18 PRELIMINARY STUDIES. 



These hairs are of the utmost importance in the economy 

 of the plant. They are the principal organs of absorption 

 for the nutrient fluids of the plant. The fluids enter them 

 from the soil by the physical process of osmosis. These 

 fluids contain the dissolved mineral and organic matters 

 which furnish food for the plant. They pass from cell to 

 cell up to the very tops of the loftiest trees. It is known 

 that osmotic action has much to do with this passage of 

 the fluid, but it is still an unsolved problem why the 

 osmosis should be so much stronger from without inward 

 than it is from within the plant outward. Because it is 

 stronger from without, there results a pressure, known as 

 root pressure, which carries the fluids up into the plant 

 and keeps up a supply of food. 



8. Trace the development of the plumule. When the 

 seedling was placed in the horizontal position in Number 

 5, the plumule soon turned upward from the earth. The 

 movement is called an apogeotropic movement. 



9. Place a well-developed seedling- in a position where 

 it will receive the light on only one side. Notice that 

 after some time it turns its leaves towards the light, or 

 exhibits heliotropism. 



10. Observe the arrangement of the leaves on the young 

 stems. Is the arrangement the same in all of these plants ? 

 Describe the leaf arrangements you find, whether the 

 leaves are opposite one another, or are arranged alter- 

 nately. If opposite, consider the positions of successive 

 pairs. If alternate, count the number of leaves in one 

 complete revolution about the stem. 



Are the leaves first produced by the seedlings of the 

 same shape as those formed after the plants have attained 

 considerable size ? If not, describe the differences for 

 each plant and illustrate by outline drawings. Are the 

 lower leaves in all cases as well developed as those formed 

 later ? As we shall consider more fully later, a large part 



