LECTURE X. 



METABOLISM (continued}. 



5. The Distribution of Organic Substance throughout the 



Plant. 



IN the last lecture we studied the processes by which the 

 formation of organic substance is effected in the plant, and we 

 found that in highly organised plants the leaves are the organs 

 in which these processes especially take place. We will now 

 proceed to ascertain in what forms and by what means the 

 organic substance thus produced is distributed throughout 

 the plant, either to serve as plastic material for the building 

 up of the structure of the plant, or to be deposited as reserve- 

 material in some part of it. 



Our knowledge on this subject depends principally upon 

 the results of micro-chemical investigation, that is, upon the 

 detection of the various substances in the cells under the 

 microscope by means of appropriate reagents. It is extremely 

 important to bear in mind, in the consideration of the results 

 obtained by this method, that although it may not be possible 

 to detect the presence of any given substance in the cells of 

 an organ, we must not conclude that this substance is there- 

 fore not being formed there. The presence of a substance in 

 the cells of an organ depends upon this condition, namely, 

 that the amount of the substance conveyed to or formed in 

 the cells is greater than the amount which is being conveyed 

 away from or consumed in the cells. If these two amounts 



