126 THE GREEN PLANT CELL 



(4) Mount a cross section of the fresh stem of Pellionia in a 

 drop of water. Notice under the high power the chloroplasts 

 in the cells near the surface of the section, with starch grains (bright 

 colourless granules) inside them. In cells further from the 

 surface note that the starch grains are larger and have burst 

 out of the chloroplasts. The chloroplast which has formed the 

 grain can often be seen attached to one end of the grain. The 

 growth of these deeper lying grains is continuous and is pro- 

 bably the result of a continuous supply of sugar, while the 

 sugar supply of the superficial cells probably depends, as in a 

 mesophyll cell, directly upon photosynthesis and therefore upon 

 light, so that the starch grain ceases to grow and is converted 

 back into sugar every night. Draw examples under the high 

 power of different relations of starch grain and chloroplast, and 

 then test with iodine. 



(5) With the hand spectroscope compare the spectrum of the 

 alcoholic solution of chlorophyll (i) with that of the translucent 

 green leaf and with the spectrum of sky light. [Compare also 

 with the demonstration spectra.] Note the absorption bands of 

 the chlorophyll spectrum. 



(6) Examine the demonstration water cultures, and observe 

 the effects of leaving out each of the essential elements in the 

 mixed solution of salts used to feed the plant. 



