264 EXPERIMENTS WITB PLANTS 



nating study. Find out all you can about it and the 

 principles underlying it.^ 



The form and arrangement of the branches deter- 

 mine the "habit" of a plant, by which we recognize 

 it even at a distance; the object of this arrangement 

 is to spread out the leaves in the best possible fashion 

 for their work, with the least expenditure of material 

 for construction. What plants do you think have the 

 most advantageous habit? How many distinct kinds 

 of habit can you distinguish ? 



What controls the habit of the plant? As we 

 have already learned, the main stem grows upward 

 in response to the influence of gravity. Do the 

 branches, especially the horizontal ones, assume their 

 positions in response to the influence of gravity ? Notice 

 whether the tip of a growing branch points in the 

 same direction as the branch itself. If not, how is 

 the change of direction effected ? Fasten tips of hori- 

 zontal branches in various positions, some pointing 

 upward, some downward, and if possible exclude the 

 light by conducting them into boxes which can be 

 kept dark inside. This experiment is not conclusive, 

 but indicates the probable force at work. The most 

 careful experiments so far made seem to indicate that 

 the principal influence is gravity. 



Does light also affect the direction of growth? Place 



1 See Bailey: "The Pruning Book"; also articles in the Year-Book of the 

 U. S. Department of Agriculture for 1895 liy Woods; for 1896 by Webber and 

 Lodeman; for 1898 by Saunders; for 1902 by Powell. 



