﻿A PPLEMA N — RES T PERIOD 



lar intervals, a plant readily adapts itself to the new demands and 

 the rest becomes a habit, which may continue to be repeated auto- 

 matically for a longer or shorter period of time. The habit of rest 

 thus induced is often very strongly fixed and is apparently trans- 

 missible. 



Euler (7) attributes the rest period to internal self-regulatory 

 processes and theorized in the following ingenious manner regarding 

 the nature of these processes. The growth of a young cell shows two 

 phases which follow each other by self -regulation. The first or 

 stretching phase shows an increase of soluble osmotically active cell 

 constituents; the second is characterized by the building up of the 

 soluble materials into insoluble or colloidal molecules. Hydrolysis 

 and synthesis, however, proceed simultaneously, and the two phases 

 are characterized by an excess of the one process over the other. 

 The synthesis of the second phase fixes the stretching of the first 

 phase. What applies to the single cell applies also to the develop- 

 ment of the entire organ and organism. In a number of cases, new 

 cycles of stretching and condensation follow one another with 

 unbroken regularity. In many other cases, a new cycle does not 

 immediately follow upon the ending of a previous one. The 

 stretching phase is prevented on account of an inability to dissolve 

 the highly complex reserve materials. A short or long rest period 

 must first take place. Finally a point is reached, and this point 

 determines the length of the rest period, when the synthetic pro- 

 cesses no longer hold the simultaneously occurring hydrolytic 

 processes in equilibrium, and as a result internal osmotic pressures 

 are increased. The resting organ is now physiologically ripe and 

 begins to germinate, external conditions being favorable. As long 

 as condensation processes predominate in the resting organ, germi- 

 nation is impossible. According to Euler, therefore, after-ripening 

 would simply consist in certain changes during the rest period which 

 weaken the synthetic processes. This theory, however, does not 

 explain the origin and character of these changes. 



The important observations of Schimper (25) gave us the first 

 real knowledge of the peculiar behavior of tropical vegetation in 

 respect to growth and rest periods. In places where moisture and 

 temperature conditions are favorable during the entire year, many 



