ROOTS AND THEIR WORK 



85 



gravity to grow downward ; hence they are said to be positively 

 geotropic. 



The force with which roots grow downward is greater than the down- 

 ward pull exerted by gravity on the growing root. 



Windsor Bean Experiment. — Place a grow- 

 ing Windsor bean so that the plant is held by 

 a pin through the cotyledons and the root 

 directed downward into a dish of mercury. 

 The root will push its way into this very dense 

 substance. 



Or a germinating bean may be so fastened 

 that the root will point downward into a groove 

 made in a little wooden or cork float which is 

 placed on the surface of a dish of water directly 

 under the bean. If the experiment is success- 

 ful, the root will push the float either under 

 water or out of the way.^ 



Effect of Gravity on the Stem. — The 



stem, on the other hand, seems to be 

 affected in the opposite manner, and is 

 therefore said to be negatively geotropic. 



Experiment to show the effect 

 of gravity upon the growth 

 of stem and root. (After 

 McDougall.) 



Figure to show the point 

 of most rapid growth 

 in the root of the 

 bean. (After McDou- 

 gall.) 



The effect of gravity on the young stem may be 

 shown by turning upside down a pocket garden in 

 which are young barley or corn plants. The stt m 

 at once turns upward again. The same thing may 

 be proved by germinating radish or grass setds on 

 the under side of a hanging sponge. After the st( m 

 has attained a length of one or two inches, turn 

 the sponge so that the young plants are upside 

 down. 



Place where Root grows Most Rapidly. — Select 

 three or four pea seedlings which have roots half an 

 inch to an inch in length. Mark such roots on their 

 outer surface at equal intervals (say one mm. 

 apart) by means of a thread dipped in India ink. 

 Allow these marks to be continued on strips of 

 paper which are placed next to the growing roots. 

 Keep them in a moist germinator for several days 

 Make drawings to show where increase in length 

 took place. 



The Root Cap and Most Rapid Point of 

 Growth. — If we examine a longitudinal section of 

 any dicotyledonous root under the microscope we 

 find that at the very tip of the root can be seen a collection of cells arranged 

 more or less loosely over the end of the root in the form of a cap. Most of 

 these cells are dead, and this root cap is simply an adaptation to protect the 



^ See Hunter and Valentine, Manual, page 232. 



