GENERAL DISCUSSION OF GROWTH 



217 



not have decreased. Extensibility of the wall is therefore the second condition 

 necessary for cell enlargement. Various external conditions are also necessary 

 for growth, such as favorable temperature condiitons, the presence of oxygen 

 in the surrounding air, and an adequate supply of water. 



§3. Apparatus for the Study of Growth.— The simplest equipment for the 

 study of plant enlargement is a millimeter rule. A horizontal microscope (Fig. 

 98) or a cathetometer may be used for finer and more accurate measurements. 

 The auxanometer, a self-registering apparatus for growth measurement, may 

 also be used (Fig. 99). A waxed thread is fastened to the top of- the stem to be 

 studied and is passed vertically 

 upward and over a pulley, and a 

 weight is attached to the free end. 

 The pulley is turned as the plant 

 elongates and the weight descends. 

 The growth increments are magni- 

 fied by introducing a larger pulley, 

 mounted on the same axis as the 

 first, over which is passed a second 



thread with a weight at either end. 



A pointer is fastened to one end 



of the second thread, its tip resting 



lightly upon a vertically placed 



drum revolved by clockwork and 



covered with smoked paper. As 



the drum revolves and as the 



pulley turns with the elongation 



of the plant, a curve is traced on 



the paper, the slope of which rep- 

 resents the time rate of this 



elongation during the period of 



operation. 



If it is required to determine 



whether all parts of an organ grow 



Fig. 100. — Apparatus for the study of growth. 



with equal rapidity, the organ may be marked into milUmeter or centimeter 

 spaces or zones, by means of India ink. After some time the spaces are 

 Tf m P3. su rc d 



The apparatus shown in Fig. 100 may also be employed to study growth. 

 One end of a waxed thread is attached to the tip of the plant and passes vertically 

 upward, ending by being wound about a smaU pulley on a horizontal axis. To 

 this pulley is attached a long, counterbalanced pointer, the free end of which 

 moves upward or downward in front of a large graduated arc, as the pulley is 

 turned. As the plant elongates the thread is released and the magnified growth 

 increments are read directly in degrees of arc. 



