PHOTOSYNTHESIS 59 



in that portion partly because of lack of light, and partly because 

 the supply of air (and therefore of carbon dioxide) was very 

 scanty under the cork. The truth of the supposition that lack of 

 carbon dioxide was responsible for the failure to make starch 

 may be tested by boring a large hole with a cork borer through 

 each cork before fastening it in place on the leaf, and cementing 

 over the hole a thin cover glass. Then some of the parts of the 

 leaf covered by the cork are lighted while others are not, and if 

 the lighted parts show starch, it was lack of light only that 

 prevented its formation in the shaded parts. 



Eefeeences. (See Experiment XXXII.) 



EXPERIMENT XXXIV 



Can starch making go on when the stomata are shut off from all 

 air supply ? Select a thrifty potted plant of some species which 

 has thin leaves, with stomata only on the under surface (e.g. prim- 

 rose, begonia). Put the plant in an absolutely dark place for 

 twenty -four hours, and then coat half of the under surface of one 

 or more leaves with vaseline and expose the plant for a day to 

 bright sunlight. Wipe off most of the vaseline with cotton wool 

 and remove the rest by washing, with a swab or soft brush, in 

 several successive quantities of benzine.^ Then boil, treat with 

 alcohol, and test for starch as directed in Exp. XXXII. Explain 

 the result. 



Eefeeence. Ganong, 10. 



EXPERIMENT XXXV 



Can squash seedlings make chlorophyll in the dark ? * * Plant 

 some squash seeds in sawdust or sand, and keep part of them in 

 a good light, while others are kept in total darkness at about the 

 same temperature. When the plumules of those in the light ar,e 

 developed into half-grown leaves, sketch both lots of seedlings 



1 Do not attempt this in the same room with a flame, or a lighted lamp or 

 gas jet. 



