THE NUTRITIOJS" OF THE ROOT 77 



when the aerial root attains a certain age. The same may be 

 observed to take place when such roots enter the soil, which 

 indicates that the velamen represents the special organ by- 

 means of which the root can derive its nutriment from the 

 atmosphere. 



This being the case, the practical treatment of plants pro- 

 vided with aerial roots is readily deduced. In all cases, 

 whether the aerial roots supplement the ordinary roots, or 

 whether they alone exist, they must be protected and increased, 

 as being the means of collecting and absorbing food material. 

 Occasional sprinkling of the aerial roots with dilute nutritive 

 solutions may be recommended as the most suitable means of 

 manuring. 



§13. How do ordinary roots obtain their necessary supply of air? 



We have already dwelt several times upon the fact that in 

 all horticultural practices the great respiratory need of the 

 plant must be satisfied. Not only must the surface of the 

 plant be in constant contact with the most essential constituent 

 of the air, namely, the oxygen, but all the internal tissues, too, 

 must be constantly supplied with oxygen, so that the necessary 

 oxidation which constitutes the respiration of plants may take 

 place. 



It is true that every green cell of a plant under the influence 

 of light is forming new organic substances from the nutritive 

 sap and the carbonic acid of the air (assimilation), and in so 

 doing liberates oxygen ; but this amount of oxygen is by no 

 means suflBcient for the respiration. This can be gathered 

 from the fact that in living green cells (cells of algge, for 

 instance), under favourable conditions of light and nutrition, the 

 functions cease, and the movement of the protoplasm is sus- 

 pended, if they do not receive renewed supplies of oxygen. 

 A living cell will recover from this state of suspended anima- 

 tion, if not very long after it enters upon it it is again pro- 

 vided with oxygen. Should the oxygen be withheld for some 

 time, the cell will become asphyxiated. During such a period 

 of lack of oxygen the living cell can take oxygen from other 

 organic or inorganic substances which may be at hand ; but 



