THE PRODUCTS OF THE ASSIMILATION OF CARBOX DIOXIDE 319 



unchecked, its products must be continually removed, for any excessive 

 accumulation may cause further assimilation to cease (Sect. 55). As a 

 matter of fact the sugar, which is probably glucose, does not under normal 

 conditions accumulate beyond a certain percentage, being then either removed 

 or converted into starch or proteid, or the glucose may be converted into 

 saccharose and thus the osmotic concentration decreased, for the inhibitory 

 influence is mainly dependent upon the osmotic concentration. Hence a 

 marked accumulation of insoluble or feebly osmotic assimilatory products is 

 possible without injury to the power of carbon dioxide assimilation l . 



Carbohydrates ; Glucosides. The presence and distribution of starch in a leaf 

 can be macroscopically determined by simple treatment with a solution of iodine 

 in chloral hydrate, or by staining with an iodine solution after immersing the leaf 

 in boiling water and subsequently extracting the chlorophyll with alcohol 2 . By 

 careful treatment even very minute starch grains can be detected. Starch may 

 appear in most chloroplastids, but it is not formed in those of Allhun cepa 3 , even 

 when assimilation is active and 

 an abundance of sugar accumu- 

 lates ; under such conditions 

 starch appears, however, in the 

 chloroplastids of Musa, Hemero- 

 callis fulva, Muscari nwschatum, 

 and many other plants in which 

 a moderate accumulation of sugar 

 does not suffice to induce a de- 

 position of Starch 4 (Sect. 55)- FIG. 49. Leaf coloured with iodine after the method described. 



A larger or smaller amount of 



sugar will always accompany starch, and in Al/ium the assimilatory products 



are stored up entirely in the form of sugar. 



The sugar is commonly a reducing one, usually dextrose or laevulose, but 

 probably all forms of sugar which are translocated or stored may also be formed 

 as direct products of carbon dioxide assimilation. Thus saccharose has often been 

 detected, mannite occurs in the leaves of certain Oleaceae, and sinistrin in those of 



Zeitung, 1885, p. 420; Chrapowicki, Bot. Centralbl., 1889, Bd. xxxix, p. 352; Schimper, Flora, 

 1890, p. 260; Saposchnikoff, Bot. Centralbl., 1895, Bd. LXIII, p. 246. 



1 Cf. Ewart, Journ. Linn. Soc., Vol. xxxi, 1896, pp. 429, 435. 



2 Sachs, Arb. d. Bot. Inst. in Wiirzburg, 1884, Bd. in, p. 3; Flora, 1862, p. 166 ; Bohm, 

 Sitzungsb. d. Wien. Akad., 1857, Bd. xxil, p. 500. Cf. Zimmermann, Bot. Mikrotechnik, 1892, 



p. 221. 



3 Bohm, Sitzungsb. d. Wien. Akad., 1857, Bd - XXII > P- 5 > Sachs > Experimentalphysiol. , 1865, 

 p. 326. Starch may, however, appear in the guard-cells of the stomata and in the bundle-sheaths 

 of the leaves. Sachs, I.e. ; A. Meyer, Bot. Zeitung, 1885, p. 456'; Rendle, Annals of Botany, 1888, 



Vol. II, p. 161. 



4 Literature: A. Meyer, Bot. Zeitung, 1885, pp. 451,467, &c. ; Schimper, ibid., p. 786 ; Nadsoii, 

 Bot. Centralbl., 1890, Bd. XLII, p. 50; Godlewski, Flora, 1877, p. 216. [A little starch may 

 apparently be deposited in the tissues of the bulb of Allinm cepa in the immediate neighbourhood of 

 young growing shoots.] 



