120 



PLANT PHYSIOLOGY 



methods of marking the growing organs by means of India mk 

 appUed with a fine brush is used. These marks are made at 

 definite distances from one another, and the increase in the 

 distance is measured at certain time intervals. This enables 

 one to obtain a precise measurement of the changes in length of 

 each portion during the experiment (Figs. 26 and 27) and to 

 ascertain the dimensions of the entire growing zone, since only 

 in this region will the distance between the marks increase. 



Fig. 26. — Two etiolated seedlings of the 

 horse bean with markings made 24 hi;, 

 before photographing. On the right, a 

 seedling with markings just made {after 

 Nathansohn). 



Fig. 27. — Maize sprouts 

 with markings made on 

 roots; a, before the experi- 

 ment; b, 24 hr. later (after 

 Molisch) . 



Investigations performed by means of this method have 

 established differences in the character of growth of various 

 organs of plants. Stems and roots grow at their ends while, in 

 many leaves, growth is mostly at the base. Leaves are limited 

 in their growth, very soon reaching their final dimensions ; while, 

 in stems and roots, growth continues throughout their life, in 

 trees sometimes during hundreds and even thousands of years, 

 thus permitting them to reach enormous sizes. Roots and shoots 

 differ in that the growth of roots is limited to a very short section, 

 not more than 1 cm. This allows the root to overcome the 

 resistance of the soil it has to push through ; while in the shoots, 

 the length of the growing zone is up to 10 cm. long. 



During recent years, wide use has been made of the photo- 

 graphic method for studying the course of growth. At definite 



