46 PRACTICAL BOTANY 



Now wash out the potash thoroughly with water, 

 and irrigate the preparation with iodine solution : 

 the swollen grains will still stain blue, though much 

 paler than before, showing that the swelling with 

 potash does not fundamentally alter the nature of the 

 starch. 



/. Mount some fresh starch in water, and heat it over 

 a spirit-lamp till it boils : on examining under the 

 microscope, the grains will be seen to have swollen and 

 lost their high refractive power, forming the starch - 

 paste such as is used for starching linen : staining with 

 iodine will produce the blue colour, and show that they 

 are only swollen, not dissolved : compare the effect of 

 potash. A temperature of about 65 C. is sufficient to 

 cause this swelling. 



g. Treat a small quantity of the fresh grains with a 

 relatively large bulk of chloral hydrate and iodine (see 

 Appendix A) : they will be seen to swell slowly and give 

 the blue reaction. 



h. Digest starch-grains in saliva for some hours at a tempera- 

 ture of about 45-55 C. Examine them subsequently under the 

 microscope : they will be found to have lost their high refractive 

 power. Stain with iodine : they give a pale blue or yellowish 

 colour. 



i. Treat some starch, which has been kept in strong alcohol for 

 some time, with tincture of iodine : the characteristic blue stain 

 will not be obtained, or only to a slight degree : thus it is to 

 be remembered that the blue reaction of starch with iodine is 

 dependent on the presence of water. 



j. Mount some starch-grains in a single drop of strong sul- 

 phuric ^,cid, and observe quickly under a high power : note that 

 the grains retain their outline, while a dark-looking star-shaped 

 crack is seen occupying the place of the hilum : this is caused 

 by the withdrawal of water from the grain by the acid, the most 



