24 VEGETABLE ELEMENTS AND TISSUES. 



Starch- grains are altered by boiling in water, be- 

 coming swollen and often changed into curious forms, 

 the rings becoming faint or disappearing. If a piece 

 of boiled potato be examined, the starch- granules 

 will seem to have vanished from the cells, which are 

 swollen and covered with an irregular kind of net- 

 work. The network consists of parts of the proto- 

 plasm situated in the interstices of the starch-granules, 

 and solidified or coagulated by the heat. On crush- 

 ing the cells by pressing upon the cover, the starch- 

 granules will escape, swollen and partly fused toge- 

 ther ; but they may easily be recognized as consist- 

 ing of starch by the iodine test. 



The granules of " tons les mois " starch are par- 

 ticularly well adapted for showing the concentric 

 rings, the granules being about twice as large as those 

 of the potato. 



Starch-granules are best examined in water; and a 

 small quantity only of the starch must be placed on 

 the slide, if the structure of the granules is to be 

 seen clearly. They may be mounted in glycerine, 

 although this makes them very transparent. 



To those who possess a polariscope, starch-gra- 

 nules are particularly interesting, as they exhibit a 

 black cross, and, with a plate of selenite laid beneath 

 the slide, a beautiful play of colours. 



In addition to the starch and chlorophyll, the 

 cells of plants contain other matters, as gum, sugar, 

 &c. ; but as they are dissolved in the cell-liquid, they 

 are not visible. In the cells of certain plants, how- 

 ever, spherical globules, with light centres and black 

 outlines, will be met with : these consist of oil. 



Raph'ides. Lastly, occurring in the cells of plants, 

 especially such as are soft and juicy (succulent), will 

 be found minute, hard, colourless crystals, called 

 r aphides (pa$>l<$ } a needle) . These are most frequently 

 needle-like or acicular (acus, a needle), but sometimes 

 prismatic or rod-like with flat sides; they are also 



