722 Fluctuations 



growing, but sooner or later reaches a maximum 

 and may afterwards decrease. 



This periodic augmentation and declination is 

 often boldly manifest, though in other cases it 

 may be hidden by the effect of alternate influ- 

 ences. Pinnate leaves generally, have their 

 lower blades smaller than the upper ones, the 

 tallest being seen sometimes near the apex 

 and sometimes at a distance from it. Branches 

 bearing their leaves in two rows often afford 

 quite as obvious examples, and shoots in gen- 

 eral comply with the same rule. Germinating 

 plants are very easy of observation on this 

 point. Wlien they are very weak they produce 

 only small leaves. But their strength gradually 

 increases and the subsequent organs reach 

 fuller dimensions, until the maximum is at- 

 tained. The phenomenon is so common that its 

 importance is usually overlooked. It should be 

 considered as only one instance of a rule, which 

 holds good for all stems and all branches, and 

 which is everj^where dependent on the relation 

 of growth to nutrition. 



The rule of periodicity not only affects the 

 size of the organs, but also their number, when- 

 ever these are largely variable. Umbellate 

 plants have numerous rays on the umbels of 

 strong stems, but the number is seen to decrease 

 and to become very small on the weakest lateral 



