VIII.] MINUTE STRUCTURE, 113 



add a small drop of weak tincture of iodine, when tliey 

 ought at once to become a deep violet ; for iodine forms 

 with starch a violet-coloured compound. 



Similar granules to those of the Potato, allowing for dif- 

 ferences in size and form, you may find in nearly all flowering 

 plants. They are especially abundant in thickened roots, 

 in underground stems, and in seeds. In these organs the 

 starch is stored as a temporary reserve, to be made 

 use of after a winter's rest, or (in the seed) at the time of 

 germination. The granules then dissolve, and may be said 

 to be eaten by the protoplasmic cell-contents. Starch is 

 identical in chemical composition with the ternary substance 

 of which the cell-wall is formed : it differs from it in being 

 a temporary deposit instead of a permanent one. It 

 originates always in direct connection with the chlorophyll- 

 granules of the leaves, whence it is conveyed, in what we 

 may here conveniently speak of as in solution, to those 

 tissues in which growth is actually taking place, or to other 

 organs destined to receive reserves of food available for 

 future use. Under high magnifying power the individual 

 grains of starch present more or less distinctly the appear- 

 ance of successive partial zones around a usually eccentric 

 focus. This is due, like the zoning in thickened cell-walls, 

 to the alternation of belts richer and poorer in water, and 

 so varying in refractive power. 



Another form in which temporary reserves are stored up 

 in the cells we find in the globules of oil, abundant in some 

 cells, especially of certain seeds and fruits. Hence we find 

 the principal source of our vegetable oils in the fruits of 

 different plants, as Sasamum (embryo), Olive (pulp of 

 drupe), Cocoa-nut (albumen), &c. 



Sugar is another food-deposit of plants, differing from 

 starch in being soluble in the watery cell-sap which filis the 



