STRUCTURE OF STAlfCTI 25 



cotyledons from seedlings that have been growing for three or four weeks. 

 Stain the sections with iodine solution, and notice how completely the clusters 

 of starch grains that filled most of the cells of the unsprouted cotyledons have 

 disappeared from the shriveled cotyledons of the seedlings. 



References. Strasburger-Hillhquse, 6; Tschirch, 74. 



14. Structure of starch.* * 



A. Cut moderately thin sections of a potato tuber, mount in 

 water, and examine with m.p. (medium power). Note the 

 starch grains inclosed in little chambers or cells. Kun in a 

 little weak iodine solution (Sec. 169) under one edge of the 

 cover glass, at the same time withdrawing water from the 

 opposite edge with a bit of blotting paper. Watch the sec- 

 tion carefully during the process and note the gradual stain- 

 ing of the starch grains. Draw. 



B. Mount in water some pulp scraped from a freshly cut sur- 

 face of potato and examine with h.p. (high power). Move the 

 fine adjustment constantly while observing, and note the lines 

 arranged, somewhat concentrically about a point called the 

 hilum, often marked by minute cracks in the grain. Draw 

 several grains. 



Is there any evidence that the starch grain is composed of suc- 

 cessive layers ? If so, how may they have been caused ? 



C. Draw to the same scale as seen under h.p. all the principal forms and 

 sizes of potato starch grains that you can find, together with grains of 

 several other kinds, as canna starch (from the rootstock), oat starch, 

 corn starch, and Euphorbia starch (from E. splendens). 



References. Strasburger-Hillhouse, 6 ; Tschirch, 74. 



EXPERIMENT VIII 



Determination of oil in flaxseed. Weigh out two ounces (or sixty 

 grams) of ground flaxseed and add an equal volume of ether or 

 benzine. Do not bring tJiese liquids near a gas Jet or an// other 

 flame. Let it stand ten or hfteen minutes and then filter. Wash 

 the meal by pouring over it, a little at a time, about the same 



