CELL-CONTENTS AND CELL-WALLS. 59 



(d) Treat another preparation with potash ; the grains 

 swell and become dull, losing their highly refractive pro- 

 perties owing to the additional water which the potash 

 causes them to absorb. Irrigate with water, and treat 

 with iodine ; the swollen grains turn blue, though not so 

 intensely coloured as usual. 



(e) Heat another preparation, holding the slide over a 

 flame till boiling occurs ; the grains swell and become dull 

 in appearance. Add iodine; the grains turn blue, as in 

 the preceding case. 



(/) To bring out more clearly the striations in the 

 grains, scrape some of the contents of a Potato tuber into 

 a watch-glass containing some 5 per cent, silver nitrate 

 solution. Let them remain in this solution for about 

 15 minutes, then transfer them to a watch-glass contain- 

 ing some 1 per cent, solution of common salt, and expose 

 to direct sunlight, in order to reduce the chloride of silver 

 which has been formed within the grains. The less dense 

 layers of the starch will take a grey colour, due to the 

 reduced silver. 



70. Half-compound and compound grains are sometimes 

 found in Potato tuber, in addition to the simple grains with a single 

 hilum. A half -grain compound grain consists of two or more small 

 grains fixed together (usually by their broader ends) and covered 

 by a common outer layered envelope of starch. A compound grain 

 consists of an aggregate of several grains without any common 

 envelope ; in the endosperm of Oats and Rice all the grains are 

 compound, with as many as 100 to 300 small grains. 



The starch-grains of other plants should be examined. Those of 

 Bean and Pea cotyledons are rounded or ovoid but centric. Those 

 of Wheat endosperm are rounded discs, and those of Maize poly- 

 gonal and densely packed in the endosperm cells. 



71. Leucoplasts are colourless plastids. They occur in various 

 tissues in which starch is being stored. In the "pseudo-bulb" of 

 Phajus, of which prepared sections can be bought, the leucoplasts 

 are long disc-like bodies. They can be found, however, in the 

 rhizome of Canna or Iris, or in Potato tubers. 



Get some young Potato tubers. To harden the tissues thoroughly 

 cut the tubers in pieces, not larger than a Pea, and place these in 

 strong picric acid. A rapid method is to cut sections from the sur- 

 face of the young tuber (the leucoplasts are most abundant in the 



