DIRECTIONS FOR LABORATORY STUDY. 397 



Darken portions of some leaves of a plant previously found to 

 show starch in its leaves (sunflower, bean, tomato, or nasturtium) 

 by attaching two plates of cork on opposite sides by means of 

 two pins driven through both and the leaf. On the afternoon of 

 the following day, if sunny, cut off the leaves and test for starch. 

 What has become of starch in cells under the cork? 



23. To show that oxygen is a by-product of photosyntax. (^[ 250.) 

 Collect the gas mixture evolved from a vessel full of aquatic 



plants by inverting over them a funnel to whose tip is connected 

 a test tube filled with water to be displaced by the rising gases. 

 Keep the plants in sunlight. When the tube is filled, test the 

 contents for oxygen by inserting a glowing splinter. 



24. To show the effect of light and temperature on photosyntax, 

 ttsing the rate of evolution of oxygen as an index. 



Fasten a shoot of a water plant (Elodea, Myriophyllum, or Cerat- 

 ophyllum) 10 cm. long to a glass rod and immerse in tap water so 

 that the cuf end is uppermost. Set in sunlight and observe the 

 bubbles rising from the end of stem.* Determine rate at which 

 they rise by counting the number given off in a certain short 

 time. Continue the observation until the rate is approximately 

 uniform. Shade the shoot and determine rate. Return to sun- 

 light and determine rate. Put a piece of ice in the water and de- 

 termine again. 



25. To show the digestion of starch by diastase. (^[ 237.) 

 Powder a handful of malt in a mortar or obtain ground malt. 



To 25 grams of the powder add 100 cc. of water; stir well to- 

 gether; allow mixture to stand (with occasional stirring) one to 

 two hours; filter; preserve the filtrate. Take i gm. of starch 

 and rub it up in a dish with 5 cc. water; pour this into 95 cc. of 

 boiling water, stirring as it enters. With 25 cc. of this paste mix 

 thoroughly 5 cc. of the filtrate (which contains diastase extracted 

 from the malt). Test a small portion of the mixture at once for 

 starch by adding a few drops of tincture of iodine, and similar 

 portions at intervals of half an hour until starch reaction ceases. 

 Taste the remaining paste. Into what has the starch been con- 

 verted ? 



26. 7^o show evolution of CO -2 by respiration of leaves and 

 flowers. (^[[239.) 



* If several bubbles arise at once, remove shoot from water, dry the cut end of stem 

 with filter paper and coat it with a *hin layer of grafting wax ; then perforate this wax 

 witli a fine needle point so as to offer one exit for gases. 



