164 THK PLANT CELL. 



leaf (Chap, ii.), it was seen that the starch-granules formed in 

 them were produced (or rather stored) during the daytime in 

 the presence of light, but in the formation of starch in the 

 plastids in the absence of light a somewhat different process goes 

 on, although the ultimate product is the same. Thus, in 

 plastids it is highly probable that elaborated fnnrl (?ngar^. 

 from the leaves is used, and gradually worked up by the proto- 

 plasm, each plastid being, as has been pointed out, a specialised 

 portion of the cytoplasm of a cell, and us such, capable of acting 

 as a "plastic" body. After the plastid or chloroplast has been 

 completely converted into starch, the further growth of each 

 granule goes on by a process of accretion, the main cytoplasm of 

 the cell forming successive layers. \The "hilum in the larger 

 granules may at times indicate the posraon of a former plastid,, 

 but is more often produced by splitting of the centre of the granule/ 

 producing a tri-radiate figure. Compound and semi-compound 

 starch-grains are also found in the cells of the potato-tuber. 



The blue reaction of starch with iodine indicates the forma- 

 tion of a definite but rather unstable chemical compound, which 

 is readily destroyed by heating or treatment with alcohol. 



Boiling in water causes the granules to swell, and finally a 

 sort of sac or shell is produced, formed by an external substance 

 known as farinose, enclosing a granular substance, or granulose 

 which takes up iodine. Caustic potash also causes a swelling of 

 the granules, and a dilute acid or a solution of diastase will 

 dissolve the granules, especially on gently warming, with the 

 formation of dextrin. 



The chemistry of starch-formation is rather complex. It is 

 not intended here to give more than a brief outline of the 

 process, which is, as yet, somewhat undetermined. It may, 

 however, be mentioned, that a good deal of the starch in plants 

 is the result of anabolic processes, and not of katabolic. 

 The main feature in these processes seems to be the elimination 

 of oxygen. Experimental evidence points to the fact that there 

 are many stages between CO^, HgO, and starch in the anabolism 

 of these substances by the chloroplasts, and between protoplasm 

 and starch during the katabolism of the living substance. 



It has been thought that formaldehyde is an intermediate 

 product during the formation of starch (or sugar) in chloroplasts 

 or plastids, and that this, by elimination of water, is converted 



