THE METABOLISM OF PLANTS. 183 



With . reference to the optical properties of starch-grains von Mohl 

 pointed out that the interference colours of starch-grains are complemen- 

 tary to those of cell-walls, the former being optically positive and the 

 latter negative. Strasburger refers this difference in optical properties to 

 the differences of the tensions in the two cases, the tensions in starch-grains 

 being of the nature of traction, in cell-walls of compression. 



A few words may be added as to the chemistry of starch. A starch- 

 grain consists of two forms of carbohydrate. If a starch-grain be treated 

 with dilute sulphuric or hydrochloric acid, a portion of it will be dissolved, 

 leaving a skeleton which retains the form of the grain, and which consists 

 of a substance termed starch-cellulose, closely allied to ordinary cellulose. 

 The dissolved substance is termed granulose j it turns blue on treatment 

 with iodine, and it is in fact to the presence of this substance that the 

 characteristic blue colour which starch-grains assume with iodine is due ; 

 it appears to be slightly dissolved when starch-grains are rubbed in a 

 mortar with cold water ; it is soluble in dilute acids and hi concentrated 

 solutions of certain salts such as potassium bromide and iodide, cal- 

 cium and zinc chloride. When a starch-grain is treated with a solution 

 of an unorganised ferment, the granulose first disappears, and then the 

 cellulose-skeleton is slowly dissolved (Fig. 28). 



From the researches of W. Naegeli and of Sachsse it appears that the 

 formula of starch (granulose ?) is 6 (C 6 H 10 O 5 ) = C se H.6oO 3(> : Pfeiffer, how- 

 ever, concludes from the compounds which it forms with alkalies, that its 

 formula is probably 



FIG. 28 (after Baranetzky). Starch-grains from a potato in various stages of 

 solution under the action of a diastatic ferment : in c and d scarcely anything 

 is left but the cellulose-skeleton. 



The reserve-proteids of seeds are stored up, as before men- 

 tioned, in the form of granules, aleurone-grains. These gran- 

 ules are especially well-developed in oily seeds, being much 

 larger than they are in starchy seeds. When a section of an 

 oily seed is examined, the aleurone-grains are seen occupying 

 the interstices of the protoplasm in some or all of the cells. 

 On the addition of water they swell-up, and, owing to the 

 solution of part of their substance, their structure becomes 



