340 LECTURE XX. 



portion of the granulose appears to pass into solution, and is precipitated by iodine 

 as fine-grained blue pellicles. Starch-grains ground up with fine sand likewise 

 appear to yield an actual solution of granulose in cold water. 



To the very characteristic properties of starch belongs the formation of paste. 

 Water of at least 55° C. causes large, watery starch-grains to swell up vigorously: 

 in the case of small denser grains this only results, according to Naegeli, at 65° C. If 

 sufficient water is present at this temperature the watery inner portion of a grain swells 

 first, and then the outer : the outermost layer scarcely swells at all, it is rather blown 

 up and remains behind in the paste produced by the swelling, in the form of pelli- 

 cular shreds. A dilute solution of potash or soda produces a similar effect in the cold. 

 The volume of a starch-grain may in this manner increase a hundred-and-twenty-five 

 fold, and so much liquid may be taken up that the swollen pasty grain only contains 

 from 2 to o'50/o of substance. With a sufiiciently large quantity of hot water the 

 paste is so distributed that it looks like a homogeneous mass, and this on cooling con- 

 geals to a translucent jelly. After long boiling the starch paste loses the property of 

 gelatinising after cooling : the starch is then converted into a modification soluble 

 even in cold water. 



Large, completely-formed, and especially simple starch-grains are found only in 

 the ripe quiescent reservoirs of reserve-materials ; in growing or vigorously vegetating 

 shoots and roots, and other organs in general where they are evidently being con- 

 tinually formed and dissolved, they are found only in small and often exceedingly 

 minute granules. In such cases however even the smallest quantities of starch may 

 easily be detected under the microscope, if thin sections of the portion of the plant 

 in question are either warmed in potash solution or left for some time lying in it in 

 the cold, then carefiiUy washed out with water neutralised with acetic acid, and very 

 dilute iodine solution finally added. The swoUen-up minute starch-granules now 

 appear as voluminous blue grains. I have employed this method thousands of times, 

 particularly in investigations upon the origin of the starch in the chlorophyll as well 

 as its translocation during the growth of the organs, and much information as to the 

 processes which occur during germination and the passage of material from place 

 to place, to be described in the following lecture,' has been obtained in this way. 



