OR ' PHOTOSYNTHESIS ' 



215 



place the thumb over the open end of the tube while it is below water, so 

 as to prevent air from getting in. Take out the tube completely, turn it 

 up, and keep the thumb over the end of the tube all the time ; then 

 remove the thumb, and plunge a smouldering match-stalk into the gas. 



Although the gas collected is not pure oxygen, 

 it contains a considerable proportion of the latter, 

 and causes a smouldering match to burst into 

 flame when placed in it. 



Ex. 120. — (i) Tie a terminal shoot of Elodea 

 4 to 6 inches long to a glass rod, and place so 

 that the broken end of the shoot is uppermost 

 in a tall glass cylinder full of well water. 



Expose the whole to bright daylight ; notice 

 and count the number of bubbles of oxygen which 

 rise from the broken end of the shoot in two or 

 three minutes. 



(ii) Move the apparatus to a badly-lighted 

 room, and count the bubbles rising in the same 

 time as before. Do more bubbles rise when the 

 plant is exposed to bright light than when exposed 

 to a dim light ? 



Ex. 121. — Repeat the above experiment, usihg 

 boiled water from which all the carbon dioxide 

 has been driven off. Notice that little or no gas 

 is evolved. Now supply carbon dioxide to the 

 water by blowing through a glass tube into it. 

 Ex. 122. — Repeat Ex. iig, using roots, flowers, or other portions of plants 

 which are not green, to show that oxygen is not evolved from such parts. 



Ex. 123. — (i) In the afternoon of a warm, bright day pluck off a leaf from 

 several common broad-leaved plants, and test for starch in them, thus : — 



First place them in boiling water for a minute, after which transfer them 

 to a vessel containing wann methylated spirits to dissolve out the chlorophyll 

 and other pigments. Leave them in the latter for a few hours until they are 

 pale in colour, and then transfer them to a saucer containing a solution of 

 iodine (see Ex. 85). 



If they contain starch they will turn black or deep purple, 

 (ii) Test for starch in leaves variegated with white patches and show that 

 none is formed in the white parts from which chloroplasts are absent. 



Ex. 124. — (i) Smear one half of a pear or poplar leaf with cacao butter or 

 best lard on both sides to block up the stomata. Leave for two days, and 

 in the afternoon of the following day, test the whole leaf for starch, after 

 removing the butter with hot water. 



