LEAVES FUNCTION OR WORK 



101 



Leaves are sometimes modified to perform other functions 

 than the vital processes: they may be tendrils, as the 

 terminal leaflets of pea and sweet pea; or spines, as in 

 barberry. Not all spines and thorns, however, represent 

 modified leaves: some of them (as of hawthorns, osage 

 orange, honey locust) are branches. 



SUGGESTIONS. To test for chlorophyll 84. Purchase about a 

 gill of wood alcohol. Secure a leaf of geranium, clover, or other 

 plant that has been exposed to sunlight for a few hours, and, after 

 dipping it for a minute in boiling water, put it in a white cup with 

 sufficient alcohol to cover. Place the cup in a shallow pan of 

 hot water on the stove where it is not hot enough for the alcohol 

 to take fire. After a time the chlorophyll is dissolved by the 

 alcohol, which has become an intense green. Save this leaf for 

 the starch experiment (Exercise 85). Without chlorophyll, the 

 plant cannot appropriate the carbon dioxid of the air. Starch 

 and photosynthesis. 85. Starch is present in the green leaves 

 which have been exposed to sunlight ; but in the dark no starch 

 can be formed from carbon dioxid. Apply iodine to the leaf from 

 which the chlorophyll was dissolved in the previous experiment 

 Note that the leaf is colored purplish brown throughout. The leaf 

 contains starch. 86. Se- 

 cure a leaf from a plant 

 which has been in the dark- 

 ness for about two days. 

 Dissolve the chlorophyll as 

 before, and attempt to stain 

 this leaf with iodine. No 

 purplish brown color is pro- 

 duced. This shows that 

 the starch manufactured in 

 the leaf may be entirely 

 removed during darkness. 

 87. Secure a plant which 

 has been kept in darkness 

 for twenty- four hours or 

 more. Split a small cork 

 and pin the two halves on opposite sides of one of the leaves, as 

 shown in Fig. 120. Place the plant in the sunlight again. After 

 a morning of bright sunshine dissolve the chlorophyll in this leaf 

 with alcohol ; then stain the leaf with the iodine. Notice that the 

 leaf is stained deeply except where the cork was ; there sunlight and 

 carbon dioxid were excluded, Fig. 121. There is no starch in the 



FIG. 120. EXCLUD- 

 ING LIGHT AND 

 CO 2 FROM PART 

 OF A LEAF. 



FIG. i2i. THE 

 RESULT. 



