PLASTICITY OF SHOOT AND ROOT 1 99 



a. How may simple be distinguished from com- 



pound leaves ? 



b. Do leaves always continue to grow straight 



out from their points of origin ? 



29. You have noticed that seedlings turn green as 

 they come into the light, and further observa- 

 tion shows that in general only parts exposed 

 to light are green. It is known to physiologists 

 that starch, which is the real food of plants, 

 is made only in green parts. Is there any 

 connection between these facts, i.e. is light 

 essential to starch formation ? Answer by 

 Experiment 4. 



The exchange of gases, particularly O and CO 2 , 

 in these processes, is important. What gas is 

 given off in starch making (photosynthesis or 

 assimilation)? Answer by Experiment 5. 

 If the gas proven by Experiment 5 is given off, 

 inferentially the other is absorbed, and hence 

 necessary to the process. Is it necessary ? 

 Answer by Experiment 6. 



Materials. - These should be preferably potted plants, 

 studied in a greenhouse or brought to the laboratory ; they 

 need not be injured. Or herbarium material, collected for 

 the purpose on the principle earlier discussed (page 106), 

 may be used. Extreme modifications in adaptation to special 

 function may best be left for another week ; here should come 

 " stemless " plants (primroses, houseleeks), flat-stemmed 



