274 Plant Physiology 



LABORATORY WORK 



Starch. Examine under the microscope and describe the 

 starch grains from a variety of sources, such as Canna (root- 

 stock), potato (tuber), rice or oat kernel, milky juice of Euphor- 

 bia, and seed of beet. Leucoplasts associated with starch grains 

 may be best observed in fixed and stained material, but they 

 are also visible without staining in such favorable material as 

 the young shoots of Canna, or in the young root-stocks of various 

 monocotyledons. 



Rub up about 1 gram of starch with a small quantity of water 

 in an evaporating dish, and when there are no more lumps dilute 

 to 50 cc. Is starch soluble in cold water ? Heat the preceding 

 to the boiling point, and when a paste is formed, examine it 

 microscopically with respect to solubility. 



With the paste above prepared, and with a weak alcoholic 

 solution of iodine, make a complete test of the iodine reaction. 

 In small test-tubes first use a few drops of a strong paste and 

 considerable iodine solution, then weaken the paste up toward 

 a dilution of one hundred times, using also less or weaker iodine. 

 Determine the effect of heating and recooling, also of a few drops 

 of strong caustic potash, upon the iodine reaction. Compare 

 the reactions of the starch paste toward iodine with that of a 

 suspension of starch in cold water. 



Study the distribution of starch in any plant available, em- 

 ploying sections, especially from fleshy roots, leaves, etc. Deter- 

 mine where in the resting twigs of apple, lilac, or maple the 

 storage of starch occurs. In order to stain starch occurring in 

 small quantities in the tissues, especially in the cells of leaves, 

 as of Elodea grown in weak light, or to bring out the starch in 

 chlorophyll bodies, the material may be stained in a concentrated 

 solution of iodine in potassium iodide, when the grains stand out 

 black. Again, a dilute solution of iodine in potassium iodide 

 may be used, and after washing, the material may be laid in a 

 strong solution of chloral hydrate which dissolves most of the 

 cell-contents, swells the stained grains, and in time decomposes 

 these last also, so that a prompt examination must be given. 



