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TEXTBOOK OF PLANT PHYSIOLOGY 



the entire growing zone, since only in this region the distance 

 between the marks will increase. On the basis of growth incre- 

 ments of separate regions, as well as whole organs, Sachs estab- 

 lished a definite law of "the grand period of growth." According 

 to this law, every organ and each definite region of a developing 

 organ at first grows slowly, then accelerates, reaches its maximum 

 speed, and finally slows down. The following figures show daily 

 growth increments of a small section in the growing roots of the 

 horse bean, which at the beginning of the experiment was 1 mm. in 

 length. 



90 



80 



10 



60 



g50 



2 40 



30 



20 



10 







0123456789 

 Days 



In the bottom row are given the figures of the whole length of 



the region at the moment of its 

 measurement. If the results of these 

 measurements are represented graphi- 

 cally, two distinct curves are obtained 

 (Fig. 109). The curve of increments 

 has an almost symmetrical aspect with 

 a rather prolonged maximum (dotted 

 lines), while the curve of changes in the 

 total length of the section is of the 

 letter S type. It rises, at first, slowly, 

 Fig. 109. — Growth curves of t h en more rapidly, and finally ap- 



roots of Vicia faba (after Rippel) . .. . , . . . . . 



proaches the horizontal direction. 

 In this growth curve one can distinguish two sections : from the 

 beginning to the maximum, or the most rapid rise, and from the 

 maximum to the cessation of growth. The first part may be 

 termed the initial phase, since here we have the manifestation of 

 growth in its purest aspect. The second part may be considered 

 as the development of retardation, which shows itself at first weakly 

 but later with increasing force, until growth ceases. For the initial 

 phase, Blackman applies the formula of compound interest, since, 

 in growth we are dealing with an increase in the number of cells 

 following a geometrical progression, in which one cell forms 2, 2 



