GROWTH OF PLANTS 



119 



can be used only when measurements are taken not oftener than 

 every 24 hrs. 



When it is desired to follow the growth of a plant at shorter 

 intervals of time, a horizontal microscope will be found con- 

 venient. It is focused upon the end of the growing organ, e.g., 

 upon the tip of a blade of grass or the end of a root ; and then, by 

 means of an ocular micrometer, the distance covered by the 

 extremity of the plant is determined (Fig. 24). If the magnifi- 

 cation of the microscope is known, it is easy to calculate the 

 growth increase. 



Fig. 25. — Wiesner's auxograph {after Molisch). Clockwork, W, rotates a 

 cylinder C, covered with lampblack on which pointer, Z, marks the growth elonga- 

 tion shown by thread g passing over small pulley r. The sliding counterweight, 

 G, attached to thread over the circumference of wheel, R, multiplies the 

 movement. 



It is very convenient to use for studies of the course of growth 

 a self-recording apparatus, the so-called ''auxograph," in which, 

 by means of pulleys and wheels, changes in the position of the 

 tip of a plant are recorded on an enlarged scale on the surface 

 of a revolving smoked paper cyhnder (Fig. 25). Auxographs 

 usually give an enlargement of 20 to 40 times. From such 

 records, one can read alterations in growth occurring in the 

 course of H or 1 hr. Recently, however, the Indian scientist 

 Bose has constructed an apparatus, the crescograph, which gives 

 enlargements of 1,000 and even 10,000 times. This apparatus 

 allows one to record the progress of growth not only by minutes 

 but even by seconds. 



Often it is important to follow the growth of certain organs 

 instead of that of the whole plant. In such cases, Sachs's 



