296 GROWTH 



If the rate of growth of every hour of the day is taken into 

 consideration it will be found that here also a periodicity is ex- 

 hibited. The organ elongates but little during certain parts of 

 the day, then begins to grow at an increased rate until a maxi- 

 mum is reached when it shows a lessening activity until the mini- 

 mum is again shown, and in some instances a total cessation may 

 take place ; the plant thus exhibits a daily period of growth. 



The growth of a massive fruit as measured by weight shows a 

 grand period and also a daily period. The irregularities attend- 

 ant upon the attainment of mature size are most marked in such 

 formations. In addition the cessation of growth is followed by 

 decrease in weight due to respiration and transpiration. 



The variations in weight of a fruit depend on both the growth 

 of storage tissues and the deposition of reserve material. In this, 

 as well as increase in volume, certain external factors such as 

 humidity and water supply in connection with root-pressure may 

 modify the daily rhythm, which is not lost however. Simple in- 

 crease in weight does not imply growth, since a deposition of 

 material in existing cells may cause it, and on the other hand the 

 use or destruction of reserve material may proceed with growth. 

 If the life of a perennial plant is taken into consideration it is to 

 be seen that the casting of leaves in the autumn, or the annual 

 dying away of the tender shoots above the soil exhibited by some 

 species may greatly decrease the weight and volume of a plant. 



373. Rhythm. Ever since the inception of protoplasm it has 

 been subject to regularly recurring changes in the external 

 seasonal conditions, and to alternating periods of daylight and 

 darkness, with attendant changes in temperature and atmospheric 

 moisture. This has stamped upon the vegetal organism the habit 

 of moving in cycles which may coincide with the year, and is 

 most marked in the flora of the temperate and arctic zones. The 

 most noticeable feature of the cycle is the period of rest, in which 

 the foliar apparatus is discarded and active growth is very much 

 diminished. So deeply is this rhythm imprinted on the plant that 

 a deciduous tree from the temperate zone will continue to shed 



