60 PHYSIOLOG Y. 



of CO, and giving off of oxygen), and that starch is a product of 

 that chemical change. 



143. Schimper's method of testing for the presence of starch. 



— Another convenient and quick method of testing for the pres- 

 ence of starch is what is known as Schimper's method. A 

 strong solution of chloral hydrate is made by taking 8 grams of 

 chloral hydrate for every $cc of water. To this solution is 

 added a little of an alcholic tincture of iodine. The threads of 

 spirogyra may be placed directly in this solution, and in a few 

 moments mounted in water on the glass slip and examined with 

 the microscope. The reaction is strong and easily seen. 



144. We may test vaucheria which has been grown under like 

 conditions in the same way. We find here also that the starch 

 is present in the threads which have been exposed to the sun- 

 light, while it is absent from those which have been for a suffi- 

 ciently long time in the dark. 



145. We should also examine the leaves of elodea, or one of 

 the higher green plants which has been for some time in the 

 sunlight. We may use here Schimper's method by placing the 

 leaves directly in the solution of chloral hydrate and iodine. 

 The leaves are made transparent by the chloral hydrate so that 

 the starch reaction from the iodine is easily detected. 



146 If we wish to use the potassium iodide of iodine the leaves should be 

 first boiled for a short time in water, then heated for some time in alcohol or 

 the alcohol changed several times. The green color is extracted slowly by 

 this process, and will be hastened if the preparation is placed in the sunlight. 

 (If care is used the leaves may be boiled in alcohol.) After the leaves are 

 decolorized they should be immersed in the potassium iodide of iodine. 



147. Green parts of plants form starch when exposed to 

 light. — Thus we find that in the case of all the green plants we 

 have examined, starch is present in the green cells of those which 

 have been standing for some time in the sunlight where the proc- 

 ess of the absorption of CO a and the giving off of oxygen can 

 go on, and that in the case of plants grown in the dark, or in 

 leaves of plants which have stood for some time in the dark 

 starch is absent. We reason from this that starch is the product 



