76 PHOTOSYNTHESIS AND RESPIRATION. 



Here again it seems necessary to emphasise the necessity 

 for making " control " experiments. The following four 

 experiments, for instance, are not conclusive or satisfactory 

 unless in each case you also test leaves on the same plant which 

 have been left untouched ; in this case, as in the experiments 

 of Art. 109, the leaves which are being investigated must be 

 left on the living plant. 



* (d) Smear the lower surface of a leaf with vaseline ; this will block the 

 stomates, which in many leaves are mostly or even entirely found on 

 the lower surface. Find out the distribution of stomates on the two 

 sides of each leaf you experiment with by tearing off and examining 

 with the microscope pieces of the lower and the upper skin. If you 

 have no microscope, dipping the leaf into hot water gives a rough idea 

 on this point. 



* (e) Smear a small circular area of the leaf with vaseline, applying it 

 to both sides, so that this part of the leaf will be shut off on both sides 

 from communication with the atmosphere through the stomates. 



* (/) Smear with vaseline the upper surface of another leaf. 



* (g) Smear both surfaces of another leaf. 



The results of these simple experiments will show that the 

 air is, somehow or other, concerned with the process of 

 starch-making (Art. 113). What about leaves which grow 

 under water, like those of submerged aquatic plants ? 



* (h) In an experiment on germination we prevented the access of air 

 to seeds by covering them with water, and in the preceding experi- 

 ments we have excluded air by covering the leaf with vaseline. Tie 

 the stalks of the leaves of some ordinary land-plants to a stone and sink 

 them under water in a glass jar. Expose to light, and afterwards test 

 the leaves for starch. 



* () Repeat the last experiment with the leaves of a plant which lives 

 submerged in water e.g. Water Starwort, Canadian Water- weed, or 

 any other plant you find growing below water in ponds or streams. Do 

 these submerged leaves possess stomates ? 



111. Is Warmth needed for Starch-Making? It is 



very easy to find out whether temperature has any influence 

 on starch-production by leaves. Place a leaf in a saucer or 

 jar kept cold by ice, expose to light, and after several hours 

 test for starch. 



