITION OF THE ROOT 71 



have exhausted the nutritive substances, and because we can re- 

 newthese by liquid manures during the life of the plant,and often 

 increase their amount above the original supply of food matter. 



In the case of pot-plants, the choice of the various soils 

 depends upon their physical properties, upon their power of 

 retaining water, of absorbing nutritive substances, and allowing 

 a sufficient amount of air to enter for the aeration of the root. 



With regard to the latter, our cultivated plants have very 

 diffei'ent requirements. As far as the development of the 

 plants is dependent upon the soil, we may say that : It is not 

 the total amount of nutritive substances contained in the soil 

 which determines the vigour of growth of a pot-plant, but it is 

 due to the degree of concentration of the nutritive solutions in 

 the soil, and to the intensity of its aeration. 



It is the knowledge of this fact, gained by experience, which 

 distinguishes successful cultures from the attempts of beginners, 

 who are always searching for rich mould. The " richness," 

 i.e., the amount of nutritive substance, we can easily supply to 

 every flower-pot, but we cannot do the same with the physical 

 conditions which are necessary for the active growth, and 

 especially for the respiration of the root. 



We have at present no scientific knowledge of the actual 

 amounts of air necessary for our various plants, and we must 

 therefore at present make use of the practical experience of 

 gardeners. Now this teaches us that a root can never have 

 too much air, but often has too little. Without taking into 

 account those plants which are provided with aerial roots, we 

 need only remember the exposed roots of trees on rocky slopes, 

 walls, and roads, and think for a moment of the habit of many 

 plants in our greenhouses of sending up delicate rootlets out of 

 the soil into the air at the edge of the pot. All plants aie 

 also able to grow in moss or sand if the disadvantages of these 

 media are remedied by abundant watering with nutritive solu- 

 tions. As soon as we are able to satisfy the wants of roots 

 with regard to water and nutritive substances, we may choose 

 beads of glass or crumbled quartz instead of ordinary soil. 

 Indeed, by timely changes of tbe nutritive solution, and by 

 constantly renewing the supply of oxygen, we can cultivate 

 plants for years in water itself. 



