viii.] CELL CONTENTS. 97 



that is to say of the chlorophyll itself, as distinguished 

 from the "granule" with which it is always associated, is 

 determined by the action of light, as may be proved by 

 growing plants in total darkness, when they become 

 blanched. The presence, moreover, of a minute quantity 

 of iron is found to be essential to the assumption of the 

 green coloration. The green colour may be easily removed 

 by a little spirits of wine, leaving the granules, which are 

 essentially but simple segregated portions of denser pro- 

 toplasm, almost unaltered. Upon the presence of the 

 green colouring matter of these granules under the in- 

 fluence of solar light depends the most characteristic phe- 

 nomenon of vegetable life the assimilation of material 

 adapted to vegetable and animal growth from the inorganic 

 binary compounds, carbonic acid and water. 



8. STARCH. In observing the tissue of a Potato, re- 

 ferred to above as well adapted to show thin-walled closely- 

 packed cells, you may notice that the cells which are 

 not cut into (and thus emptied), in making a very thin 

 slice, are filled with very minute, oblong, translucent gran- 

 ules. If the slice be too thick, the granules are so nume- 

 rous that they entirely conceal the delicate cell-walls. 

 These granules, which are stored up in nearly all the 

 cells of the Potato, are called starch granules. To 

 compare with potato-starch, you may take the smallest 

 possible pinch of dry arrowroot and dust it upon your 

 slide, and you will find that the granules of which arrow- 

 root consists, though they differ a little in form, are, in 

 other respects, like those of potato-starch. You may 

 make quite sure of it, if you add a small drop of weak 

 tincture of iodine, when they ought at once to assume a 

 deep violet colour : for iodine forms with starch a violet- 

 coloured compound. 



Similar granules to those of the Potato, allowing for 

 differences 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 away as a tempo- 

 rary 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 proto- 

 plasmic cell-contents. Starch is identical in chemical 

 O.B. H 



