u6 



SEEDS AND SEEDLINGS 



and sketch it. After the stem has directed itself well up- 

 ward, invert the pot again, 

 and watch the growth. 

 After a week remove the 

 plant and notice the direc- 

 tion of the root. Sketch it 

 entire, showing the changes 

 of direction. 



At the same time that 

 this experiment is arranged, 

 lay another pot with a 

 rapidly growing plant on 

 one side, and every forty- 

 eight hours reverse the 



252, 253. Experiment showing the position of the pot, laying 



^-."S^^Z! it on the opposite side. At 



253, the same after an interval of eight hQ end of ten Or twelve 



days remove the plant and 



examine. How has the growth of root and stem been 

 affected ? 



What do we learn from these experiments and from 

 those in Sections 153 and 154, as to the normal direction 

 of growth in these two organs respectively ? 



160. Geotropism. This general tendency of the grow- 

 ing axes of plants to take an upward and downward course 

 in other words to point to and from the center of the 

 earth is called geotropism. It is positive when the 

 growing organs point downwards, as most primary roots 

 do ; negative when they point upwards, as in most primary 

 stems ; and transverse or lateral, when they extend hori- 

 zontally, as is the case with most secondary roots and 

 branches. 



161. Gravity and Growth. It has been proved by ex- 

 periment that geotropism is due to gravity. It must be 

 carefully noted, however, that the influence here alluded to 

 is not the mere mechanical effect of gravity due to weight 

 of parts, as when the bough of a peach or an orange tree 



