CHAPTER II 



GROWTH PHENOMENA THAT ARE CONTROLLED BY INTERNAL 



CONDITIONS 



§1. The Grand Period of Growth. — Plants and their component organs and 

 tissues do not enlarge at the same rate throughout the period of their develop- 

 ment. Enlargement begins at a slow rate, which gradually increases until a 

 maximum is reached, after which the rate progressively decreases until enlarge- 

 ment ceases altogether. The time period corresponding to this march of rate 

 of enlargement was designated by Sachs^ as the grand period of growth, and the 

 same author called the graph representing this march, the grand curve of growth. 

 This peculiar march of the growth rate is due to the fact that each individual 

 cell of the plant body passes through a similar grand period of development. 

 External conditions can lengthen or shorten the period of growth, but the general 

 character of the curve is not altered. Thus, in one experiment, the daily incre- 

 ments of elongation of the terminal internode (3.5 mm. in length) of a seedling 

 of Phaseolus muUiflorus , with a temperature of 12.8 to I3.8°C., had the values 

 shown below. 



Increment or 

 Number oe Elongation, 



Day ' mm. 



1 1.2 



2 1. 5 



3 2.5 



4 S-S 



5 7.0 



6 9.0 



7 14.0 



8 10. o 



9 7.0 



10 2.0 



§2. Growth of Root, Stem and Leaf.— While it is generally true that'the 

 three most important organs of the plant (roots, stems and leaves) all pass 

 through a grand period of growth, nevertheless there are individual pecularities 

 to be observed in each case. 



In roots,2 the elongating region is restricted to a portion near the tip, usually 

 not more than 10 mm. when the roots are surrounded by soil. Aerial roots are 

 an exception to this; the elongating regions of the aerial roots of Monster a 



1 Sachs, J., Ueber den Einfluss der Lufttemperatur und des Tageslichts auf die stundlichen und taglichon 

 Aenderungen des Langenwachsthums (Streckung) der Internodien. Arbeit. Bot. Inst. Wuraburg i : 99- 

 192. 1874. 



' Sachs, J., Ueber das Wachsthum der Haupt- und Nebenwurzeln. Arbeit. Bot. Inst. Wtozburg 1; 

 38S-474, 584-634- 1874. 



218 



