68 VEGETABLE HISTOLOGY. 



of simple grains, while oat, rice and colchicum starch are com- 

 pound. 



Nearly all starch grains possess a nucleus or Jiilum, around 

 which the granule is built up in layers, which differ from each 

 other in transparency, owing to different amounts of water in 

 the different layers. The layers are concentric or eccentric 

 according as the nucleus is central or placed to one side. Exam- 

 ples of these are bean and potato starch respectively. 



Starch grains differ from one another in a number of ways, 

 some of which are the following : 



Size and shape of grains. 



Position of hilum (central or eccentric). 



Number and distinctness of stratification lines. 



Degree to which the hilum is fissured. 



Character of fissure. 



POTATO STARCH. 



To see the manner in which starch is packed in the cells of 

 the various plant organs, examine cross-sections of the potato 

 tuber and podophyllum rhizome. 



Cut an oblong block about a half-inch square on the end from 

 a potato, having the outer coat on one side. Cut thin sections 

 perpendicular to the coat and mount one in water. Examine 

 first under low power and then under high power. 



There will be seen on the exterior a layer of cork cells like 

 rows of bricks. Next these are some small, closely packed par- 

 enchyma cells, rich in protoplasm, but containing few starch 

 grains and these are small. There may also be seen cubical 

 crystals which look like mineral crystals, but are proteid, as 

 they stain with iodine and are dissolved by caustic potash. 

 Farther interior the cells are very large and filled with large- 

 sized starch grains, and the cubical crystals are wanting. 



Apply dilute iodine solution. The grains assume a dark 

 blue color, the protoplasm stains brown and the cubes the same. 

 It will be observed that the younger starch grains in the outer 

 small parenchyma cells are mostly found in groups. By a spe- 

 cial method of preparation, it would be seen that the grains are 

 grouped around a mass of proteid granules, each one of which 

 is attached to a starch grain. These proteid granules are the 

 starch-builders and are called amyloplasts. They are similar 

 to chlorophyll bodies, and except in a few instances, all starch 

 grains are formed by one or the other kind of these bodies. 



Scrape a slice of potato with a knife and mount in water. 

 The grains are ovate, with one end smaller than the other and 

 a hilum or nucleus at the smaller end. The hilum is sur- 

 rounded first by concentric lines which, farther away, become 

 eccentric. This shows that at first the growth of the grain was 



