Exercise XXI 



PLANT GROWTH AND TROPISMS 113 



in the three leaves as displayed by your film. 

 What do you conclude about the effects of light 

 on the incorporation of CO2? What evidence do 

 you observe of the translocation of recently 

 synthesized organic molecules? 



In the intervals of waiting for things to hap- 

 pen, study the prepared slides of the apex of the 

 flowering plant Coleus. The small, darkly stained 

 cells that form a small mound at the apex of the 

 stem represent the apical meristem. Remember 

 that these cells are responsible for the further 

 growth of all the remaining plant structures that 

 are above ground. Note the young leaves de- 



veloping around the apex. Careful observation 

 will reveal much of the differentiation of the 

 tissues that compose the lateral stems and leaves. 

 Note the area that will become vascular tissue. 

 Follow bundles of vascular tissue back to the 

 apex from the largest leaves and the base of the 

 stem. 



Tissue differentiation is much easier to study 

 in plants than animals, because all stages of 

 differentiation appear in a linear sequence start- 

 ing at the apical meristem and working toward 

 whatever type of tissue interests you. 



EQUIPMENT 



Per student 



8 Avena seedlings, grown 3-5 days 



2 petri dishes 



2% sucrose solution (5 ml) 



2% sucrose solution with 2 mg/1 indole acetic acid, 



brought to pH 6.5 with KH2PO4 (5 ml) 



razor blade 



graph paper 



0.5-ml centrifuge tube containing BaCos 



wooden sticks (12" long) 



piece of 5" X 7" (no screen) x-ray film 



piece of 5" X 7" cardboard 



2 rubber bands 



black paper envelope (8" X 10") 

 prepared slide of apex of coleus 



Per 2 students 



bean plant {2-A weeks old) 



black wooden box with colored light (red, green, 



blue, and yellow) filter 



Per 8 students 



roll of Scotch tape 



H2SO4, 1 M (5 ml) and capillary pipet 



jar of vaseline 



Per laboratory 



space outfitted as a darkroom 



developer and fixer for processing x-ray film 



