364 PHYSIOLOGY OF GROWTH. 



given in 59 and 148.* The detached flower-stalks are now placed 

 vertically in a cylinder filled with spring water, so that they are 

 completely covered, while the bean-stalks are not yet cut off. 

 After twelve or twenty-four hours we determine the distances 

 between the marks ; and we shall find that these are no longer 

 10 or 20 mm., but have become greater. This is due to the 

 growth which has taken place. It can now be easily determined, 

 and this is of special interest, that the growth of the individual 

 partial zones is by no means the same. The most rapid growth 

 has taken place either in the very youngest partial zone (so I 

 found in harmony with H. de Vries, using for investigation a 

 young flower-stalk of Plantago, divided into partial zones 20 mm. 

 in length), or in one of the youngest zones (e.g. the third). I 

 found this to be the case in experiments with the epicotyl of 

 Phaseolus seedlings grown in sawdust in the dark. The epicotyl 

 was 70 mm. in length, and the marks were made at distances of 

 only 5 mm. from each other. In the course of forty-eight hours at 

 15 C., the youngest partial zone increased in length by 1 mm., 

 the second by 3 mm., the third by 8 mm., the fourth by 6 mm., 

 the fifth by 5 mm., the sixth by 3 mm., and the seventh by 1 mm. 

 It is always found that the rate of growth of the cells gradually 

 diminishes with advancing age, until finally their growth com- 

 pletely comes to an end. 



After determining the distribution of the rate of growth in the 

 objects investigated, we plasmolyse them (see 59) by placing them 

 in a 10 per cent, solution of common salt or Potassium nitrate. 

 Pieces of stem 2-3 mm. thick may be at once placed in the salt 

 solution ; thicker pieces must first be halved. After a shorter or 

 longer time (three to twelve hours), plasmolysis is complete. The 

 partial zones have become shorter owing to loss of turgidity ; and 

 if we calculate the shortening with reference to the original 

 length of the zones (5, 10 or 20 mm., as the case may be), it 

 appears that it is greatest in extent, roughly or exactly, in the 

 regions where the most vigorous growth took place. There is a 

 clear relation between the turgor-extension of [the cells of the 

 different partial zones and their rate of growth, a result which leads 

 to the view that the rate of surface growth of cells is dependent 

 upon the magnitude of their turgor-extension. This last is deter- 



* If it is required to place ink-marks on curved plant structures e.f/. the 

 epicotyls of Phaseolus it can be done by means of a strip of paper divided 

 into millimetres. 



