Lecture VI. 



43 



centrating our attention on the chloroplasts we can see that they 

 have a higher refractive index than the surrounding protoplasm 

 and that the green colour is rigorously limited to their substance. 

 Within each chloroplast one or more very minute specks can 

 usually be seen. If we replace the water surrounding our pre- 

 paration with alcohol or methylated spirit, we will soon observe 

 that the green pigment, which was evidently insoluble in water, 

 quickly dissolves out, leaving the chloroplasts much paler or 

 colourless. If now a little liquor iodi be run into the prepara- 

 tion, the characteristic reaction for starch will be shown by the 

 specks within the chloroplasts. Evidently the chloroplasts have 

 been behaving just like the . chloroplasts of Chlamydomonas 



FIG. 8. Glass cell, within which is 

 seen submerged in water a branch 

 of Elodca canadensis. On expos- 

 ure to light bubbles of oxygen are 

 given off. 



FIG. g. Elodea canadensis, cell of 

 leaf, x 350. , cytoplasm ; , nuc- 

 leus; p, chloroplast; v, vacuole ; 

 w, cell-wall. 



and have built up starch from the carbon dioxide and surrounding 

 water. The oxygen set free by the process has been liberated 

 as bubbles, either at the external surface of the cell, or injected 

 into one of the passages of the leaf and from thence finds its 

 way into the stem and emerges at its cut or broken surface. 



The process taking place in an illuminated green cell may be 

 represented in a simplified form as follows : 



6CO 2 + sH 2 O = C 6 H 10 O 5 + 6O,>. 



Carbon Water Starch Oxygen 



dioxide 



In this process the volume of carbon dioxide dealt with is equal 

 to the volume of oxygen set free. Inasmuch as oxygen is much 



