THE MATERIAL AND THE EXPERIMENTS 11 



and shoots becomes considerable, since it is not possible to cut off 

 these organs at their base with absolute accuracy. On the other 

 hand, when the shoots reach a considerable size, they participate 

 to an increasing degree in the assimilation. As long as their 

 assimilating mass is small compared with the assimilating mass 

 of the leaf on which they grow, the error committed in neglecting 

 this influence is small; if necessary it can be corrected by deduct- 

 ing the mass of the shoots from that of the leaf in calculating the 

 influence of the active mass of the leaf on the production of roots 

 or shoots. The writer carried on his experiments for about three 

 or four weeks; in this time the mass of roots and shoots produced 

 under the conditions of temperature and light prevailing in the 

 greenhouse was sufficiently large to make the error committed 

 in cutting off the roots and shoots comparatively small. In 

 order to rule out accidental variations, each experiment was 

 made on a larger number of leaves or stems, rarely fewer than six 

 in one experiment. 



Since the amount of water in leaves, roots, and shoots is vari- 

 able, it was impossible to base conclusions on measurements of 

 the fresh weight of the mass of roots or shoots produced and the 

 dry weight had to be used instead. The dry weight was deter- 

 mined after the organs had been kept in a drying oven for about 

 24 hours at a temperature of about 100°C. Tests showed that 

 this sufficed to give a constant dry weight. 



Having stated these preliminaries we may now begin to give 

 the data which show that the production of roots and shoots in 

 sister leaves is in proportion to the active mass (or the dry weight) 

 of the sister leaves. 



