20 COMPENDIUM OF GENERAL BOTANY. 



valuable microcbemical tests, namely, bine coloration with an 

 aqueous iodine solution. Previous boiling of the starch-bearing 

 substance in water is recommended. When the quantity of starch 

 is very small, Arthur Meyer's plan will be found useful. It 

 consists in decolorizing and extracting the plant-substance by means 

 of alcohol and then adding iodine in a chloral-hydrate solution. 



Only the percentage composition is known which corresponds 

 to that of cellulose: n{CJl^fi^. According to JSTageli ^im. and 

 Akthue Meyer starch-grains do not consist of two or more starch 

 modifications as was formerly believed by IS^ageli sen. Red or 

 reddish-brown coloration with iodine simply demonstrates an 

 association with amylodextrin. 



The ferment diastase, which plays such an important part in 

 germinating cereals by virtue of its starch-dissolving effect, has 

 already been mentioned. Its true nature and method of action is 

 still unknown. Other substances, such as dilute acids, through long- 

 continued action will also dissolve the entire mass of starch-grains 

 (Niigeli jun.). 



The behavior of starch-grains with water is especially interesting. 

 Due to internal (molecular) causes, dry but otherwise intact starch- 

 grains " imbibe" a definite amount of water. There is besides this 

 imhihitlon a sioelling due to external causes. Tiiis swelling is 

 caused by a greater or lesser absorption of water at a high tempera- 

 ture, or by the addition of acids or alkalies. If such a swollen 

 starch-grain is dried and again supplied with water it no longer 

 assumes its former volume ; its structure has become modified, 

 something which never follows '"imbibition" (Niigeli, Correns). 

 Our foods (as boiled potatoes, peas, bread) contain paste (that is, 

 swollen starch), since they have been exposed to high temperatures. 

 This swollen starch is subsequently converted into soluble sugar by 

 means of the saliva (ptyalin) and the pancreatic juice (amylopsin). 



We shall now add a few remarks on the morphology of starch- 

 grains, especially as to their form and size. In many instances not 

 only genera and species but entire families may be recognized by 

 characteristic starch-grain>. Potato-starch is characterized by an ex- 

 centric nucleus (hilum), and is of oval or conical form. Starch- 

 grains of the Leguminosce (seeds) are oval with a concentric hilum ; 

 those of our indigenous cereals are lentil-shaped, very small and with 

 concentric hilum ; those from the milky juice of Euphorhia species 

 are dumbbell-shaped. The smallest starch-grains approach the 



