EFFECTS OF EXTERNAL CONDITIONS. 107 



67. Where in the most highly differentiated plants is growth 

 localized or most active? 



Your studies in earlier courses have given you data for 

 answering- this question ; review the facts, supplement from 

 other sources, and express conclusions in a diagram showing 

 the growth areas of a higher plant shaded in proportion to the 

 activity of each. 



B. His tological Differentiation and Assumption of Form. 



68. What are the factors controlling histological differentia- 

 tion and assumption of form ? 



Xo experiment upon this subject is practicable ; answer 

 from your various sources of information. Particularly note the 

 processes concerned in the maturation of embryos and vegeta- 

 tive points. 



C. Effects of External Conditions. 



69. In what way is growth directly (i.e., non-irritably) af- 

 fected by the principal external physical agencies ? 



a. By presence, absence, and different qualities of light? 



Answer by Experiment 37. 



b. By differences in temperature? 

 Answer by Experiment 38. 



c. By electricity? 



Answer from study of the references below. 



d. By moisture in various degrees? 

 Answer by Experiment 39. 



EXPERIMENT 37. This may be tested by using parts in which 

 growth occurs uncomplicated by photosynthesis, as in germinating 

 seeds, and compelling them to grow under light of different colors. 

 Prepare an even-moisture germination apparatus as shown by Fig. 26 

 (consult page 44). In each of the four Zurich germinators sow ten 

 soaked seeds of oats and cover respectively with (a} clear glass, (&) 

 glass blackened by asphalt varnish, (c) red glass, and (d) blue glass. 

 Set the apparatus in bright though not direct sunlight, and com- 

 pare the rates (a) of germination, (b} of early growth, and (c) of later 

 prowth. Express results quantitatively in your records. 



