64 PLANT PHYSIOLOGY 



united on the left to H, on the right to OH, the corresponding C-atom of the 

 enantiomorph has the H and OH arranged in exactly the reverse order. Along 

 with the chemical structure runs an optical character, i.e. the way in which 

 the compound affects polarized light ; 8 hexoses (/-) turn the beam to the left, 

 8 (d-) to the right. It has been shown that only the dextro-rotatory isomers, 

 but not all of them, are fermentable, viz. ^-glucose, d-mannose, and ^-galac- 

 tose. Comparing their constitutional formulae, rf-mannose shows an exchange 

 of the H and OH groups attached to the first C-atom and ^-galactose of those 

 attached to the third, as contrasted with rf -glucose. In d-talose, already referred 

 to, an exchange of the groups attached to both first and third C-atoms takes 

 place, and thus removes ^-talose from the series of fermentable substances. 

 The structure of the ketohexoses is indicated by that of d-fructose (laevu- 

 lose) : 



CH 2 OH 

 I 



c=o 



I 



HO C H 

 H C OH 

 H C OH 

 CH 2 OH 



One sees that the groupings round the three asymmetric C-atoms entirely 

 correspond with those round the three lower C-atoms of ^-glucose and 

 d-mannose ; hence laevulose is also fermentable, but it is the only one of all 

 the ketohexoses with this capacit}'. We have already called attention to the 

 fact that, in spite of its being called ^-fructose, it turns the polarized ray to 

 the left. 



There are altogether four fermentable hexoses known. ^-Glucose (dex- 

 trose) and laevulose are almost equally fermentable, and mannose is apparently 

 scarcely less so. On the other hand, the various species of Saccharomyces 

 behave differently towards galactose. S. pastorianus (I) ferments it approxi- 

 mately as rapidly as it does the three other hexoses ; S. ellipsoideus ferments 

 it only slowly, and S. productivus and 5. apiculatus not at all. As to the real 

 connexion between constitution and fermentability we are still in the dark. 



210, 11. 9-10, for which together can also hydro lyse maltose and saccharose 

 read into d-galactose and ^-glucose. 



11. 21-30, for There is another question . . . circumstances read On the 

 other hand, the yeasts split up glycogen, and some of them also dextrine. 



According to H. PRINGSHEIM (1907) a definite source of nitrogen must 

 be present as well as a fermentable sugar, if fermentation is to take place. 

 For this purpose ammonia, amino-acids, or peptone are suitable, but many 

 other organic nitrogenous compounds, which in their structure are far removed 

 from amino-acids, are quite unsuitable ; still these substances often induce 

 very apparent growth in yeasts. 



1. 42 P. 213, 1. 23, for It is quite possible ... by that gas read The split- 

 ting of sugar into alcohol and carbon-dioxide is remarkably thorough, as may 

 be best seen by comparing the formula of /-glucose with those of its fermenta- 

 tion products : 



COH . (CHOH) 4 . CH 2 OH = (CH 2 OH . CH 3 ) 2 +(CO 2 ) 2 



This splitting can also be effected outside the cell by the action of caustic 

 potash in sunlight on glucose (DUCLAUX quoted in LAFAR'S Mycologie, IV. 



