GROWTH OF PLANTS 



121 



time intervals, the growing parts are photographed, and all the 

 measurements are carried out on these photographs. They 

 may be used also for motion-picture projection of the growth 

 phenomena. The photographic method possesses the advantage 

 of permitting measurements of any degree of precision without 

 the growing organ being touched, all disturbances of its normal 

 development thus being eliminated. The drawback of this 

 method is the necessity of applying hght. Light, as will be seen 

 later, greatly affects growth. Therefore, in cases when the effect 

 of Ught must be eliminated, the photographs must be made in 

 red light, which affects growth very httle. 



On the basis of growth increments of separate regions as well 

 as whole organs, Sachs established 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. 



Days 



Growth increase, millimeters, 

 Total length, millimeters. . . . 







1.8 



2.8 



3.7 



6.5 



17.5 

 24.0 



16.5 

 40.5 



17.0 

 57.5 



(3 



14.5 

 72.0 



7.0 

 80.0 



8 



0.0 

 80.0 



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

 the region at the time of its measurement. If the results of 

 these measurements are represented graphically, two distinct 

 curves are obtained (Fig. 28). The curve of the rate of growth 

 has an almost symmetrical aspect with a rather prolonged maxi- 

 mum (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, 

 then more rapidly, passes through a point of inflection at the 

 time of most rapid growth, and finally approaches 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 is the manifestation of 

 growth in its simplest aspect. The second part may be con- 



